MCD C-Term Signs, Saturation Behavior, and Determination of Band Polarizations in Randomly Oriented Systems with Spin S >/= (1)/(2). Applications to S = (1)/(2) and S = (5)/(2)

Mechanisms of Photoredox Catalysis Featuring Nickel-Bipyridine Complexes

Metallaphotoredox catalysis can unlock useful pathways for transforming organic reactants into desirable products, largely due to the conversion of photon energy into chemical potential to drive redox and bond transformation processes. Despite the importance of these processes for cross-coupling reactions and other transformations, their mechanistic details are only superficially understood. In this review, we have provided a detailed summary of various photoredox mechanisms that have been proposed to date for Ni-bipyridine (bpy) complexes, focusing separately on photosensitized and direct excitation reaction processes. By highlighting multiple bond transformation pathways and key findings, we depict how photoredox reaction mechanisms, which ultimately define substrate scope, are themselves defined by the ground- and excited-state geometric and electronic structures of key Ni-based intermediates. We further identify knowledge gaps to motivate future mechanistic studies and the development of synergistic research approaches spanning the physical, organic, and inorganic chemistry communities.

Identification, Characterization, and Electronic Structures of Interconvertible Cobalt-Oxygen TAML Intermediates

The reaction of Li[(TAML)CoIII]·3H2O (TAML = tetraamido macrocyclic tetraanionic ligand) with iodosylbenzene at 253 K in acetone in the presence of redox-innocent metal ions (Sc(OTf)3 and Y(OTf)3) or triflic acid affords a blue species 1, which is converted reversibly to a green species 2 upon cooling to 193 K. The electronic structures of 1 and 2 have been determined by combining advanced spectroscopic techniques (X-band electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR), X-ray absorption spectroscopy/extended X-ray absorption fine structure (XAS/EXAFS), and magnetic circular dichroism (MCD)) with ab initio theoretical studies. Complex 1 is best represented as an S = 1/2 [(Sol)(TAML•+)CoIII---OH(LA)]- species (LA = Lewis/Brønsted acid and Sol = solvent), where an S = 1 Co(III) center is antiferromagnetically coupled to S = 1/2 TAML•+, which represents a one-electron oxidized TAML ligand. In contrast, complex 2, also with an S = 1/2 ground state, is found to be multiconfigurational with contributions of both the resonance forms [(H-TAML)CoIV═O(LA)]- and [(H-TAML•+)CoIII═O(LA)]-; H-TAML and H-TAML•+ represent the protonated forms of TAML and TAML•+ ligands, respectively. Thus, the interconversion of 1 and 2 is associated with a LA-associated tautomerization event, whereby H+ shifts from the terminal -OH group to TAML•+ with the concomitant formation of a terminal cobalt-oxo species possessing both singlet (SCo = 0) Co(III) and doublet (SCo = 1/2) Co(IV) characters. The reactivities of 1 and 2 at different temperatures have been investigated in oxygen atom transfer (OAT) and hydrogen atom transfer (HAT) reactions to compare the activation enthalpies and entropies of 1 and 2.

Correlating Valence and 2p3d RIXS Spectroscopies: A Ligand-Field Study of Spin-Crossover Iron(II)

The molecular spin-crossover phenomenon between high-spin (HS) and low-spin (LS) states is a promising route to next-generation information storage, sensing applications, and molecular spintronics. Spin-crossover complexes also provide a unique opportunity to study the ligand field (LF) properties of a system in both HS and LS states while maintaining the same ligand environment. Presently, we employ complementing valence and core-level spectroscopic methods to probe the electronic excited-state manifolds of the spin-crossover complex [FeII(H2B(pz)2)2phen]0. Light-induced excited spin-state trapping (LIESST) at liquid He temperatures is exploited to characterize magnetic and spectroscopic properties of the photoinduced HS state using SQUID magnetometry and magnetic circular dichroism spectroscopy. In parallel, Fe 2p3d RIXS spectroscopy is employed to examine the ΔS = 0, 1 excited LF states. These experimental studies are combined with state-of-the-art CASSCF/NEVPT2 and CASCI/NEVPT2 calculations characterizing the ground and LF excited states. Analysis of the acquired LF information further supports the notion that the spin-crossover of [FeII(H2B(pz)2)2phen]0 is asymmetric, evidenced by a decrease in eπ in the LS state. The results demonstrate the power of cross-correlating spectroscopic techniques with high and low LF information content to make accurate excited-state assignments, as well as the current capabilities of ab initio theory in interpreting these electronic properties.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Magnetic Exchange Coupling in Zeolite Copper Dimers and Its Contribution to Methane Activation

The highly reactive binuclear [Cu2O]2+ active site in copper zeolites activates the inert C-H bond of methane at low temperatures, offering a potential solution to reduce methane flaring and mitigate atmospheric methane levels. While substantial progress has been made in understanding the activation of methane by this core, one critical aspect, the active site's spin, has remained undetermined. In this study, we use variable-temperature, variable-field magnetic circular dichroism spectroscopy to define the ground state spin of the [Cu2O]2+ active sites in Cu-CHA and Cu-MFI. This novel approach allows for site-selective determination of the magnetic exchange coupling between the two copper centers of specific [Cu2O]2+ cores in a heterogeneous mixture, circumventing the drawbacks of bulk magnetic techniques. These experimental findings are coupled to density functional theory calculations to elucidate magnetostructural correlations in copper zeolites that are different from those of homogeneous binuclear Cu(II) complexes. The different spin states for the [Cu2O]2+ cores have different reactivities governed by how methane approaches the active site. This introduces a new understanding of zeolite topological control on active site reactivity.

