Oxygen Activation and Radical Transformations in Heme Proteins and Metalloporphyrins

Flavin-N5OOH Functions as both a Powerful Nucleophile and a Base in the Superfamily of Flavoenzymes

Flavoenzymes can mediate a large variety of oxidation reactions through the activation of oxygen. However, the O2 activation chemistry of flavin enzymes is not yet fully exploited. Normally, the O2 activation occurs at the C4a site of the flavin cofactor, yielding the flavin C4a-(hydro)hydroperoxyl species in monooxygenases or oxidases. Using extensive MD simulations, QM/MM calculations and QM calculations, our studies reveal the formation of the common nucleophilic species, Flavin-N5OOH, in two distinct flavoenzymes (RutA and EncM). Our studies show that Flavin-N5OOH acts as a powerful nucleophile that promotes C-N cleavage of uracil in RutA, and a powerful base in the deprotonation of substrates in EncM. We reason that Flavin-N5OOH can be a common reactive species in the superfamily of flavoenzymes, which accomplish generally selective general base catalysis and C-X (X=N, S, Cl, O) cleavage reactions that are otherwise challenging with solvated hydroxide ion base. These results expand our understanding of the chemistry and catalysis of flavoenzymes.

Low-Temperature Adaptive Single-Atom Iron Nanozymes against Viruses in the Cold Chain

Outbreaks of viral infectious diseases, such as the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) and influenza A virus (IAV), pose a great threat to human health. Viral spread is accelerated worldwide by the development of cold chain logistics; Therefore, an effective antiviral approach is required. In this study, it is aimed to develop a distinct antiviral strategy using nanozymes with low-temperature adaptability, suitable for cold chain logistics. Phosphorus (P) atoms are added to the remote counter position of Fe-N-C center to prepare FeN4P2-single-atom nanozymes (SAzymes), exhibiting lipid oxidase (OXD)-like activity at cold chain temperatures (-20, and 4 °C). This feature enables FeN4P2-SAzymes to disrupt multiple enveloped viruses (human, swine, and avian coronaviruses, and H1-H11 subtypes of IAV) by catalyzing lipid peroxidation of the viral lipid envelope. Under the simulated conditions of cold chain logistics, FeN4P2-SAzymes are successfully applied as antiviral coatings on outer packaging and personal protective equipment; Therefore, FeN4P2-SAzymes with low-temperature adaptability and broad-spectrum antiviral properties may serve as key materials for developing specific antiviral approaches to interrupt viral transmission through the cold chain.

An organic O donor for biological hydroxylation reactions

All biological hydroxylation reactions are thought to derive the oxygen atom from one of three inorganic oxygen donors, O2, H2O2, or H2O. Here, we have identified the organic compound prephenate as the oxygen donor for the three hydroxylation steps of the O2-independent biosynthetic pathway of ubiquinone, a widely distributed lipid coenzyme. Prephenate is an intermediate in the aromatic amino acid pathway and genetic experiments showed that it is essential for ubiquinone biosynthesis in Escherichia coli under anaerobic conditions. Metabolic labeling experiments with 18O-shikimate, a precursor of prephenate, demonstrated the incorporation of 18O atoms into ubiquinone. The role of specific iron-sulfur enzymes belonging to the widespread U32 protein family is discussed. Prephenate-dependent hydroxylation reactions represent a unique biochemical strategy for adaptation to anaerobic environments.

A frontier-orbital view of the initial steps of lytic polysaccharide monooxygenase reactions

Lytic polysaccharide monooxygenases (LPMOs) are copper enzymes that oxidatively cleave the strong C-H bonds in recalcitrant polysaccharide substrates, thereby playing a crucial role in biomass degradation. Recently, LPMOs have also been shown to be important for several pathogens. It is well established that the Cu(II) resting state of LPMOs is inactive, and the electronic structure of the active site needs to be altered to transform the enzyme into an active form. Whether this transformation occurs due to substrate binding or due to a unique priming reduction has remained speculative. Starting from four different crystal structures of the LPMO LsAA9A with well-defined oxidation states, we use a frontier molecular orbital approach to elucidate the initial steps of the LPMO reaction. We give an explanation for the requirement of the unique priming reduction and analyse electronic structure changes upon substrate binding. We further investigate how the presence of the substrate could facilitate an electron transfer from the copper active site to an H2O2 co-substrate. Our findings could help to control experimental LPMO reactions.

Functional Model of Compound II of Cytochrome P450: Spectroscopic Characterization and Reactivity Studies of a FeIV-OH Complex

Herein, we show that the reaction of a mononuclear FeIII(OH) complex (1) with N-tosyliminobenzyliodinane (PhINTs) resulted in the formation of a FeIV(OH) species (3). The obtained complex 3 was characterized by an array of spectroscopic techniques and represented a rare example of a synthetic FeIV(OH) complex. The reaction of 1 with the one-electron oxidizing agent was reported to form a ligand-oxidized FeIII(OH) complex (2). 3 revealed a one-electron reduction potential of -0.22 V vs Fc+/Fc at -15 °C, which was 150 mV anodically shifted than 2 (Ered = -0.37 V vs Fc+/Fc at -15 °C), inferring 3 to be more oxidizing than 2. 3 reacted spontaneously with (4-OMe-C6H4)3C• to form (4-OMe-C6H4)3C(OH) through rebound of the OH group and displayed significantly faster reactivity than 2. Further, activation of the hydrocarbon C-H and the phenolic O-H bond by 2 and 3 was compared and showed that 3 is a stronger oxidant than 2. A detailed kinetic study established the occurrence of a concerted proton-electron transfer/hydrogen atom transfer reaction of 3. Studying one-electron reduction of 2 and 3 using decamethylferrocene (Fc*) revealed a higher ket of 3 than 2. The study established that the primary coordination sphere around Fe and the redox state of the metal center is very crucial in controlling the reactivity of high-valent Fe-OH complexes. Further, a FeIII(OMe) complex (4) was synthesized and thoroughly characterized, including X-ray structure determination. The reaction of 4 with PhINTs resulted in the formation of a FeIV(OMe) species (5), revealing the presence of two FeIV species with isomer shifts of -0.11 mm/s and = 0.17 mm/s in the Mössbauer spectrum and showed FeIV/FeIII potential at -0.36 V vs Fc+/Fc couple in acetonitrile at -15 °C. The reactivity studies of 5 were investigated and compared with the FeIV(OH) complex (3).

Controlling product selectivity during dioxygen reduction with Mn complexes using pendent proton donor relays and added base

The catalytic reduction of dioxygen (O2) is important in biological energy conversion and alternative energy applications. In comparison to Fe- and Co-based systems, examples of catalytic O2 reduction by homogeneous Mn-based systems is relatively sparse. Motivated by this lack of knowledge, two Mn-based catalysts for the oxygen reduction reaction (ORR) containing a bipyridine-based non-porphyrinic ligand framework have been developed to evaluate how pendent proton donor relays alter activity and selectivity for the ORR, where Mn(p-tbudhbpy)Cl (1) was used as a control complex and Mn(nPrdhbpy)Cl (2) contains a pendent -OMe group in the secondary coordination sphere. Using an ammonium-based proton source, N,N'-diisopropylethylammonium hexafluorophosphate, we analyzed catalytic activity for the ORR: 1 was found to be 64% selective for H2O2 and 2 is quantitative for H2O2, with O2 binding to the reduced Mn(ii) center being the rate-determining step. Upon addition of the conjugate base, N,N'-diisopropylethylamine, the observed catalytic selectivity of both 1 and 2 shifted to H2O as the primary product. Interestingly, while the shift in selectivity suggests a change in mechanism for both 1 and 2, the catalytic activity of 2 is substantially enhanced in the presence of base and the rate-determining step becomes the bimetallic cleavage of the O-O bond in a Mn-hydroperoxo species. These data suggest that the introduction of pendent relay moieties can improve selectivity for H2O2 at the expense of diminished reaction rates from strong hydrogen bonding interactions. Further, although catalytic rate enhancements are observed with a change in product selectivity when base is added to buffer proton activity, the pendent relays stabilize dimer intermediates, limiting the maximum rate.

Binuclear Metal Phthalocyanines with Enhanced Activity in the Oxygen Evolution Reaction: A First-Principles Study

The rational design of efficient catalysts for the electrochemical oxygen evolution reaction (OER) critically relies on a comprehensive understanding of the reaction mechanisms. Herein, the alkaline OER on planar mononuclear metal phthalocyanines (MPc, where M = Mn, Co, Fe, and Ni) and binuclear metal phthalocyanines (bi-MPc) is studied using density functional theory (DFT) methods. Both FePc and bi-CoPc exhibit enhanced stability and OER activity, with the energy required for the leaching of central metal being as high as 2.28 and 2.45 eV and the overpotentials of the OER being 0.48 and 0.57 V, respectively. Through electronic structure analysis, it is found that, in the OER process of bi-MPc, the large macrocyclic ligand and metal ions not bonding with the intermediate can serve as hole reservoirs. Intermediate species are further stabilized by the dispersal of a positive charge, reducing the free energy. These findings underscore the significance of macrocyclic ligands in the rate-determining step of the OER catalyst.