Chiroptical activity of benzannulated N-heterocyclic carbene rhenium(I) tricarbonyl halide complexes: towards efficient circularly polarized luminescence emitters

The design of enantiomerically pure circularly polarized luminescent (CPL) emitters would enormously benefit from the accurate and in-depth interpretation of the chiroptical properties by means of jointly (chiroptical) photophysical measurements and state-of-the-art theoretical investigation. Herein, computed and experimental (chiro-)optical properties of a series of eight enantiopure phosphorescent rhenium(I) tricarbonyl complexes are systematically compared in terms of electronic circular dichroism (ECD) and CPL. The compounds have general formula fac-[ReX(CO)3(N^CNHC)], where N^CNHC is a pyridyl benzannulated N-heterocyclic carbene deriving from a (substituted) 2-(pyridin-2-yl)imidazo[1,5-a]pyridin-2-ium proligand and X = Cl, Br and I, and display structured red phosphorescence with long-lived (τ = 7.0-19.1 μs) excited-state lifetime and dissymmetry factors |gLum| up to 4 × 10-3. The mixing of the character of the lowest-lying emitting triplet excited state is finely modulated between ligand centred (3LC), metal-to-ligand charge transfer (3MLCT) and halogen-to-ligand charge transfer (3XLCT) by the nature of the ancillary halogen and the chromophoric N^CNHC ligand. The study unravels the effect exerted by the nature of the excited state onto the ECD and CPL activity and will help to pave the way to construct efficient CPL emitters by chemical design.

Second-sphere tuning of analogues for the ferric-hydroperoxoheme form of Mycobacterium tuberculosis MhuD

Mycobacterium tuberculosis MhuD catalyzes the oxygenation of heme to mycobilin; experimental data presented here elucidates the novel hydroxylation reaction catalyzed by this enzyme. Analogues for the critical ferric-hydroperoxoheme (MhuD-heme-OOH) intermediate of this enzyme were characterized using UV/Vis absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectroscopies. In order to extract electronic transition energies from these spectroscopic data, a novel global fitting model was developed for analysis of UV/Vis Abs, CD, and MCD data. A variant of MhuD was prepared, N7S, which weakens the affinity of heme-bound enzyme for a hydroperoxo analogue, azide, without significantly altering the protein secondary structure. Global fitting of spectroscopic data acquired in this study revealed that the second-sphere N7S substitution perturbs the electronic structure of two analogues for MhuD-heme-OOH: azide-inhibited MhuD (MhuD-heme-N₃) and cyanide-inhibited MhuD (MhuD-heme-CN). The ground state electronic structures of MhuD-heme-N₃ and MhuD-heme-CN were assessed using variable-temperature, variable-field MCD. Altogether, these data strongly suggest that there is a hydrogen bond between the Asn7 side-chain and the terminal oxygen of the hydroperoxo ligand in MhuD-heme-OOH. As discussed herein, this finding supports a novel hydroxylation reaction mechanism where the Asn7 side-chain guides a transient hydroxyl radical derived from homolysis of the OO bond in MhuD-heme-OOH to the β- or δ-meso carbon of the porphyrin ligand yielding β- or δ-meso-hydroxyheme, respectively.

The Evolution from Superatom- to Plasmon-Mediated Magnetic Circular Dichroism in Colloidal Metal Nanoparticles Spanning the Nonmetallic to Metallic Limits

The magneto-optical absorption properties of colloidal metal nanoclusters spanning nonmetallic to metallic regimes were examined using variable-temperature variable-field magnetic circular dichroism (VTVH-MCD) spectroscopy. Charge neutral Au₂₅(SC₈H₉)₁₈ exhibited MCD spectra dominated by Faraday C-terms, consistent with expectations for a nonmetallic paramagnetic nanocluster. This response is reconciled by the open-shell superatom configuration of Au₂₅(SC₈H₉)₁₈. Metallic and plasmon-supporting Au₄₅₉(pMBA)₁₇₀ exhibited temperature-independent VTVH-MCD spectra dominated by Faraday A-terms. Au₁₄₄(SC₈H₉)₆₀, which is intermediate to the metallic and nonmetallic limits, showed the most complex VTVH-MCD response of the three nanoclusters, consisting of 19 distinguishable peaks spanning the visible and near-infrared (3.0-1.4 eV). Variable-temperature analysis suggested that none of these transitions originated from plasmon excitation. However, evidence for both paramagnetic and mixed (i.e., nondiscrete) transitions of Au₁₄₄(SC₈H₉)₆₀ was observed. These results highlight the complexity of gold nanocluster electronic transitions that emerge as sizes approach metallic length scales. Nanoclusters in this regime may provide opportunities for tailoring the magneto-optical properties of colloidal nanostructures.

Fully Delocalized Mixed-Valent Cu1.5 Cu1.5 Complex: Strong Cu-Cu interaction and Fast Electron Self-Exchange Rate Despite Large Structural Changes

A flexible macrocyclic ligand with two tridentate {CNC} compartments can host two Cu ions in reversibly interconvertible states, Cu^I Cu^I (1) and mixed-valent Cu^1.5 Cu^1.5 (2). They were characterized by XRD and multiple spectroscopic methods, including EPR, UV/Vis absorption and MCD, in combination with TD-DFT and CASSCF calculations. 2 features a short Cu⋅⋅⋅Cu distance (≈2.5 Å; compared to ≈4.0 Å in 1) and a very high delocalization energy of 13 000 cm^-1 , comparable to the mixed-valent state of the biological Cu_A site. Electron self-exchange between 1 and 2 is rapid despite large structural reorganization, and is proposed to proceed via a sequential mechanism involving an active conformer of 1, viz. 1'; the latter has been characterized by XRD. Such electron transfer (ET) process is reminiscent of the conformationally gated ET proposed for biological systems. This redox couple is a unique pair of flexible dicopper complexes, achieving fast electron self-exchange closely related to the function of the Cu_A site.