Hydrogen Atom Transfer (HAT)-Mediated Remote Desaturation Enabled by Fe/Cr-H Cooperative Catalysis

An iron/chromium system (Fe(OAc)2, CpCr(CO)3H) catalyzes the preparation of β,γ- or γ,δ-unsaturated amides from 1,4,2-dioxazol-5-ones. An acyl nitrenoid iron complex seems likely to be responsible for C-H activation. A cascade of three H• transfer steps appears to be involved: (i) the abstraction of H• from a remote C-H bond by the nitrenoid N, (ii) the transfer of H• from Cr to N, and (iii) the abstraction of H• from a radical substituent by the Cr•. The observed kinetic isotope effects are consistent with the proposed mechanism if nitrenoid formation is the rate-determining step. The Fe/Cr catalysts can also desaturate substituted 1,4,2-dioxazol-5-ones to 3,5-dienamides.

Oxidase-like manganese oxide nanoparticles: a mechanism of organic acids/aldehydes as electron acceptors and potential application in cancer therapy

Identifying the underlying catalytic mechanisms of synthetic nanocatalysts or nanozymes is important in directing their design and applications. Herein, we revisited the oxidation process of 4,4'-diamino-3,3',5,5'-tetramethylbiphenyl (TMB) by Mn3O4 nanoparticles and revealed that it adopted an organic acid/aldehyde-triggered catalytic mechanism at a weakly acidic or neutral pH, which is O2-independent and inhibited by the pre-addition of H2O2. Importantly, similar organic acid/aldehyde-mediated oxidation was applied to other substrates of peroxidase in the presence of nanoparticulate or commercially available MnO2 and Mn2O3 but not MnO. The selective oxidation of TMB by Mn3O4 over MnO was further supported by density functional theory calculations. Moreover, Mn3O4 nanoparticles enabled the oxidation of indole 3-acetic acid, a substrate that can generate cytotoxic singlet oxygen upon single-electron transfer oxidation, displaying potential in nanocatalytic tumor therapy. Overall, we revealed a general catalytic mechanism of manganese oxides towards the oxidation of peroxidase substrates, which could boost the design and various applications of these manganese-based nanoparticles.

Regulating the Heme Active Site by Covalent Modifications: Two Case Studies of Myoglobin

Using myoglobin (Mb) as a model protein, we herein developed a facial approach to modifying the heme active site. A cavity was first generated in the heme distal site by F46 C mutation, and the thiol group of Cys46 was then used for covalently linked to exogenous ligands, 1H-1,2,4-triazole-3-thiol and 1-(4-hydroxyphenyl)-1H-pyrrole-2,5-dione. The engineered proteins, termed F46C-triazole Mb and F46C-phenol Mb, respectively, were characterized by X-ray crystallography, spectroscopic and stopped-flow kinetic studies. The results showed that both the heme coordination state and the protein function such as H2 O2 activation and peroxidase activity could be efficiently regulated, which suggests that this approach might be generally applied to the design of functional heme proteins.

Bifunctional porphyrin-based metal-organic polymers for electrochemical water splitting

Electrochemical water splitting offers the potential for environmentally friendly hydrogen and oxygen gas generation. Here, we present the synthesis, characterization, and electrochemical analyses of four organic polymers where metalloporphyrins are the active center nodes. These materials were obtained from the polymerization reaction of poly(p-phenylene terephtalamide) (PPTA) with the respective amino-functionalized metalloporphyrins, where M = Fe, 1; Co, 2; Ni, 3; Cu, 4. Scanning and transmission electron microscopy images (SEM and TEM) show that these polymers exhibit a layer-type morphology, which is attributed to hydrogen bonding and π-π stacking between the metalloporphyrin nodes. The synthesized materials were characterized by X-ray photoelectron spectroscopy (XPS), X-ray powder diffraction (XRD), energy-dispersive X-ray spectroscopy (EDX), UV-Vis spectroscopy, and Fourier-transform infrared spectroscopy (FT-IR). Among the materials studied, the cobalt-based polymer, 2, demonstrates a bifunctional electrocatalytic activity for oxygen (OER) and hydrogen (HER) evolution reactions with overpotentials (η10) of 337 mV and 435 mV, respectively. The Fe, 1, and Ni, 2, polymers are less active for HER with maximum current densities (jmax) of 12.6 and 19.1 mA cm-2 and η10 678 mV, 644 mV. Polymer 2 achieves a jmax of 37.7 mA cm-2 for HER and 133 mA cm-2 for OER. The copper-based material, 4, on the other hand, shows selectivity towards HER with an overpotential (η) of 436 mV and a maximum current density (j) of 45.5 mA cm-2. The bifunctional electrocatalytic performance was tested in the overall water-splitting setup, where polymer 2 requires a cell voltage of 1.64 V at 10 mA cm-2. This work presents a novel approach to heterogenized molecular systems, providing materials with exceptional structural characteristics and enhanced electrocatalytic capabilities.

Metal-ligand synergy driven functionalisation of alkylene linked bis(aldimine) on a diruthenium(II) platform. Cyclisation versus oxygenation

This article addresses the impact of metal-ligand redox cooperativity on the functionalisation of coordinated ligands. It demonstrates the structure-reactivity correlation of bis(aldimine) derived bis-bidentate L (Py-CHN-(CH2)n-NCH-Py, with n = 2 (L1), 3 (L2), 4 (L3)) as a function of the conformation (syn/anti) of its alkylene linker as well as the overall structural form (cis/trans) of (acac)2RuII(μ-L)RuII(acac)2 complex moieties (1-5) possessing an electron-rich acetylacetonate (acac) co-ligand. A systematic variation of the bridging alkylene unit of L in RuII/RuII-derived 1-5 led to the following reactivity/redox events, which were validated through structural, spectroscopic, electrochemical and theoretical evaluations: (i) Cyclisation of the ethylene linked (syn conformation) bis-aldimine unit of L1 via C-C coupling yielded pyrazine bridged (acac)2RuII(μ-L1')RuII(acac)2, 1a, while the corresponding anti-form (ethylene linker) of the metal-bound L1 in 2 ((acac)2RuII(μ-L1)RuII(acac)2) led to oxygenation at the ligand backbone (bis-aldimine (L) → bis(carboxamido) (L'')) via O2 activation to generate RuIIIRuIII-derived (acac)2RuIII(μ-L1''2-)RuIII(acac)2 (2a). (ii) Consequently, propylene and butylene linked L2 and L3 bridged between two {Ru(acac)2} units in 3 and 4/5 underwent oxygenation of L to L'' to yield diruthenium(III) complexes 3a and 4a/5a, respectively. (iii) In contrast, analogous L bridged oxidised [(acac)2RuIII(μ-L)RuIII(acac)2](ClO4)2 ([2](ClO4)2-[5](ClO4)2) and [{(PPh3)2(CO)(H)RuII}2(μ-L)](ClO4)2 ([6](ClO4)2-[8](ClO4)2) involving electron poor co-ligands failed to undergo the oxygenation of L irrespective of its n value, reemphasising the effective role of redox interplay between RuII and L particularly in the presence of an electron-rich acac co-ligand in the functionalisation of the latter in 1a-5a.

Altering the Localization of an Unpaired Spin in a Formal Ni(V) Species

The participation of both ligand and the metal center in the redox events has been recognized as one of the ways to attain the formal high valent complexes for the late 3d metals, such as Ni and Cu. Such an approach has been employed successfully to stabilize a Ni(III) bisphenoxyl diradical species in which there exist an equilibrium between the ligand and the Ni localized resultant spin. The present work, however, broadens the scope of the previously reported three oxidized equivalent species by conveying the approaches that tend to affect the reported equilibrium in CH3 CN at 233 K. Various spectroscopic characterization revealed that employing exogenous N-donor ligands like 1-methyl imidazole and pyridine favors the formation of the Ni centered localized spin though axial binding. In contrast, due to its steric hinderance, quinoline favors an exclusive ligand localized radical species. DFT studies shed light on the novel intermediates' complex electronic structure. Further, the three oxidized equivalent species with the Ni centered spin was examined for its hydrogen atom abstraction ability stressing their key role in alike reactions.