Preparation and Characterization of a Formally NiIV-Oxo Complex with a Triplet Ground State and Application in Oxidation Reactions

High-valent first-row transition-metal-oxo complexes are important intermediates in biologically and chemically relevant oxidative transformations of organic molecules and in the water splitting reaction in (artificial) photosynthesis. While high-valent Fe- and Mn-oxo complexes have been characterized in detail, much less is known about their analogues with late transition metals. In this study, we present the synthesis and detailed characterization of a unique mononuclear terminal Ni-O complex. This compound, [Ni(TAML)(O)(OH)]^3-, is characterized by an intense charge-transfer (CT) band around 730 nm and has an S_t = 1 ground state, as determined by magnetic circular dichroism spectroscopy. From extended X-ray absorption fine structure (EXAFS), the Ni-O bond distance is 1.84 Å. Ni K edge XAS data indicate that the complex contains a Ni(III) center, which results from an unusually large degree of Ni-O π-bond inversion, with one hole located on the oxo ligand. The complex is therefore best described as a low-spin Ni(III) complex (S = 1/2) with a bound oxyl (O^•-) ligand (S = 1/2), where the spins of Ni and oxyl are ferromagnetically coupled, giving rise to the observed S_t = 1 ground state. This bonding description is roughly equivalent to the presence of a Ni-O single (σ) bond. Reactivity studies show that [Ni(TAML)(O)(OH)]^3- is a strong oxidant capable of oxidizing thioanisole and styrene derivatives with large negative ρ values in the Hammett plot, indicating its electrophilic nature. The intermediate also shows high reactivity in C-H bond activation of hydrocarbons with a kinetic isotope effect of 7.0(3) in xanthene oxidation.

Particle Swarm Fitting of Spin Hamiltonians: Magnetic Circular Dichroism of Reduced and NO-Bound Flavodiiron Protein

A particle swarm optimization (PSO) algorithm is described for the fitting of ground-state spin Hamiltonian parameters from variable-temperature/variable-field (VTVH) magnetic circular dichroism (MCD) data. This PSO algorithm is employed to define the ground state of two catalytic intermediates from a flavodiiron protein (FDP), a class of enzymes with nitric oxide reductase activity. The bimetallic iron active site of this enzyme proceeds through a biferrous intermediate and a mixed ferrous-{FeNO}⁷ intermediate during the catalytic cycle, and the MCD spectra of these intermediates are presented and analyzed. The fits of the spin Hamiltonians are shown to provide important geometric and electronic insight into these species that is compared and contrasted with previous reports.

Effective Mass for Holes in Paramagnetic, Plasmonic Cu5FeS4 Semiconductor Nanocrystals

The impact of a magneto-structural phase transition on the carrier effective mass in Cu5FeS4 plasmonic semiconductor nanocrystals was examined using Magnetic Circular Dichroism (MCD). Through MCD, the sample was confirmed as p-type from variable temperature studies from 1.8 - 75 K. Magnetic field dependent behavior is observed, showing an asymptotic behavior at high field with an m ∗ value 5.98 m ∗ ∕ m e at 10 T and 2.73 m ∗ ∕ m e at 2 T. Experimentally obtained results are holistically compared to SQUID magnetization data and DFT results, highlighting a dependency on vacancy driven polaronic coupling, magnetocrystalline anisotropy, and plasmon coupling of the magnetic field all contributing to an overall decrease in the hole mean free path dependent on the magnetic field applied to Cu5FeS4.

Spectroscopic Studies of Mononuclear Molybdenum Enzyme Centers

A concise review is provided of the contributions that various spectroscopic methods have made to our understanding of the physical and electronic structures of mononuclear molybdenum enzymes. Contributions to our understanding of the structure and function of each of the major families of these enzymes is considered, providing a perspective on how spectroscopy has impacted the field.

Going beyond the Electric-Dipole Approximation in the Calculation of Absorption and (Magnetic) Circular Dichroism Spectra including Scalar Relativistic and Spin-Orbit Coupling Effects

In this work, a time-dependent density functional theory (TD-DFT) scheme for computing optical spectroscopic properties in the framework of linearly and circularly polarized light is presented. The scheme is based on a previously formulated theory for predicting optical absorption and magnetic circular dichroism (MCD) spectra. The scheme operates in the framework of the full semi-classical field-matter interaction operator, thus generating a powerful and general computational scheme capable of computing the absorption (ABS), circular dichroism (CD), and MCD spectra. In addition, our implementation includes the treatment of relativistic effects in the framework of quasidegenerate perturbation theory, which accounts for scalar relativistic effects (in the self-consistent field step) and spin-orbit coupling (in the TD-DFT step), as well as external magnetic field perturbations. Hence, this formalism is also able to probe spin-forbidden transitions. The random orientations of molecules are taken into account by a semi-numerical approach involving a Lebedev numerical quadrature alongside analytical integration. It is demonstrated the numerical quadrature requires as few as 14 points for satisfactory converged results thus leading to a highly efficient scheme, while the calculation of the exact transition moments creates no computational bottlenecks. It is demonstrated that at zero magnetic field, the CD spectrum is recovered while the sum of left and right circularly polarized light contributions provides the linear absorption spectrum. The virtues of this efficient and general protocol are demonstrated on a selected set of organic molecules where the various contributions to the spectral intensities have been analyzed in detail.

Electronic structure studies of free and enzyme-bound B12 species by magnetic circular dichroism and complementary spectroscopic techniques

Electronic absorption (Abs) and circular dichroism (CD) spectroscopic techniques have been used successfully for over half a century in studies of free and enzyme-bound B₁₂ species. More recently, magnetic circular dichroism (MCD) spectroscopy and other complementary techniques have provided an increasingly detailed understanding of the electronic structure of cobalamins. While CD spectroscopy measures the difference in the absorption of left- and right-circularly polarized light, MCD spectroscopy adds the application of a magnetic field parallel to the direction of light propagation. Transitions that are formally forbidden according to the Abs and CD selection rules, such as ligand field (or d→d) transitions, can gain MCD intensity through spin-orbit coupling. As such, MCD spectroscopy provides a uniquely sensitive probe of the different binding modes, Co oxidation states, and axial ligand environments of B₁₂ species in enzyme active sites, and thus the distinct reactivities displayed by these species. This chapter summarizes representative MCD studies of free and enzyme-bound B₁₂ species, including those present in adenosyltransferases, isomerases, and reductive dehalogenases. Complementary spectroscopic and computational data are also presented and discussed where appropriate.