Interfacial coupling effect promotes selective electrocatalytic oxidation of 5-hydroxymethylfurfural into the value-added products under neutral conditions

Owing to the sluggish reaction kinetics, it is a promising yet challenging task to achieve the adequate electricity-driven catalytic oxidation of biomass-derived 5-hydroxymethylfurfural (HMF) in neutral conditions. Herein, we have prepared an elelctrocatalyst with interfacial coupling effect through in-situ growth of Cu phthalocyanine (CuPc) on Co3O4 spinel (Co3O4/CuPc), which constructs an effective electrocatalytic system of HMF oxidation with overall oxidation value-added products yield and total Faraday efficiency up to 80% and 70%, respectively. The interfacial coupling effect between CuPc and Co3O4 spinel improve catalytic activity by effectively boosting the interfacial charge transfer and reducing the formation energy of key *C6H3O4 in the catalytic pathway according to the in situ Raman spectroscopy and DFT simulation. This work illustrates the significance of interfacial coupling effect for developing highly efficient electrocatalysts applied for neutral system of biomass oxidation.

Bimetallic Porphyrin-Based Metal-Organic Framework as a Superior Photocatalyst for Enhanced Photocatalytic Hydrogen Production

The meaningful and rational engineering of porphyrin-based catalysts with multimetallic active sites is very attractive toward photocatalytic hydrogen generation from water decomposition. Herein, three metal organic frameworks (MOFs) based on meso-tetrakis(4-carboxylphenyl)porphyrin (TCPP) were successfully constructed under solvothermal conditions. As a novel architectured photocatalyst (triclinic, C48H29N4O10PdYb), Pd/Yb-PMOF manifested diverse metal active sites, suitable bandgap positions, prominent visible light-collecting capacity, excellent carrier transfer efficiency, and obvious synergistic effect between ytterbium and palladium ions. Consequently, such a bimetallic MOF exhibited strengthened photocatalytic hydrogen evolution performance. Concretely, its hydrogen generation efficiency was up to 3196.42 μmol g-1 h-1 with 2 wt % Pt as a cocatalyst under visible light illumination. Our work demonstrates a promising strategy for highly efficient visible-light catalysts based on bimetallic-trimmed porphyrin MOFs.

A high-spin alkylperoxo-iron(iii) complex with cis-anionic ligands: implications for the superoxide reductase mechanism

The N3O macrocycle of the 12-TMCO ligand stabilizes a high spin (S = 5/2) [FeIII(12-TMCO)(OOtBu)Cl]+ (3-Cl) species in the reaction of [FeII(12-TMCO)(OTf)2] (1-(OTf)2) with tert-butylhydroperoxide (tBuOOH) in the presence of tetraethylammonium chloride (NEt4Cl) in acetonitrile at -20 °C. In the absence of NEt4Cl the oxo-iron(iv) complex 2 [FeIV(12-TMCO)(O)(CH3CN)]2+ is formed, which can be further converted to 3-Cl by adding NEt4Cl and tBuOOH. The role of the cis-chloride ligand in the stabilization of the FeIII-OOtBu moiety can be extended to other anions including the thiolate ligand relevant to the enzyme superoxide reductase (SOR). The present study underlines the importance of subtle electronic changes and secondary interactions in the stability of the biologically relevant metal-dioxygen intermediates. It also provides some rationale for the dramatically different outcomes of the chemistry of iron(iii)peroxy intermediates formed in the catalytic cycles of SOR (Fe-O cleavage) and cytochrome P450 (O-O bond lysis) in similar N4S coordination environments.

Synthetic Advances for Mechanistic Insights: Metal-Oxygen Intermediates with a Macrocyclic Pyridinophane System

ConspectusMetalloenzymes, which are proteins containing earth-abundant transition-metal ions as cofactors in the active site, generate various metal-oxygen intermediates via activating a dioxygen molecule (O2) to mediate vital metabolic functions, such as the oxidative metabolism of xenobiotics and the biotransformation of naturally occurring molecules. By replicating the active sites of metalloenzymes, many bioinorganic chemists have studied the geometric and electronic properties and reactivities of model complexes to understand the nature of enzymatic intermediates and develop bioinspired metal catalysts. Among the reported model complexes, nonporphyrinic macrocyclic ligands are the predominant coordination system widely used in stabilizing and isolating diverse metal-oxygen intermediates, which allows us to extensively investigate the physicochemical characteristics of the analogs of reactive intermediates of metalloenzymes. In particular, it has been reported that the ring size of the macrocyclic ligands, defined by the number of atoms in the macrocyclic ring, drastically affects the identity of the metal-oxygen intermediate. Thus, systematic modification of the macrocyclic ligands has been a great subject being examined in various inorganic fields.In this Account, we describe synthetic advances of a macrocyclic ligand system by introducing pyridine donors into a 12-membered tetraazamacrocyclic ligand (12-TMC) that initially has 4 amine donors. Interestingly, the backbone of the pyridinophane ligand with 2 pyridine and 2 amine donors in a 12-membered ring is shown to be much more folded than in other macrocyclic ligands, thereby allowing the axial and equatorial donors to separately control the electronic structure of metal complexes. Then, we looked over independent electronic and steric effects on metal-oxygen species with thorough physicochemical analysis. The NiIII-peroxo complexes exhibit nucleophilic reactivity dependent on the steric hindrance of the second coordination sphere. Furthermore, the C-H bond strength of the second coordination sphere has also been an important factor in determining the stability of MnIV-bis(hydroxo) intermediates. Electronic tuning on CoIII-hydroperoxo intermediates results in a trend between the electron-donating abilities of para-substituents on pyridine in the pyridinophane ligand and electrophilic reactivities, from which mechanistic insights into the metal-hydroperoxo species have been gained. Importantly, the metal-oxygen intermediates supported by the pyridinophane ligand system have revealed quite challenging chemical reactions, including dioxygenase-like nitrile activation by CoIII-peroxo intermediates and the oxidation of aldehyde and aromatic compounds by manganese-oxygen intermediates. Based on the fine substitution of donors, we have addressed that those novel reactions originated from the unique framework of the pyridinophane system incorporating spin-crossover behavior and high redox potentials of the metal-oxygen intermediates. These results will be valuable for the structure-activity relationship of metal-oxygen intermediates, giving a better understanding on the enzymatic coordination system where amino acid ligands vary for specific chemical reactions.

Efficiency aspects of regioselective testosterone hydroxylation with highly active CYP450-based whole-cell biocatalysts

Steroid hydroxylations belong to the industrially most relevant reactions catalysed by cytochrome P450 monooxygenases (CYP450s) due to the pharmacological relevance of hydroxylated derivatives. The implementation of respective bioprocesses at an industrial scale still suffers from several limitations commonly found in CYP450 catalysis, that is low turnover rates, enzyme instability, inhibition and toxicity related to the substrate(s) and/or product(s). Recently, we achieved a new level of steroid hydroxylation rates by introducing highly active testosterone-hydroxylating CYP450 BM3 variants together with the hydrophobic outer membrane protein AlkL into Escherichia coli-based whole-cell biocatalysts. However, the activity tended to decrease, which possibly impedes overall productivities and final product titres. In this study, a considerable instability was confirmed and subject to a systematic investigation regarding possible causes. In-depth evaluation of whole-cell biocatalyst kinetics and stability revealed a limitation in substrate availability due to poor testosterone solubility as well as inhibition by the main product 15β-hydroxytestosterone. Instability of CYP450 BM3 variants was disclosed as another critical factor, which is of general significance for CYP450-based biocatalysis. Presented results reveal biocatalyst, reaction and process engineering strategies auguring well for industrial implementation of the developed steroid hydroxylation platform.

Structure and Function of Alkane Monooxygenase (AlkB)

Every year, perhaps as much as 800 million tons of hydrocarbons enters the environment; alkanes make up a large percentage of it. Most are transformed by organisms that utilize these molecules as sources of energy and carbon. Both aerobic and anaerobic alkane transformation chemistries exist, capitalizing on the presence of alkanes in both oxic and anoxic environments. Over the past 40 years, tremendous progress has been made in understanding the structure and mechanism of enzymes that catalyze the transformation of methane. By contrast, progress involving enzymes that transform liquid alkanes has been slower with the first structures of AlkB, the predominant aerobic alkane hydroxylase in the environment, appearing in 2023. Because of the fundamental importance of C-H bond activation chemistries, interest in understanding how biology activates and transforms alkanes is high.In this Account, we focus on steps we have taken to understand the mechanism and structure of alkane monooxygenase (AlkB), the metalloenzyme that dominates the transformation of liquid alkanes in the environment (not to be confused with another AlkB that is an α-ketogluturate-dependent enzyme involved in DNA repair). First, we briefly describe what is known about the prevalence of AlkB in the environment and its role in the carbon cycle. Then we review the key findings from our recent high-resolution cryoEM structure of AlkB and highlight important similarities and differences in the structures of members of class III diiron enzymes. Functional studies, which we summarize, from a number of single residue variants enable us to say a great deal about how the structure of AlkB facilitates its function. Next, we overview work from our laboratories using mechanistically diagnostic radical clock substrates to characterize the mechanism of AlkB and contextualize the results we have obtained on AlkB with results we have obtained on other alkane-oxidizing enzymes and explain these results in light of the enzyme's structure. Finally, we integrate recent work in our laboratories with information from prior studies of AlkB, and relevant model systems, to create a holistic picture of the enzyme. We end by pointing to critical questions that still need to be answered, questions about the electronic structure of the active site of the enzyme throughout the reaction cycle and about whether and to what extent the enzyme plays functional roles in biology beyond simply initiating the degradation of alkanes.