Magnetic interaction of photoexcited terbium-porphyrin complexes with non-aromatic ligands having different symmetries

Interaction between the total angular momentum (J) of 4f electrons in lanthanides and the orbital angular momentum (L) of porphyrins in the photoexcited states was investigated by temperature-dependent and magnetic circular dichroism (MCD) for 5,10,15,20-tetraphenylporphyrinato (TPP) terbium(III) complexes with two different non-aromatic ligands i.e. 12-crown-4(1,4,7,10-tetraoxacyclododecane) and 1-aza-12-crown-4(1,4,7-trioxa-10-azacyclododecane). The two cases with different ligands were examined in order to understand how magnetic interaction depends on the symmetry of the non-aromatic ligands. The three absorption bands in the visible region, B(0,0), Q(1,0), and Q(0,0) bands, showed temperature-dependent MCD A term. For each band, the J-L interaction was determined from the simulation-based fitting to experimental ratios. An increase in the magnitude of the J-L interaction was observed when the second ligand was the aza-crown with a lower symmetry. Ab initio RASSCF/RASSI calculations were performed to explore the effect of the difference in the second ligand to the ligand centred excited states and the ligand-field-splitting structure on the metal-centred ground multiplet of J = 6.

Ruffling is essential for Staphylococcus aureus IsdG-catalyzed degradation of heme to staphylobilin

Non-canonical heme oxygenases are enzymes that degrade heme to non-biliverdin products within bacterial heme iron acquisition pathways. These enzymes all contain a conserved second-sphere Trp residue that is essential for enzymatic turnover. Here, UV/Vis absorption (Abs) and circular dichroism (CD) spectroscopies were employed to show that the W67F variant of IsdG perturbs the heme substrate conformation. In general, a dynamic equilibrium between "planar" and "ruffled" substrate conformations exists within non-canonical heme oxygenases, and that the second-sphere Trp favors population of the "ruffled" substrate conformation. ¹H nuclear magnetic resonance and magnetic CD spectroscopies were used to characterize the electronic structures of IsdG and IsdI variants with different substrate conformational distributions. These data revealed that the "ruffled" substrate conformation promotes partial porphyrin-to‑iron electron transfer, which makes the meso carbons of the porphyrin ring susceptible to radical attack. Finally, UV/Vis Abs spectroscopy was utilized to quantify the enzymatic rates, and electrospray ionization mass spectrometry was used to identify the product distributions, for variants of IsdG with altered substrate conformational distributions. In general, the rate of heme oxygenation by non-canonical heme oxygenases depends upon the population of the "ruffled" substrate conformation. Also, the production of staphylobilin or mycobilin by these enzymes is correlated with the population of the "ruffled" substrate conformation, since variants that favor population of the "planar" substrate conformation yield significant amounts of biliverdin. These data can be understood within the framework of a concerted rearrangement mechanism for the monooxygenation of heme to meso-hydroxyheme by non-canonical heme oxygenases.

The Biologically Relevant Coordination Chemistry of Iron and Nitric Oxide: Electronic Structure and Reactivity

Nitric oxide (NO) is an important signaling molecule that is involved in a wide range of physiological and pathological events in biology. Metal coordination chemistry, especially with iron, is at the heart of many biological transformations involving NO. A series of heme proteins, nitric oxide synthases (NOS), soluble guanylate cyclase (sGC), and nitrophorins, are responsible for the biosynthesis, sensing, and transport of NO. Alternatively, NO can be generated from nitrite by heme- and copper-containing nitrite reductases (NIRs). The NO-bearing small molecules such as nitrosothiols and dinitrosyl iron complexes (DNICs) can serve as an alternative vehicle for NO storage and transport. Once NO is formed, the rich reaction chemistry of NO leads to a wide variety of biological activities including reduction of NO by heme or non-heme iron-containing NO reductases and protein post-translational modifications by DNICs. Much of our understanding of the reactivity of metal sites in biology with NO and the mechanisms of these transformations has come from the elucidation of the geometric and electronic structures and chemical reactivity of synthetic model systems, in synergy with biochemical and biophysical studies on the relevant proteins themselves. This review focuses on recent advancements from studies on proteins and model complexes that not only have improved our understanding of the biological roles of NO but also have provided foundations for biomedical research and for bio-inspired catalyst design in energy science.

The three-spin intermediate at the O-O cleavage and proton-pumping junction in heme-Cu oxidases

[Figure: see text].

Spin-Polarized Photoluminescence in Au25 (SC8 H9 )18 Monolayer-Protected Clusters

Here, the observation of spin-polarized emission for the Au₂₅ (SC₈ H₉ )₁₈ monolayer-protected cluster (MPC) is reported. Variable-temperature variable-field magnetic circular photoluminescence (VTV H ⇀ -MCPL) measurements are combined with VT-PL spectroscopy to provide state-resolved characterization of the transient electronic structure and spin-polarized electron-hole recombination dynamics of Au₂₅ (SC₈ H₉ )₁₈ . Through analysis of VTV H ⇀ -MCPL measurements, a low energy (1.64 eV) emission peak is assigned to intraband relaxation between core-metal-localized superatom-D to -P orbitals. Two higher energy interband components (1.78 eV, 1.94 eV) are assigned to relaxation from superatom-D orbitals to states localized to the inorganic semirings. For both intraband superatom-based or interband relaxation mechanisms, the extent of spin-polarization, quantified as the degree of circular polarization (DOCP), is determined by state-specific electron-vibration coupling strengths and energy separations of bright and dark electronic fine-structure levels. At low temperatures (<60 K), metal-metal superatom-based intraband transitions dominate the global PL emission. At higher temperatures (>60 K), interband ligand-based emission is dominant. In the low-temperature PL regime, increased sample temperature results in larger global PL intensity. In the high-temperature regime, increased temperature quenches interband radiative recombination. The relative intensity for each PL mechanism is discussed in terms of state-specific electronic-vibrational coupling strengths and related to the total angular momentum, quantified by Landé g-factors.

Role of a Tyrosine Radical in Human Ceruloplasmin Catalysis

Multicopper oxidases (MCOs) are a large family of diverse enzymes found in both eukaryotes and prokaryotes that couple one-electron oxidations of various substrates to the four-electron reduction of O₂ to H₂O, functioning through a set of metallocofactors consisting of one type 1 copper (T1 Cu) and one trinuclear copper cluster (TNC). Human serum ceruloplasmin (Cp) is a unique member of MCOs composed of six cupredoxin domains and harbors six Cu ions arranged as three T1 Cu and one TNC. The native substrate of Cp is Fe²⁺. It is an essential ferroxidase critical for iron homeostasis and is closely associated with metal-mediated diseases and metal neurotoxicity. In human serum, Cp operates under substrate-limiting low [Fe²⁺] but high [O₂] conditions, implying the possible involvement of partially reduced intermediates in Cp catalysis. In this work, we studied for the first time Cp reactivities at defined partially reduced states and discovered a tyrosine radical weakly magnetically coupled to the native intermediate (NI) of the TNC via a hydrogen bond. Our results lead to a new hypothesis that human iron transport is regulated as the paired transfer of iron from ferroportin to Cp to transferrin, and the tyrosine residue in Cp acts as a gate to avoid reactive oxygen species (ROS) formation when Fe²⁺ delivery is dysregulated.

Concerted proton-electron transfer reactions of manganese-hydroxo and manganese-oxo complexes

The enzymes manganese superoxide dismutase and manganese lipoxygenase use MnIII-hydroxo centres to mediate proton-coupled electron transfer (PCET) reactions with substrate. As manganese is earth-abundant and inexpensive, manganese catalysts are of interest for synthetic applications. Recent years have seen exciting reports of enantioselective C-H bond oxidation by Mn catalysts supported by aminopyridyl ligands. Such catalysts offer economic and environmentally-friendly alternatives to conventional reagents and catalysts. Mechanistic studies of synthetic catalysts highlight the role of Mn-oxo motifs in attacking substrate C-H bonds, presumably by a concerted proton-electron transfer (CPET) step. (CPET is a sub-class of PCET, where the proton and electron are transferred in the same step.) Knowledge of geometric and electronic influences for CPET reactions of Mn-hydroxo and Mn-oxo adducts enhances our understanding of biological and synthetic manganese centers and informs the design of new catalysts. In this Feature article, we describe kinetic, spectroscopic, and computational studies of MnIII-hydroxo and MnIV-oxo complexes that provide insight into the basis for the CPET reactivity of these species. Systematic perturbations of the ligand environment around MnIII-hydroxo and MnIV-oxo motifs permit elucidation of structure-activity relationships. For MnIII-hydroxo centers, electron-deficient ligands enhance oxidative reactivity. However, ligand perturbations have competing consequences, as changes in the MnIII/II potential, which represents the electron-transfer component for CPET, is offset by compensating changes in the pKa of the MnII-aqua product, which represents the proton-transfer component for CPET. For MnIV-oxo systems, a multi-state reactivity model inspired the development of significantly more reactive complexes. Weakened equatorial donation to the MnIV-oxo unit results in large rate enhancements for C-H bond oxidation and oxygen-atom transfer reactions. These results demonstrate that the local coordination environment can be rationally changed to enhance reactivity of MnIII-hydroxo and MnIV-oxo adducts.

The Spectroscopy of Nitrogenases

Nitrogenases are responsible for biological nitrogen fixation, a crucial step in the biogeochemical nitrogen cycle. These enzymes utilize a two-component protein system and a series of iron-sulfur clusters to perform this reaction, culminating at the FeMco active site (M = Mo, V, Fe), which is capable of binding and reducing N2 to 2NH3. In this review, we summarize how different spectroscopic approaches have shed light on various aspects of these enzymes, including their structure, mechanism, alternative reactivity, and maturation. Synthetic model chemistry and theory have also played significant roles in developing our present understanding of these systems and are discussed in the context of their contributions to interpreting the nature of nitrogenases. Despite years of significant progress, there is still much to be learned from these enzymes through spectroscopic means, and we highlight where further spectroscopic investigations are needed.

Electron Paramagnetic Resonance and Magnetic Circular Dichroism Spectra of the Nitrogenase M Cluster Precursor Suggest Sulfur Migration upon Oxidation: A Proposal for Substrate and Inhibitor Binding

The active site of the nitrogen-fixing enzyme Mo-nitrogenase is the M cluster ([MoFe7 S9 C⋅R-homocitrate]), also known as the FeMo cofactor or FeMoco. The biosynthesis of this highly complex metallocluster involves a series of proteins. Among them, NifB, a radical-SAM enzyme, is instrumental in the assembly of the L cluster ([Fe8 S9 C]), a precursor and all-iron core of the M cluster. In the absence of sulfite, NifB assembles a precursor form of the L cluster called the L* cluster ([Fe8 S8 C]), which lacks the final ninth sulfur. EPR and MCD spectroscopies are used to probe the electronic structures of the paramagnetic, oxidized forms of both the L and L* clusters, labeled LOx and [L*]Ox . This study shows that both LOx and [L*]Ox have nearly identical EPR and MCD spectra, thus suggesting that the two clusters have identical structures upon oxidation; in other words, a sulfur migrates away from LOx following oxidation, thereby rendering the cluster identical to [L*]Ox . It is proposed that a similar migration could occur to the M cluster upon oxidation, and that this is an instrumental part of both M cluster formation and nitrogenase substrate/inhibitor binding.

Electronic Structure and Magnetic Properties of a Titanium(II) Coordination Complex

Stable coordination complexes of Ti^II (3d²) are relatively uncommon, but are of interest as synthons for low oxidation state titanium complexes for application as potential catalysts and reagents for organic synthesis. Specifically, high-spin Ti^II ions supported by redox-inactive ligands are still quite rare due to the reducing power of this soft ion. Among such Ti^II complexes is trans-[TiCl₂(tmeda)₂], where tmeda = N,N,N',N'-tetramethylethane-1,2-diamine. This complex was first reported by Gambarotta and co-workers almost 30 years ago, but it was not spectroscopically characterized and theoretical investigation by quantum chemical theory (QCT) was not feasible at that time. As part of our interest in low oxidation state early transition metal complexes, we have revisited this complex and report a modified synthesis and a low temperature (100 K) crystal structure that differs slightly from that originally reported at ambient temperature. We have used magnetometry, high-frequency and -field EPR (HFEPR), and variable-temperature variable-field magnetic circular dichroism (VTVH-MCD) spectroscopies to characterize trans-[TiCl₂(tmeda)₂]. These techniques yield the following S = 1 spin Hamiltonian parameters for the complex: D = -5.23(1) cm^-1, E = -0.88(1) cm^-1, (E/D = 0.17), g = [1.86(1), 1.94(2), 1.77(1)]. This information, in combination with electronic transitions from MCD, was used as input for both classical ligand-field theory (LFT) and detailed QCT studies, the latter including both density functional theory (DFT) and ab initio methods. These computational methods are seldom applied to paramagnetic early transition metal complexes, particularly those with S > 1/2. Our studies provide a complete picture of the electronic structure of this complex that can be put into context with the few other high-spin and mononuclear Ti^II species characterized to date.