Aerobic Oxidation of 5-Hydroxymethylfurfural (HMF) in Aqueous Medium over Fe-Doped-Poly(heptazine imide) Photocatalysts: Unveiling the Bad Role of Hydroxyl Radical Generation on the Catalytic Performance

5-hydroxymethylfurfural (HMF) oxidation in aqueous media using visible photocatalysis is a green and sustainable route for the valorization of lignocellulosic biomass derivatives. Several semiconductors have already been applied for this purpose; however, the use of Poly(heptazine imides), which has high crystallinity and a special cation exchange property that allows the replacement of the cation held between the layers of C3N4 structure by transition metal ions (TM), remains scarce. In this study, PHI(Na) was synthesized using a melamine/NaCl method and used as precursor to prepare metal (Fe, Co, Ni, or Cu)-doped PHI catalysts. The catalysts were tested for selective oxidation of HMF to 2,5-diformylfuran (DFF) in water and O2 atmosphere under blue LED radiation. The catalytic results revealed that the 0.1 wt% PHI(Fe) catalyst is the most efficient photocatalyst while higher Fe loading (1 and 2 wt%) favors the formation of Fe3+ clusters, which are responsible for the drop in HMF oxidation. Moreover, the 0.1 wt% PHI(Fe) photocatalyst has strong oxidative power due to its efficiency in H2O2 production, thus boosting the generation of nonselective hydroxyl radicals (●OH) via different pathways that can destroy HMF. We found that using 50 mM, the highest DFF production rate (393 μmol·h-1·g-1) was obtained in an aqueous medium under visible light radiation.

Heme-copper and Heme O2-derived synthetic (bioinorganic) chemistry toward an understanding of cytochrome c oxidase dioxygen chemistry

Cytochrome c oxidase (CcO), also widely known as mitochondrial electron-transport-chain complex IV, is a multi-subunit transmembrane protein responsible for catalyzing the last step of the electron transport chain, dioxygen reduction to water, which is essential to the establishment and maintenance of the membrane proton gradient that drives ATP synthesis. Although many intermediates in the CcO catalytic cycle have been spectroscopically and/or computationally authenticated, the specifics regarding the IP intermediate, hypothesized to be a heme-Cu (hydro)peroxo species whose O-O bond homolysis is supported by a hydrogen-bonding network of water molecules, are largely obscured by the fast kinetics of the A (FeIII-O2•-/CuI/Tyr) → PM (FeIV=O/CuII-OH/Tyr•) step. In this review, we have focused on the recent advancements in the design, development, and characterization of synthetic heme-peroxo‑copper model complexes, which can circumvent the abovementioned limitation, for the investigation of the formation of IP and its O-O cleavage chemistry. Novel findings regarding (a) proton and electron transfer (PT/ET) processes, together with their contributions to exogenous phenol induced O-O cleavage, (b) the stereo-electronic tunability of the secondary coordination sphere (especially hydrogen-bonding) on the geometric and spin state alteration of the heme-peroxo‑copper unit, and (c) a plausible mechanism for the Tyr-His cofactor biogenesis, are discussed in great detail. Additionally, since the ferric-superoxide and the ferryl-oxo (Compound II) species are critically involved in the CcO catalytic cycle, this review also highlights a few fundamental aspects of these heme-only (i.e., without copper) species, including the structural and reactivity influences of electron-donating trans-axial ligands and Lewis acid-promoted H-bonding.

New insight into biodegradation mechanism of phenylurea herbicides by cytochrome P450 enzymes: Successive N-demethylation mechanism

Phenylurea herbicides (PUHs) present one of the most important herbicides, which have cause serious effects on ecological environment and humans. Nowadays enzyme strategy shows great advantages in degradation of PUHs. Here density functional theory (DFT), quantitative structure - activity relationship (QSAR) and quantum mechanics/molecular mechanics (QM/MM) approaches are used to investigate the degradation mechanism of PUHs catalyzed by P450 enzymes. Two successive N-demethylation pathways are identified and two hydrogen abstraction (H-abstraction) reaction pathways are identified as the rate-determining step through high-throughput DFT calculations. The Boltzmann-weighted average energy barrier of the second H-abstraction pathway (19.95 kcal/mol) is higher than that of the first H-abstraction pathway (16.80 kcal/mol). Two QSAR models are established to predict the energy barriers of the two H-abstraction pathways based on the quantum chemical descriptors and mordred molecular descriptors. The determination coefficient (R2) values of QSAR models are > 0.9, which reveal that the established QSAR models have great predictive capability. QM/MM calculations indicate that human P450 enzymes are more efficient in degradation of PUHs than crop and weed P450 enzymes. Correlations between energy barriers and key structural/charge parameters are revealed and key parameters that have influence on degradation efficiency of PUHs are identified. This study provides lateral insights into the biodegradation strategy and removal method of PUHs and valuable information for designing or engineering of highly efficient degradation enzymes and genetically modified crops.

Characterisation of the heme aqua-ligand coordination environment in an engineered peroxygenase cytochrome P450 variant

The cytochrome P450 enzymes (CYPs) are heme-thiolate monooxygenases that catalyse the insertion of an oxygen atom into the C-H bonds of organic molecules. In most CYPs, the activation of dioxygen by the heme is aided by an acid-alcohol pair of residues located in the I-helix of the enzyme. Mutation of the threonine residue of this acid-alcohol pair of CYP199A4, from the bacterium Rhodospeudomonas palustris HaA2, to a glutamate residue induces peroxygenase activity. In the X-ray crystal structures of this variant an interaction of the glutamate side chain and the distal aqua ligand of the heme was observed and this results in this ligand not being readily displaced in the peroxygenase mutant on the addition of substrate. Here we use a range of bulky hydrophobic and nitrogen donor containing ligands in an attempt to displace the distal aqua ligand of the T252E mutant of CYP199A4. Ligand binding was assessed by UV-visible absorbance spectroscopy, native mass spectrometry, electron paramagnetic resonance and X-ray crystallography. None of the ligands tested, even the nitrogen donor ligands which bind directly to the iron in the wild-type enzyme, resulted in displacement of the aqua ligand. Therefore, modification of the I-helix threonine residue to a glutamate residue results in a significant strengthening of the ferric distal aqua ligand. This ligand was not displaced using any of the ligands during this study and this provides a rationale as to why this mutant can shutdown the monooxygenase pathway of this enzyme and switch to peroxygenase activity.

Ten-Fold Solvent Kinetic Isotope Effect for the Nonradiative Relaxation of the Aqueous Ferrate(VI) Ion

Hypervalent iron intermediates have been invoked in the catalytic cycles of many metalloproteins, and thus, it is crucial to understand how the coupling between such species and their environment can impact their chemical and physical properties in such contexts. In this work, we take advantage of the solvent kinetic isotope effect (SKIE) to gain insight into the nonradiative deactivation of electronic excited states of the aqueous ferrate(VI) ion. We observe an exceptionally large SKIE of 9.7 for the nanosecond-scale relaxation of the lowest energy triplet ligand field state to the ground state. Proton inventory studies demonstrate that a single solvent O-H bond is coupled to the ion during deactivation, likely due to the sparse vibrational structure of ferrate(VI). Such a mechanism is consistent with that reported for the deactivation of f-f excited states of aqueous trivalent lanthanides, which exhibit comparably large SKIE values. This phenomenon is ascribed entirely to dissipation of energy into a higher overtone of a solvent acceptor mode, as any impact on the apparent relaxation rate due to a change in solvent viscosity is negligible.

Bioinspired Non-Heme Mn Catalysts for Regio- and Stereoselective Oxyfunctionalizations with H2 O2

In recent years, metalloenzymes-mediated highly selective oxidations of organic substrates under mild conditions have been inspiration for developing synthetic bioinspired catalyst systems, capable of conducting such processes in the laboratory (and, in the future, in industry), relying on easy-to-handle and environmentally benign oxidants such as H2 O2 . To date, non-heme manganese complexes with chiral bis-amino-bis-pyridylmethyl and structurally related ligands are considered as possessing the highest synthetic potential, having demonstrated the ability to mediate a variety of chemo- and stereoselective oxidative transformations, such as epoxidations, C(sp3 )-H hydroxylations and ketonizations, oxidative desymmetrizations, kinetic resolutions, etc. Furthermore, in the past few years non-heme Mn based catalysts have become the major platform for studies focused on getting insight into the molecular mechanisms of oxidant activation and (stereo)selective oxygen transfer, testing non-traditional hydroperoxide oxidants, engineering catalytic sites with enzyme-like substrate recognition-based selectivity, exploration of catalytic regioselectivity trends in the oxidation of biologically active substrates of natural origin. This contribution summarizes the progress in manganese catalyzed C-H oxygenative transformations of organic substrates, achieved essentially in the past 5 years (late 2018-2023).