Mechanism of L2,3-edge x-ray magnetic circular dichroism intensity from quantum chemical calculations and experiment-A case study on V(IV)/V(III) complexes

In this work, we present a combined experimental and theoretical study on the V L_2,3-edge x-ray absorption (XAS) and x-ray magnetic circular dichroism (XMCD) spectra of V^IVO(acac)₂ and V^III(acac)₃ prototype complexes. The recorded V L_2,3-edge XAS and XMCD spectra are richly featured in both V L₃ and L₂ spectral regions. In an effort to predict and interpret the nature of the experimentally observed spectral features, a first-principles approach for the simultaneous prediction of XAS and XMCD spectra in the framework of wavefunction based ab initio methods is presented. The theory used here has previously been formulated for predicting optical absorption and MCD spectra. In the present context, it is applied to the prediction of the V L_2,3-edge XAS and XMCD spectra of the V^IVO(acac)₂ and V^III(acac)₃ complexes. In this approach, the spin-free Hamiltonian is computed on the basis of the complete active space configuration interaction (CASCI) in conjunction with second order N-electron valence state perturbation theory (NEVPT2) as well as the density functional theory (DFT)/restricted open configuration interaction with singles configuration state functions based on a ground state Kohn-Sham determinant (ROCIS/DFT). Quasi-degenerate perturbation theory is then used to treat the spin-orbit coupling (SOC) operator variationally at the many particle level. The XAS and XMCD transitions are computed between the relativistic many particle states, considering their respective Boltzmann populations. These states are obtained from the diagonalization of the SOC operator along with the spin and orbital Zeeman operators. Upon averaging over all possible magnetic field orientations, the XAS and XMCD spectra of randomly oriented samples are obtained. This approach does not rely on the validity of low-order perturbation theory and provides simultaneous access to the calculation of XMCD A, B, and C terms. The ability of the method to predict the XMCD C-term signs and provide access to the XMCD intensity mechanism is demonstrated on the basis of a generalized state coupling mechanism based on the type of the excitations dominating the relativistically corrected states. In the second step, the performance of CASCI, CASCI/NEVPT2, and ROCIS/DFT is evaluated. The very good agreement between theory and experiment has allowed us to unravel the complicated XMCD C-term mechanism on the basis of the SOC interaction between the various multiplets with spin S' = S, S ± 1. In the last step, it is shown that the commonly used spin and orbital sum rules are inadequate in interpreting the intensity mechanism of the XAS and XMCD spectra of the V^IVO(acac)₂ and V^III(acac)₃ complexes as they breakdown when they are employed to predict their magneto-optical properties. This conclusion is expected to hold more generally.

CfbA promotes insertion of cobalt and nickel into ruffled tetrapyrroles in vitro

The nickel chelatase CfbA is the smallest member of the chelatase family, but the mechanism by which this enzyme inserts nickel into sirohydrochlorin is unknown. In order to gain mechanistic insight, metal binding, tetrapyrrole binding, and enzyme activity were characterized for a variety of substrates using several spectroscopic and computational approaches. Mass spectrometery and magnetic circular dichroism experiments revealed that CfbA binds an octahedral, high-spin metal substrate. UV/Vis absorption spectroscopy demonstrated that the enzyme binds a wide range of tetrapyrrole substrates and perturbs their electronic structures. Based upon activity assays, CfbA promotes insertion of cobalt and nickel into several tetrapyrroles, including cobalt insertion into protopophyrin IX. Finally, density functional theory models were developed which strongly suggest that observed spectral changes upon binding to the enzyme can be explained by tetrapyrrole ruffling, but not deprotonation or saddling. The observation of an octahedral, high-spin metal bound to CfbA leads to a generalization for all class II chelatases: these enzymes bind labile metal substrates and metal desolvation is not a rate-limiting step. The conclusion that CfbA ruffles its tetrapyrrole substrate reveals that the CfbA mechanism is different from that currently proposed for ferrochelatase, and identifies an intriguing correlation between metal substrate specificity and tetrapyrrole distortion mode in chelatases.

O2 Reduction to Water by High Potential Multicopper Oxidases: Contributions of the T1 Copper Site Potential and the Local Environment of the Trinuclear Copper Cluster

High potential multicopper oxidases (MCOs) have T1 reduction potentials >600 mV (vs normal hydrogen electrode), making them important catalysts for O₂ reduction in various biotechnological applications. The oxygen reduction mechanism for the low potential MCOs is well-characterized; however, O₂ reactivity of high potential MCOs is not well understood. In this study, we have shown that laccase from Trametes versicolor, where the T1 redox potential is increased by ∼350 mV over that of the low potential MCOs corresponding to an 8 kcal/mol decrease in the driving force, exhibits a slower intramolecular electron transfer (IET) rate to the trinuclear Cu cluster (TNC) in the native intermediate (NI), relative to the low potential MCO from Rhus vernicifera laccase. This IET rate is, however, >10² times faster than the decay rate of the NI, demonstrating that this intermediate form of the enzyme is catalytically relevant enabling fast turnover. However, in contrast to the low potential MCOs where T1 reduction by substrate is rate limiting, the rate limiting step in turnover of high potential MCOs is the first IET to NI. Part of the reduction potential difference of the T1 sites in high vs low potential MCOs is balanced by an ∼100 mV higher reduction potential of NI due to the more positive protein environment in the vicinity of the TNC.