The elusive reaction mechanism of Mn(II)-mediated benzylic oxidation of alkylarene by H2O2: a gem-diol mechanism or a dual hydrogen abstraction mechanism?

The direct oxygenation of alkylarenes at the benzylic position employing bioinspired nonheme catalysts has emerged as a promising strategy for the production of bioactive arene ketone scaffolds in drugs. However, the structure-activity relationship of the active species and the mechanism of these reactions remain elusive. Herein, the reaction mechanism of the Mn(II)-mediated benzylic oxygenation of phenylbutanoic acid (PBA) to 4-oxo-4-phenylbutyric acid (4-oxo-PBA) by H2O2 was investigated using density functional theory calculations. The calculated results demonstrated that the MnIII-OOH species (1) is a sluggish oxidant and needs to be converted to a high-valent manganese-oxo species (2). The conversion of PBA to 4-oxo-PBA by 2 occurs via the consecutive hydroxylation of PBA to 4-hydroxyl-4-phenylbutyric acid (4-OH-PBA) and the alcohol oxidation of 4-OH-PBA to 4-oxo-PBA. The hydroxylation of PBA proceeds via a novel hydride transfer/hydroxyl-rebound mechanism and the alcohol oxidation of 4-OH-PBA occurs via three pathways (gem-diol, dual hydrogen abstraction (DHA), and reversed-DHA pathways). The regio-selectivity of benzylic oxidations was caused by a strong π-π stacking interaction between the pyridine ring of the nonheme ligand and the phenyl ring of the substrate. These mechanistic findings enrich the knowledge of biomimetic alcohol oxidations and play a positive role in the rational design of new non-heme catalysts.

In Situ Structural Observation of a Substrate- and Peroxide-Bound High-Spin Ferric-Hydroperoxo Intermediate in the P450 Enzyme CYP121

The P450 enzyme CYP121 from Mycobacterium tuberculosis catalyzes a carbon-carbon (C-C) bond coupling cyclization of the dityrosine substrate containing a diketopiperazine ring, cyclo(l-tyrosine-l-tyrosine) (cYY). An unusual high-spin (S = 5/2) ferric intermediate maximizes its population in less than 5 ms in the rapid freeze-quenching study of CYP121 during the shunt reaction with peracetic acid or hydrogen peroxide in acetic acid solution. We show that this intermediate can also be observed in the crystalline state by EPR spectroscopy. By developing an on-demand-rapid-mixing method for time-resolved serial femtosecond crystallography with X-ray free-electron laser (tr-SFX-XFEL) technology covering the millisecond time domain and without freezing, we structurally monitored the reaction in situ at room temperature. After a 200 ms peracetic acid reaction with the cocrystallized enzyme-substrate microcrystal slurry, a ferric-hydroperoxo intermediate is observed, and its structure is determined at 1.85 Å resolution. The structure shows a hydroperoxyl ligand between the heme and the native substrate, cYY. The oxygen atoms of the hydroperoxo are 2.5 and 3.2 Å from the iron ion. The end-on binding ligand adopts a near-side-on geometry and is weakly associated with the iron ion, causing the unusual high-spin state. This compound 0 intermediate, spectroscopically and structurally observed during the catalytic shunt pathway, reveals a unique binding mode that deviates from the end-on compound 0 intermediates in other heme enzymes. The hydroperoxyl ligand is only 2.9 Å from the bound cYY, suggesting an active oxidant role of the intermediate for direct substrate oxidation in the nonhydroxylation C-C bond coupling chemistry.

New insights into controlling radical migration pathways in heme enzymes gained from the study of a dye-decolorising peroxidase

In heme enzymes, such as members of the dye-decolorising peroxidase (DyP) family, the formation of the highly oxidising catalytic Fe(iv)-oxo intermediates following reaction with hydrogen peroxide can lead to free radical migration (hole hopping) from the heme to form cationic tyrosine and/or tryptophan radicals. These species are highly oxidising (∼1 V vs. NHE) and under certain circumstances can catalyse the oxidation of organic substrates. Factors that govern which specific tyrosine or tryptophan the free radical migrates to in heme enzymes are not well understood, although in the case of tyrosyl radical formation the nearby proximity of a proton acceptor is a recognised facilitating factor. By using an A-type member of the DyP family (DtpAa) as an exemplar, we combine protein engineering, X-ray crystallography, hole-hopping calculations, EPR spectroscopy and kinetic modelling to provide compelling new insights into the control of radical migration pathways following reaction of the heme with hydrogen peroxide. We demonstrate that the presence of a tryptophan/tyrosine dyad motif displaying a T-shaped orientation of aromatic rings on the proximal side of the heme dominates the radical migration landscape in wild-type DtpAa and continues to do so following the rational engineering into DtpAa of a previously identified radical migration pathway in an A-type homolog on the distal side of the heme. Only on disrupting the proximal dyad, through removal of an oxygen atom, does the radical migration pathway then switch to the engineered distal pathway to form the desired tyrosyl radical. Implications for protein design and biocatalysis are discussed.

A breath of sunshine: oxygenic photosynthesis by functional molecular architectures

The conversion of light into chemical energy is the game-changer enabling technology for the energetic transition to renewable and clean solar fuels. The photochemistry of interest includes the overall reductive/oxidative splitting of water into hydrogen and oxygen and alternatives based on the reductive conversion of carbon dioxide or nitrogen, as primary sources of energy-rich products. Devices capable of performing such transformations are based on the integration of three sequential core functions: light absorption, photo-induced charge separation, and the photo-activated breaking/making of molecular bonds via specific catalytic routes. The key to success does not rely simply on the individual components' performance, but on their optimized integration in terms of type, number, geometry, spacing, and linkers dictating the photosynthetic architecture. Natural photosynthesis has evolved along this concept, by integrating each functional component in one specialized "body" (from the Greek word "soma") to enable the conversion of light quanta with high efficiency. Therefore, the natural "quantasome" represents the key paradigm to inspire man-made constructs for artificial photosynthesis. The case study presented in this perspective article deals with the design of artificial photosynthetic systems for water oxidation and oxygen production, engineered as molecular architectures then rendered on electrodic surfaces. Water oxidation to oxygen is indeed the pervasive oxidative reaction used by photosynthetic organisms, as the source of reducing equivalents (electrons and protons) to be delivered for the processing of high-energy products. Considering the vast and abundant supply of water (including seawater) as a renewable source on our planet, this is also a very appealing option for photosynthetic energy devices. We will showcase the progress in the last 15 years (2009-2023) in the strategies for integrating functional building blocks as molecular photosensitizers, multi-redox water oxidation catalysts and semiconductor materials, highlighting how additional components such as redox mediators, hydrophilic/hydrophobic pendants, and protective layers can impact on the overall photosynthetic performance. Emerging directions consider the modular tuning of the multi-component device, in order to target a diversity of photocatalytic oxidations, expanding the scope of the primary electron and proton sources while enhancing the added-value of the oxidation product beyond oxygen: the selective photooxidation of organics combines the green chemistry vision with renewable energy schemes and is expected to explode in coming years.

Radical and Cationic Pathways in C(sp3)-H Bond Oxygenation by Dioxiranes of Bicyclic and Spirocyclic Hydrocarbons Bearing Cyclopropane Moieties

A product and DFT computational study on the reactions of 3-ethyl-3-(trifluoromethyl)dioxirane (ETFDO) with bicyclic and spirocyclic hydrocarbons bearing cyclopropyl groups was carried out. With bicyclo[n.1.0]alkanes (n = 3-6), diastereoselective formation of the alcohol product derived from C2-H bond hydroxylation was observed, accompanied by smaller amounts of products derived from oxygenation at other sites. With 1-methylbicyclo[4.1.0]heptane, rearranged products were also observed in addition to the unrearranged products deriving from oxygenation at the most activated C2-H and C5-H bonds. With spiro[2.5]octane and 6-tert-butylspiro[2.5]octane, reaction with ETFDO occurred predominantly or exclusively at the axial C4-H to give unrearranged oxygenation products, accompanied by smaller amounts of rearranged bicyclo[4.2.0]octan-1-ols. The good to outstanding site-selectivities and diastereoselectivities are paralleled by the calculated activation free energies for the corresponding reaction pathways. Computations show that the σ* orbitals of the bicyclo[n.1.0]alkane cis or trans C2-H bonds and spiro[2.5]octanes axial C4-H bond hyperconjugatively interact with the Walsh orbitals of the cyclopropane ring, activating these bonds toward HAT to ETFDO. The detection of rearranged oxygenation products in the oxidation of 1-methylbicyclo[4.1.0]heptane, spiro[2.5]octane, and 6-tert-butylspiro[2.5]octane provides unambiguous evidence for the involvement of cationic intermediates in these reactions, representing the first examples on the operation of ET pathways in dioxirane-mediated C(sp3)-H bond oxygenations. Computations support these findings, showing that formation of cationic intermediates is associated with specific stabilizing hyperconjugative interactions between the incipient carbon radical and the cyclopropane C-C bonding orbitals that trigger ET to the incipient dioxirane derived 1,1,1-trifluoro-2-hydroxy-2-butoxyl radical.

Bioinspired porous three-coordinated single-atom Fe nanozyme with oxidase-like activity for tumor visual identification via glutathione

Inspired by structures of natural metalloenzymes, a biomimetic synthetic strategy is developed for scalable synthesis of porous Fe-N3 single atom nanozymes (pFeSAN) using hemoglobin as Fe-source and template. pFeSAN delivers 3.3- and 8791-fold higher oxidase-like activity than Fe-N4 and Fe3O4 nanozymes. The high catalytic performance is attributed to (1) the suppressed aggregation of atomically dispersed Fe; (2) facilitated mass transfer and maximized exposure of active sites for the created mesopores by thermal removal of hemoglobin (2 ~ 3 nm); and (3) unique electronic configuration of Fe-N3 for the oxygen-to-water oxidation pathway (analogy with natural cytochrome c oxidase). The pFeSAN is successfully demonstrated for the rapid colorimetric detection of glutathione with a low limit of detection (2.4 nM) and wide range (50 nM-1 mM), and further developed as a real-time, facile, rapid (~6 min) and precise visualization analysis methodology of tumors via glutathione level, showing its potentials for diagnostic and clinic applications.

Adapting Synthetic Models of Heme/Cu Sites to Energy-Efficient Electrocatalytic Oxygen Reduction Reaction

In nature, cytochrome c oxidases catalyze the 4e- oxygen reduction reaction (ORR) at the heme/Cu site, in which CuI is used to assist O2 activation. Because of the thermodynamic barrier to generate CuI , synthetic Fe-porphyrin/Cu complexes usually show moderate electrocatalytic ORR activity. We herein report on a Co-corrole/Co complex 1-Co for energy-efficient electrocatalytic ORR. By hanging a CoII ion over Co corrole, 1-Co realizes electrocatalytic 4e- ORR with a half-wave potential of 0.89 V versus RHE, which is outstanding among corrole-based electrocatalysts. Notably, 1-Co outperforms Co corrole hanged with CuII or ZnII . We revealed that the hanging CoII ion can provide an electron to improve O2 binding thermodynamically and dynamically, a function represented by the biological CuI ion of the heme/Cu site. This work is significant to present a remarkable ORR electrocatalyst and to show the vital role of a second-sphere redox-active metal ion in promoting O2 binding and activation.

Factors controlling the reactivity of synthetic compound-I Analogs

A high-valent iron(IV)-oxo porphyrin radical cation (FeIV(O)(porph+•) serves as a key, reactive intermediate for a range of heme enzymes, including cytochrome P450 (CYP), horseradish peroxidase (HRP), and catalase (CAT). Synthetic analogs of this intermediate, known as Compound-I (Cpd-I) in the heme enzyme literature, have been generated with different tetrapyrrolic, macrocyclic ligands, including porphyrin derivatives, and the closely related ring-contracted macrocycles, corroles and corrolazines. These synthetic analogs have been useful for assigning and understanding structural and spectroscopic features and examining the reactivity of Cpd-I-like species in controlled and well-defined environments. This review focuses on summarizing recent developments in the synthesis and reactivity of high-valent iron-oxo porphyrinoid complexes in two main classes of reactions, proton-coupled electron transfer (PCET) and oxygen atom transfer (OAT). The relationship between the structure of the complexes and their reactivity is emphasized, including the influence of axial ligation and peripheral macrocyclic substitution, as well as the effects of solvent and secondary coordination spheres on the reactivity of the Cpd-I analogs. In bringing together the latest findings on Cpd-I analogs, this review intends to broaden our current understanding of the factors that control the stability and reactivity of Cpd-I species. This new knowledge should, in turn, point toward new synthetic strategies for constructing catalysts that rely on Cpd-I-like reactive intermediates.

Reactivities and Electronic Structures of μ-1,2-Peroxo and μ-1,2-Superoxo CoIIICoIII Complexes: Electrophilic Reactivity and O2 Release Induced by Oxidation

Peroxo complexes are key intermediates in water oxidation catalysis (WOC). Cobalt plays an important role in WOC, either as oxides CoOx or as {CoIII(μ-1,2-peroxo)CoIII} complexes, which are the oldest peroxo complexes known. The oxidation of {CoIII(μ-1,2-peroxo)CoIII} complexes had usually been described to form {CoIII(μ-1,2-superoxo)CoIII} complexes; however, recently the formation of {CoIV(μ-1,2-peroxo)CoIII} species were suggested. Using a bis(tetradentate) dinucleating ligand, we present here the synthesis and characterization of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} and {CoIII(μ-OH)2CoIII} complexes. Oxidation of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} at -40 °C in CH3CN provides the stable {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} species and activates electrophilic reactivity. Moreover, {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} catalyzes water oxidation, not molecularly but rather via CoOx films. While {CoIII(μ-1,2-peroxo)(μ-OH)CoIII} can be reversibly deprotonated with DBU at -40 °C in CH3CN, {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} undergoes irreversible conversions upon reaction with bases to a new intermediate that is also the decay product of {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} in aqueous solution at pH > 2. Based on a combination of experimental methods, the new intermediate is proposed to have a {CoII(μ-OH)CoIII} core formed by the release of O2 from {CoIII(μ-1,2-superoxo)(μ-OH)CoIII} confirmed by a 100% yield of O2 upon photocatalytic oxidation of {CoIII(μ-1,2-peroxo)(μ-OH)CoIII}. This release of O2 by oxidation of a peroxo intermediate corresponds to the last step in molecular WOC.

The Nature of the Chemical Bonds of High-Valent Transition-Metal Oxo (M=O) and Peroxo (MOO) Compounds: A Historical Perspective of the Metal Oxyl-Radical Character by the Classical to Quantum Computations

This review article describes a historical perspective of elucidation of the nature of the chemical bonds of the high-valent transition metal oxo (M=O) and peroxo (M-O-O) compounds in chemistry and biology. The basic concepts and theoretical backgrounds of the broken-symmetry (BS) method are revisited to explain orbital symmetry conservation and orbital symmetry breaking for the theoretical characterization of four different mechanisms of chemical reactions. Beyond BS methods using the natural orbitals (UNO) of the BS solutions, such as UNO CI (CC), are also revisited for the elucidation of the scope and applicability of the BS methods. Several chemical indices have been derived as the conceptual bridges between the BS and beyond BS methods. The BS molecular orbital models have been employed to explain the metal oxyl-radical character of the M=O and M-O-O bonds, which respond to their radical reactivity. The isolobal and isospin analogy between carbonyl oxide R2C-O-O and metal peroxide LFe-O-O has been applied to understand and explain the chameleonic chemical reactivity of these compounds. The isolobal and isospin analogy among Fe=O, O=O, and O have also provided the triplet atomic oxygen (3O) model for non-heme Fe(IV)=O species with strong radical reactivity. The chameleonic reactivity of the compounds I (Cpd I) and II (Cpd II) is also explained by this analogy. The early proposals obtained by these theoretical models have been examined based on recent computational results by hybrid DFT (UHDFT), DLPNO CCSD(T0), CASPT2, and UNO CI (CC) methods and quantum computing (QC).

Insights into Cytochrome P450 Enzyme Catalyzed Defluorination of Aromatic Fluorides

Density functional calculations establish a novel mechanism of aromatic defluorination by P450 Compound I. This is achieved via either an initial epoxide intermediate or through a 1,2-fluorine shift in an electrophilic intermediate, which highlights that the P450s can defluorinate fluoroarenes. However, in the absence of a proton donor a strong Fe-F bond can be obtained as shown from the calculations.