Geometric and Electronic Structural Contributions to Fe/O2 Reactivity

While two classes of non-heme iron enzymes use ferric centers to activate singlet organic substrates for the spin forbidden reaction with ³O₂, most classes use high spin ferrous sites to activate dioxygen. These Fe^II active sites do not exhibit intense absorption bands and have an integer spin ground state thus are mostly EPR inactive. We have developed new spectroscopic methodologies that provide geometric and electronic structural insight into the ferrous centers and their interactions with cosubstrates for dioxygen activation and into the nature of the intermediates generated in these reactions. First, we present our variable-temperature variable-field magnetic circular dichroism (VTVH MCD) methodology to experimentally define the geometric and electronic structure of the high spin ferrous active site. Then, we focus on using Nuclear Resonance Vibrational Spectroscopy (NRVS, performed at SPring-8) to define geometric structure and VTVH MCD to define the electronic structure of the Fe^III-OOH and Fe^IV=O intermediates generated in O₂ activation and the spin state dependence of their frontier molecular orbitals (FMOs) in controlling reactivity. Experimentally validated reaction coordinates are derived for the anticancer drug bleomycin in its cleavage of DNA and for an alpha- ketoglutarate dependent dioxygenase in its selective halogenation over the thermodynamically favored hydroxylation of substrate.

A Biochemical Nickel(I) State Supports Nucleophilic Alkyl Addition: A Roadmap for Methyl Reactivity in Acetyl Coenzyme A Synthase

Nickel-containing enzymes such as methyl coenzyme M reductase (MCR) and carbon monoxide dehydrogenase/acetyl coenzyme A synthase (CODH/ACS) play a critical role in global energy conversion reactions, with significant contributions to carbon-centered processes. These enzymes are implied to cycle through a series of nickel-based organometallic intermediates during catalysis, though identification of these intermediates remains challenging. In this work, we have developed and characterized a nickel-containing metalloprotein that models the methyl-bound organometallic intermediates proposed in the native enzymes. Using a nickel(I)-substituted azurin mutant, we demonstrate that alkyl binding occurs via nucleophilic addition of methyl iodide as a methyl donor. The paramagnetic Ni^III-CH₃ species initially generated can be rapidly reduced to a high-spin Ni^II-CH₃ species in the presence of exogenous reducing agent, following a reaction sequence analogous to that proposed for ACS. These two distinct bioorganometallic species have been characterized by optical, EPR, XAS, and MCD spectroscopy, and the overall mechanism describing methyl reactivity with nickel azurin has been quantitatively modeled using global kinetic simulations. A comparison between the nickel azurin protein system and existing ACS model compounds is presented. Ni^III-CH₃ Az is only the second example of two-electron addition of methyl iodide to a Ni^I center to give an isolable species and the first to be formed in a biologically relevant system. These results highlight the divergent reactivity of nickel across the two intermediates, with implications for likely reaction mechanisms and catalytically relevant states in the native ACS enzyme.

A bioorganometallic model for acetyl coenzyme A synthase has been developed. This model protein is able to bind a cationic methyl group via direct addition to the nickel(I) center. The resultant nickel(III)-methyl species has been characterized via optical and electron paramagnetic resonance spectroscopy, and the reduced nickel(II)-methyl state has been characterized using magnetic circular dichroism and X-ray spectroscopy. Implications for further reactivity with CO are gleaned from electronic structure analysis of the nickel-methyl species.

Low-Temperature Magnetism in Nanoscale Gold Revealed through Variable-Temperature Magnetic Circular Dichroism Spectroscopy

The low-temperature (0.35-4.2 K) steady-state electronic absorption of the monolayer-protected cluster (MPC) Au₁₀₂( pMBA)₄₄ was studied using magnetic circular dichroism (MCD) spectroscopy to investigate previously reported low-temperature (<50 K) magnetism in d¹⁰ nanogold systems. Variable-temperature variable-field analysis of resolvable MCD extinction components revealed two distinct magnetic anisotropic behaviors. A low-energy, diamagnetic component was correlated to excitation from states localized to the passivating ligands. A high-energy, paramagnetic component was attributed to excitation from the d-band of the Au core. The temperature dependence of the magnetic anisotropy for each component is discussed in terms of previously reported structural parameters of the atomically precise Au₁₀₂( pMBA)₄₄ MPC. It is concluded that temperature-sensitive structure-dependent Au d-d orbital interactions result in the promotion of 5d-band electrons to the 6sp-band via orbital rehybridization, inducing a 15× increase in the Landé g-factor over the temperature range spanning from 0.35 to 4.2 K.

Magnetic circular dichroism spectra of transition metal complexes calculated from restricted active space wavefunctions

Multiconfigurational restricted active space (RAS) self-consistent field (SCF) or configuration interaction (CI) approaches, augmented with a treatment of spin–orbit coupling by state interaction, were used to calculate the magnetic circular dichroism , , and/or for closed- and open-shell transition metal complexes.

Spectroscopic and Electronic Structure Study of ETHE1: Elucidating the Factors Influencing Sulfur Oxidation and Oxygenation in Mononuclear Nonheme Iron Enzymes

ETHE1 is a member of a growing subclass of nonheme Fe enzymes that catalyzes transformations of sulfur-containing substrates without a cofactor. ETHE1 dioxygenates glutathione persulfide (GSSH) to glutathione (GSH) and sulfite in a reaction which is similar to that of cysteine dioxygenase (CDO), but with monodentate (vs bidentate) substrate coordination and a 2-His/1-Asp (vs 3-His) ligand set. In this study, we demonstrate that GSS^- binds directly to the iron active site, causing coordination unsaturation to prime the site for O₂ activation. Nitrosyl complexes without and with GSSH were generated and spectroscopically characterized as unreactive analogues for the invoked ferric superoxide intermediate. New spectral features from persulfide binding to the Fe^III include the appearance of a low-energy Fe^III ligand field transition, an energy shift of a NO^- to Fe^III CT transition, and two new GSS^- to Fe^III CT transitions. Time-dependent density functional theory calculations were used to simulate the experimental spectra to determine the persulfide orientation. Correlation of these spectral features with those of monodentate cysteine binding in isopenicillin N synthase (IPNS) shows that the persulfide is a poorer donor but still results in an equivalent frontier molecular orbital for reactivity. The ETHE1 persulfide dioxygenation reaction coordinate was calculated, and while the initial steps are similar to the reaction coordinate of CDO, an additional hydrolysis step is required in ETHE1 to break the S-S bond. Unlike ETHE1 and CDO, which both oxygenate sulfur, IPNS oxidizes sulfur through an initial H atom abstraction. Thus, factors that determine oxygenase vs oxidase reactivity were evaluated. In general, sulfur oxygenation is thermodynamically favored and has a lower barrier for reactivity. However, in IPNS, second-sphere residues in the active site pocket constrain the substrate, raising the barrier for sulfur oxygenation relative to oxidation via H atom abstraction.