Iron-Catalyzed Intermolecular C-H Amination Assisted by an Isolated Iron-Imido Radical Intermediate

Here we report the use of a base metal complex [(tBu pyrpyrr2 )Fe(OEt2 )] (1-OEt2 ) (tBu pyrpyrr2 2- =3,5-tBu2 -bis(pyrrolyl)pyridine) as a catalyst for intermolecular amination of Csp3 -H bonds of 9,10-dihydroanthracene (2 a) using 2,4,6-trimethyl phenyl azide (3 a) as the nitrene source. The reaction is complete within one hour at 80 °C using as low as 2 mol % 1-OEt2 with control in selectivity for single C-H amination versus double C-H amination. Catalytic C-H amination reactions can be extended to other substrates such as cyclohexadiene and xanthene derivatives and can tolerate a variety of aryl azides having methyl groups in both ortho positions. Under stoichiometric conditions the imido radical species [(tBu pyrpyrr2 )Fe{=N(2,6-Me2 -4-tBu-C6 H2 )] (1-imido) can be isolated in 56 % yield, and spectroscopic, magnetometric, and computational studies confirmed it to be an S = 1 FeIV complex. Complex 1-imido reacts with 2 a to produce the ferrous aniline adduct [(tBu pyrpyrr2 )Fe{NH(2,6-Me2 -4-tBu-C6 H2 )(C14 H11 )}] (1-aniline) in 45 % yield. Lastly, it was found that complexes 1-imido and 1-aniline are both competent intermediates in catalytic intermolecular C-H amination.

Repurposing Iron- and 2-Oxoglutarate-Dependent Oxygenases to Catalyze Olefin Hydration

Mononuclear nonheme iron(II) and 2-oxoglutarate (Fe/2OG)-dependent oxygenases and halogenases are known to catalyze a diverse set of oxidative reactions, including hydroxylation, halogenation, epoxidation, and desaturation in primary metabolism and natural product maturation. However, their use in abiotic transformations has mainly been limited to C-H oxidation. Herein, we show that various enzymes of this family, when reconstituted with Fe(II) or Fe(III), can catalyze Mukaiyama hydration-a redox neutral transformation. Distinct from the native reactions of the Fe/2OG enzymes, wherein oxygen atom transfer (OAT) catalyzed by an iron-oxo species is involved, this nonnative transformation proceeds through a hydrogen atom transfer (HAT) pathway in a 2OG-independent manner. Additionally, in contrast to conventional inorganic catalysts, wherein a dinuclear iron species is responsible for HAT, the Fe/2OG enzymes exploit a mononuclear iron center to support this reaction. Collectively, our work demonstrates that Fe/2OG enzymes have utility in catalysis beyond the current scope of catalytic oxidation.

Melatonin Activation by Human Cytochrome P450 Enzymes: A Comparison between Different Isozymes

Cytochrome P450 enzymes in the human body play a pivotal role in both the biosynthesis and the degradation of the hormone melatonin. Melatonin plays a key role in circadian rhythms in the body, but its concentration is also linked to mood fluctuations as well as emotional well-being. In the present study, we present a computational analysis of the binding and activation of melatonin by various P450 isozymes that are known to yield different products and product distributions. In particular, the P450 isozymes 1A1, 1A2, and 1B1 generally react with melatonin to provide dominant aromatic hydroxylation at the C6-position, whereas the P450 2C19 isozyme mostly provides O-demethylation products. To gain insight into the origin of these product distributions of the P450 isozymes, we performed a comprehensive computational study of P450 2C19 isozymes and compared our work with previous studies on alternative isozymes. The work covers molecular mechanics, molecular dynamics and quantum mechanics approaches. Our work highlights major differences in the size and shape of the substrate binding pocket amongst the different P450 isozymes. Consequently, substrate binding and positioning in the active site varies substantially within the P450 isozymes. Thus, in P450 2C19, the substrate is oriented with its methoxy group pointing towards the heme, and therefore reacts favorably through hydrogen atom abstraction, leading to the production of O-demethylation products. On the other hand, the substrate-binding pockets in P450 1A1, 1A2, and 1B1 are tighter, direct the methoxy group away from the heme, and consequently activate an alternative site and lead to aromatic hydroxylation instead.

Full Spectroscopic Characterization of the Molecular Oxygen-Based Methane to Methanol Conversion over Open Fe(II) Sites in a Metal-Organic Framework

Iron-based enzymes efficiently activate molecular oxygen to perform the oxidation of methane to methanol (MTM), a reaction central to the contemporary chemical industry. Conversely, a very limited number of artificial catalysts have been devised to mimic this process. Herein, we employ the MIL-100(Fe) metal-organic framework (MOF), a material that exhibits isolated Fe sites, to accomplish the MTM conversion using O2 as the oxidant under mild conditions. We apply a diverse set of advanced operando X-ray techniques to unveil how MIL-100(Fe) can act as a catalyst for direct MTM conversion. Single-phase crystallinity and stability of the MOF under reaction conditions (200 or 100 °C, CH4 + O2) are confirmed by X-ray diffraction measurements. X-ray absorption, emission, and resonant inelastic scattering measurements show that thermal treatment above 200 °C generates Fe(II) sites that interact with O2 and CH4 to produce methanol. Experimental evidence-driven density functional theory (DFT) calculations illustrate that the MTM reaction involves the oxidation of the Fe(II) sites to Fe(III) via a high-spin Fe(IV)═O intermediate. Catalyst deactivation is proposed to be caused by the escape of CH3• radicals from the relatively large MOF pore cages, ultimately resulting in the formation of hydroxylated triiron units, as proven by valence-to-core X-ray emission spectroscopy. The O2-based MTM catalytic activity of MIL-100(Fe) in the investigated conditions is demonstrated for two consecutive reaction cycles, proving the MOF potential toward active site regeneration. These findings will desirably lay the groundwork for the design of improved MOF catalysts for the MTM conversion.

Enzymatic Nitrogen Incorporation Using Hydroxylamine

Hydroxylamine-derived reagents have enabled versatile nitrene transfer reactions for introducing nitrogen-containing functionalities in small-molecule catalysis, as well as biocatalysis. These reagents, however, result in a poor atom economy and stoichiometric organic waste. Activating hydroxylamine (NH2OH) for nitrene transfer offers a low-cost and sustainable route to amine synthesis, since water is the sole byproduct. Despite its presence in nature, hydroxylamine is not known to be used for enzymatic nitrogen incorporation in biosynthesis. Here, we report an engineered heme enzyme that can utilize hydroxylammonium chloride, an inexpensive commodity chemical, for nitrene transfer. Directed evolution of Pyrobaculum arsenaticum protoglobin generated efficient enzymes for benzylic C-H primary amination and styrene aminohydroxylation. Mechanistic studies supported a stepwise radical pathway involving rate-limiting hydrogen atom transfer. This unprecedented activity is a useful addition to the "nitrene transferase" repertoire and hints at possible future discovery of natural enzymes that use hydroxylamine for amination chemistry.

Elusive Active Intermediates and Reaction Mechanisms of ortho-/ipso-Hydroxylation of Benzoic Acid by Hydrogen Peroxide Mediated by Bioinspired Iron(II) Catalysts

Aromatic hydroxylation of benzoic acids (BzOH) to salicylates and phenolates is fundamentally interesting in industrial chemistry. However, key mechanistic uncertainties and dichotomies remain after decades of effort. Herein, the elusive mechanism of the competitive ortho-/ipso-hydroxylation of BzOH by H2O2 mediated by a nonheme iron(II) catalyst was comprehensively investigated using density functional theory calculations. Results revealed that the long-postulated FeV(O)(anti-BzO) oxidant is an FeIV(O)(anti-BzO•) species 2 (anti- and syn- are defined by the orientation of the carboxyl oxygen of BzO to the oxo), which rules out the noted two-oxidant mechanism proposed previously. We propose a new mechanism in which, following the formation of an FeV(O)(syn-BzO) species (3) and its electromer FeIV(O)(syn-BzO•) (3'), 3/3' either converts to salicylate and phenolate via intramolecular self-hydroxylation (route A) or acts as an oxidant to oxygenate another BzOH to generate the same products (route B). In route A, the rotation of the BzO group along the C-O bond forms 2, in which the BzO group is orientated by π-π stacking interactions. An electrophilic ipso-addition forms a phenolate by concomitant decarboxylation or an ortho-attack forms a cationic complex, which readily undergoes an NIH shift and a BzOH-assisted proton shift to form a salicylate. In route B, 3 oxidizes an additional BzOH molecule directed by hydrogen bonding and π-π stacking interactions. In both routes, selectivity is determined by the chemical property of the BzO ring. These mechanistic findings provide a clear mechanistic scenario and enrich the knowledge of hydroxylation of aromatic acids.

Diversifying the functions of heme proteins with non-porphyrin cofactors

Heme proteins perform diverse biochemical functions using a single iron porphyrin cofactor. This versatility makes them attractive platforms for the development of new functional proteins. While directed evolution and metal substitution have expanded the properties, reactivity, and applications of heme proteins, the incorporation of porphyrin analogs remains an underexplored approach. This review discusses the replacement of heme with non-porphyrin cofactors, such as porphycene, corrole, tetradehydrocorrin, phthalocyanine, and salophen, and the attendant properties of these conjugates. While structurally similar, each ligand exhibits distinct optical and redox properties, as well as unique chemical reactivity. These hybrids serve as model systems to elucidate the effects of the protein environment on the electronic structure, redox potentials, optical properties, or other features of the porphyrin analog. Protein encapsulation can confer distinct chemical reactivity or selectivity of artificial metalloenzymes that cannot be achieved with the small molecule catalyst alone. Additionally, these conjugates can interfere with heme acquisition and uptake in pathogenic bacteria, providing an inroad to innovative antibiotic strategies. Together, these examples illustrate the diverse functionality that can be achieved by cofactor substitution. The further expansion of this approach will access unexplored chemical space, enabling the development of superior catalysts and the creation of heme proteins with emergent properties.

Titanium Salalen Catalyzed Enantioselective Benzylic Hydroxylation

The titanium complex of the cis-1,2-diaminocyclohexane (cis-DACH) derived Berkessel-salalen ligand is a highly efficient and enantioselective catalyst for the asymmetric epoxidation of terminal olefins with hydrogen peroxide ("Berkessel-Katsuki catalyst"). We herein report that this epoxidation catalyst also effects the highly enantioselective hydroxylation of benzylic C-H bonds with hydrogen peroxide. Mechanism-based ligand optimization identified a novel nitro-salalen Ti-catalyst of the highest efficiency ever reported for asymmetric catalytic benzylic hydroxylation, with enantioselectivities of up to 98 % ee, while overoxidation to ketone is marginal. The novel nitro-salalen Ti-catalyst also shows enhanced epoxidation efficiency, as evidenced by e.g. the conversion of 1-decene to its epoxide in 90 % yield with 94 % ee, at a catalyst loading of 0.1 mol-% only.

The importance of and difficulties involved in creating molecular probes for a carbon monoxide gasotransmitter

As one of the triumvirate of recognized gasotransmitter molecules, namely NO, H2S, and CO, the physiological effects of CO and its potential as a biomarker have been widely investigated, garnering particular attention due to its reported hypotensive, anti-inflammatory, and cytoprotective properties, making it a promising therapeutic agent. However, the development of CO molecular probes has remained relatively stagnant in comparison with the fluorescent probes for NO and H2S, owing to its inert molecular state under physiological conditions. In this review, starting from elucidating the definition and significance of CO as a gasotransmitter, the imperative for the advancement of CO probes, especially fluorescent probes, is expounded. Subsequently, the current state of development of CO probe methodologies is comprehensively reviewed, with an overview of the challenges and prospects in this burgeoning field of research.

Direct Electrophilic Attack of Compound I on the Indole Ring in the Peroxygenase Mechanism of Dehaloperoxidase DHP B in Degrading Haloindole: Electron Transfer Promotes the Reaction

The H2O2-dependent degradation of haloindole catalyzed by the dehaloperoxidase (DHP) from Amphitrite ornate has been reported to employ the peroxygenase mechanism, and the two oxidized products 5-halo-2-oxindole and 5-halo-3-oxindole have a similar amount. According to a previous experimental study, compound I (Cpd I) was suggested to be responsible for triggering the reaction, and the reaction may undergo three possible intermediates; however, the reaction details are still unclear. To clarify the reaction mechanism of DHP, the computational model was constructed on the basis of the high-resolution crystal structure, and a series of the quantum mechanical/molecular mechanical calculations were performed. Based on our calculation results, it is confirmed that the reaction starts from the direct electrophilic attack of Cpd I on the indole ring of the substrate, and the resulted intermediate contains both a carbocation and an oxygen anion, whereas the common hydrogen abstraction by Cpd I was calculated to correspond to a relatively higher barrier. In addition, a net electron transfer from the substrate to the iron center is observed during the attack of Cpd I on the indole ring; therefore, the carbocation/oxygen anion intermediate can easily undergo an intramolecular hydride transfer to form the product 5-halo-2-oxindole or isomerize to the epoxide intermediate which finally generates another product 5-halo-3-oxindole. It is the zwitterionic characteristic of the intermediate that makes the intermolecular hydride transfer quite easy, and it is the high electron affinity of the iron center that promotes the single-electron oxidation of the reaction intermediate. Our calculations well explain the formation of two oxidized products 5-halo-2-oxindole and 5-halo-3-oxindole.

Optimizing the Synthetic Potential of O2: Implications of Overpotential in Homogeneous Aerobic Oxidation Catalysis

Molecular oxygen is the quintessential oxidant for organic chemical synthesis, but many challenges continue to limit its utility and breadth of applications. Extensive historical research has focused on overcoming kinetic challenges presented by the ground-state triplet electronic structure of O2 and the various reactivity and selectivity challenges associated with reactive oxygen species derived from O2 reduction. This Perspective will analyze thermodynamic principles underlying catalytic aerobic oxidation reactions, borrowing concepts from the study of the oxygen reduction reaction (ORR) in fuel cells. This analysis is especially important for "oxidase"-type liquid-phase catalytic aerobic oxidation reactions, which proceed by a mechanism that couples two sequential redox half-reactions: (1) substrate oxidation and (2) oxygen reduction, typically affording H2O2 or H2O. The catalysts for these reactions feature redox potentials that lie between the potentials associated with the substrate oxidation and oxygen reduction reactions, and changes in the catalyst potential lead to variations in effective overpotentials for the two half reactions. Catalysts that operate at low ORR overpotential retain a more thermodynamic driving force for the substrate oxidation step, enabling O2 to be used in more challenging oxidations. While catalysts that operate at high ORR overpotential have less driving force available for substrate oxidation, they often exhibit different or improved chemoselectivity relative to the high-potential catalysts. The concepts are elaborated in a series of case studies to highlight their implications for chemical synthesis. Examples include comparisons of (a) NOx/oxoammonium and Cu/nitroxyl catalysts, (b) high-potential quinones and amine oxidase biomimetic quinones, and (c) Pd aerobic oxidation catalysts with or without NOx cocatalysts. In addition, we show how the reductive activation of O2 provides a means to access potentials not accessible with conventional oxidase-type mechanisms. Overall, this analysis highlights the central role of catalyst overpotential in guiding the development of aerobic oxidation reactions.

The role of basicity in selective C-H bond activation by transition metal-oxidos

The development of (bio)catalysts capable of selectively activating strong C-H bonds is a continuing challenge in modern chemistry. In both metalloenzymes and synthetic systems capable of activating C-H bonds, transition metal-oxido intermediates serve as the active species for reactivity whose thermodynamic properties influence the bond strengths they are capable of activating. In this Frontier article, we present current ideas of how the basicity of transition metal-oxidos impacts their reactivity with C-H bonds and present new opportunities within this field. We highlight recent insights into the role basicity plays in the activation process and its influence on mechanism, as well as the important role that secondary coordination sphere effects, such as hydrogen bonds, in tuning the basicity of the metal-oxido species is discussed.

Applications and Potential of In Silico Approaches for Psychedelic Chemistry

Molecular-level investigations of the Central Nervous System have been revolutionized by the development of computational methods, computing power, and capacity advances. These techniques have enabled researchers to analyze large amounts of data from various sources, including genomics, in vivo, and in vitro drug tests. In this review, we explore how computational methods and informatics have contributed to our understanding of mental health disorders and the development of novel drugs for neurological diseases, with a special focus on the emerging field of psychedelics. In addition, the use of state-of-the-art computational methods to predict the potential of drug compounds and bioinformatic tools to integrate disparate data sources to create predictive models is also discussed. Furthermore, the challenges associated with these methods, such as the need for large datasets and the diversity of in vitro data, are explored. Overall, this review highlights the immense potential of computational methods and informatics in Central Nervous System research and underscores the need for continued development and refinement of these techniques and more inclusion of Quantitative Structure-Activity Relationships (QSARs).

Interactions of Mn complexes with DNA: the relevance of therapeutic applications towards cancer treatment

Manganese (Mn) is one of the most significant bio-metals that helps the body to form connective tissue, bones, blood clotting factors, and sex hormones. It is necessary for fat and carbohydrate metabolism, calcium absorption, blood sugar regulation, and normal brain and nerve functions. It accelerates the synthesis of proteins, vitamin C, and vitamin B. It is also involved in the catalysis of hematopoiesis, regulation of the endocrine level, and improvement of immune function. Again, Mn metalloenzymes like arginase, glutamine synthetase, phosphoenolpyruvate decarboxylase, and Mn superoxide dismutase (MnSOD) contribute to the metabolism processes and reduce oxidative stress against free radicals. Recent investigations have revealed that synthetic Mn-complexes act as antibacterial and antifungal agents. As a result, chemists and biologists have been actively involved in developing Mn-based drugs for the treatment of various diseases including cancer. Therefore, any therapeutic drugs based on manganese complexes would be invaluable for the treatment of cancer/infectious diseases and could be a better substitute for cisplatin and other related platinum based chemotherapeutic drugs. From this perspective, attempts have been made to discuss the interactions and nuclease activities of Mn(II/III/IV) complexes with DNA through which one can evaluate their therapeutic applications.