Non-heme High-Spin {FeNO}6-8 Complexes: One Ligand Platform Can Do It All

Heme and non-heme iron-nitrosyl complexes are important intermediates in biology. While there are numerous examples of low-spin heme iron-nitrosyl complexes in different oxidation states, much less is known about high-spin (hs) non-heme iron-nitrosyls in oxidation states other than the formally ferrous NO adducts ({FeNO}⁷ in the Enemark-Feltham notation). In this study, we present a complete series of hs-{FeNO}^6-8 complexes using the TMG₃tren coligand. Redox transformations from the hs-{FeNO}⁷ complex [Fe(TMG₃tren)(NO)]²⁺ to its {FeNO}⁶ and {FeNO}⁸ analogs do not alter the coordination environment of the iron center, allowing for detailed comparisons between these species. Here, we present new MCD, NRVS, XANES/EXAFS, and Mössbauer data, demonstrating that these redox transformations are metal based, which allows us to access hs-Fe(II)-NO^-, Fe(III)-NO^-, and Fe(IV)-NO^- complexes. Vibrational data, analyzed by NCA, directly quantify changes in Fe-NO bonding along this series. Optical data allow for the identification of a "spectator" charge-transfer transition that, together with Mössbauer and XAS data, directly monitors the electronic changes of the Fe center. Using EXAFS, we are also able to provide structural data for all complexes. The magnetic properties of the complexes are further analyzed (from magnetic Mössbauer). The properties of our hs-{FeNO}^6-8 complexes are then contrasted to corresponding, low-spin iron-nitrosyl complexes where redox transformations are generally NO centered. The hs-{FeNO}⁸ complex can further be protonated by weak acids, and the product of this reaction is characterized. Taken together, these results provide unprecedented insight into the properties of biologically relevant non-heme iron-nitrosyl complexes in three relevant oxidation states.

Development of a Rubredoxin-Type Center Embedded in a de Dovo-Designed Three-Helix Bundle

Protein design is a powerful tool for interrogating the basic requirements for the function of a metal site in a way that allows for the selective incorporation of elements that are important for function. Rubredoxins are small electron transfer proteins with a reduction potential centered near 0 mV (vs normal hydrogen electrode). All previous attempts to design a rubredoxin site have focused on incorporating the canonical CXXC motifs in addition to reproducing the peptide fold or using flexible loop regions to define the morphology of the site. We have produced a rubredoxin site in an utterly different fold, a three-helix bundle. The spectra of this construct mimic the ultraviolet-visible, Mössbauer, electron paramagnetic resonance, and magnetic circular dichroism spectra of native rubredoxin. Furthermore, the measured reduction potential suggests that this rubredoxin analogue could function similarly. Thus, we have shown that an α-helical scaffold sustains a rubredoxin site that can cycle with the desired potential between the Fe(II) and Fe(III) states and reproduces the spectroscopic characteristics of this electron transport protein without requiring the classic rubredoxin protein fold.

Variable-temperature variable-field magnetic circular photoluminescence (VTVH-MCPL) spectroscopy for electronic-structure determination in nanoscale chemical systems

In this Letter, we describe variable-temperature variable-field magnetic circular photoluminescence (VTVH-MCPL) spectroscopy as a complementary technique to absorption-based magnetic circular dichroism. A paramagnetic model system, Au₂₅(SC₈H₉)₁₈, is chosen to demonstrate the information content that is obtained from VTVH-MCPL. Specifically, the methods and analyses for the determination of electronic Landé g-factors, zero-field energy splittings, and relative A-, B-, and C-term contributions to the MCPL response are detailed. The determination of these system properties from photoluminescence data suggests the feasibility of point-source-based super-resolution magneto-optical microscopy.

A Restricted Open Configuration Interaction with Singles Method To Calculate Valence-to-Core Resonant X-ray Emission Spectra: A Case Study

In this work, a new protocol for the calculation of valence-to-core resonant X-ray emission (VtC RXES) spectra is introduced. The approach is based on the previously developed restricted open configuration interaction with singles (ROCIS) method and its parametrized version, based on a ground-state Kohn-Sham determinant (DFT/ROCIS) method. The ROCIS approach has the following features: (1) In the first step approximation, many-particle eigenstates are calculated in which the total spin is retained as a good quantum number. (2) The ground state with total spin S and excited states with spin S' = S, S ± 1, are obtained. (3) These states have a qualitatively correct multiplet structure. (4) Quasi-degenerate perturbation theory is used to treat the spin-orbit coupling operator variationally at the many-particle level. (5) Transition moments are obtained between the relativistic many-particle states. The method has shown great potential in the field of X-ray spectroscopy, in particular in the field of transition-metal L-edge, which cannot be described correctly with particle-hole theories. In this work, the method is extended to the calculation of resonant VtC RXES [alternatively referred to as 1s-VtC resonant inelastic X-ray scattering (RIXS)] spectra. The complete Kramers-Dirac-Heisenerg equation is taken into account. Thus, state interference effects are treated naturally within this protocol. As a first application of this protocol, a computational study on the previously reported VtC RXES plane on a molecular managanese(V) complex is performed. Starting from conventional X-ray absorption spectra (XAS), we present a systematic study that involves calculations and electronic structure analysis of both the XAS and non-resonant and resonant VtC XES spectra. The very good agreement between theory and experiment, observed in all cases, allows us to unravel the complicated intensity mechanism of these spectroscopic techniques as a synergic function of state polarization and interference effects. In general, intense features in the RIXS spectra originate from absorption and emission processes that involve nonorthogonal transition moments. We also present a graphical method to determine the sign of the interference contributions.