Free radical catalysis by galactose oxidase

Formation Mechanism of Cofactor Cys-Tyr in the Cysteine Dioxygenases (CDO and F2-CDO) and Its Influence on Catalysis: A QM/MM Study

Cysteine dioxygenase (CDO) is a nonheme mononuclear iron enzyme, which catalyzes the oxidation of cysteine to cysteine sulfinic acid. Crystal structure studies of mammalian CDO showed that there is a cross-linked cysteine-tyrosine (Cys-Tyr) cofactor in its active site. Moreover, the formation of the Cys-Tyr cofactor requires the metal cofactor (Fe²⁺) and O₂, and it was previously considered to substantially enhance the catalytic efficiency and half-life of CDO. Recently, a pure human CDO (F₂-CDO) without including the Cys-Tyr cofactor was crystalized by the site-directed mutagenesis approach in the anaerobic condition. In this work, to gain insights into the formation mechanism of the Cys-Tyr cofactor and whether it can really promote the catalytic reactivity of CDO, a series of computational models have been constructed, and quantum mechanical/molecular mechanical (QM/MM) calculations have been performed. Our calculation results reveal that WT-CDO and F₂-CDO follow different mechanisms for the formation of the Cys-Tyr cofactor. In F₂-CDO, the cofactor formation contains the H-abstraction, C-S bond formation, intramolecular F migration, and aromatization of the residue F₂Y157, in which the Fe-coordinate dioxygen can be recovered after the formation cofactor; however, in the WT-CDO, the cofactor formation shows some differences. During the reaction, hydrogen peroxide is generated, and the final aromatization requires the assistance of one water molecule. Furthermore, the overall barriers of cofactor formation are always higher than l-cysteine oxidation for both WT-CDO and F₂-CDO irrespective of the absence or presence of the cofactor. Thus, we can theoretically confirm that the Cys-Tyr cofactor is not essential for the oxidation activity of CDO, and cofactor formation is just an accompanying reaction but not a prerequisite for the oxidation reaction. These results may provide useful information for understanding the catalysis of CDO.

Substituent Effects in Carbon-Nanotube-Supported Copper Phenolato Complexes for Oxygen Reduction Reaction

Unprotected mononuclear pyrene-modified (bispyridylaminomethyl)methylphenol copper complexes were designed to be immobilized at multiwalled carbon nanotube (MWCNT) electrodes and form dinuclear bis(μ-phenolato) complexes on the surface. These complexes exhibit a high oxygen reduction reaction activity of 12.7 mA cm^-2 and an onset potential of 0.78 V versus reversible hydrogen electrode. The higher activity of these complexes compared to that of mononuclear complexes with bulkier groups is induced by the favorable early formation of a dinuclear catalytic species on MWCNT.

Framework of the kinetic analysis of O2-dependent oxidative biocatalysts for reaction intensification

A framework for kinetic modelling and evaluation of the reaction intensification of O2-dependent enzyme catalyzed reactions is built from measurements of consumption rates of the initially dissolved O2 in homogeneous liquid phase.

Biocatalytic oxidation of alcohols using galactose oxidase and a manganese(iii) activator for the synthesis of islatravir

Manganese(iii) acetate activates galactose oxidase (GOase), a Cu-dependent metalloenzyme that catalyzes the oxidation of alcohols to aldehydes.

Kβ X-ray Emission Spectroscopy as a Probe of Cu(I) Sites: Application to the Cu(I) Site in Preprocessed Galactose Oxidase

Cu(I) active sites in metalloproteins are involved in O₂ activation, but their O₂ reactivity is difficult to study due to the Cu(I) d¹⁰ closed shell which precludes the use of conventional spectroscopic methods. Kβ X-ray emission spectroscopy (XES) is a promising technique for investigating Cu(I) sites as it detects photons emitted by electronic transitions from occupied orbitals. Here, we demonstrate the utility of Kβ XES in probing Cu(I) sites in model complexes and a metalloprotein. Using Cu(I)Cl, emission features from double-ionization (DI) states are identified using varying incident X-ray photon energies, and a reasonable method to correct the data to remove DI contributions is presented. Kβ XES spectra of Cu(I) model complexes, having biologically relevant N/S ligands and different coordination numbers, are compared and analyzed, with the aid of density functional theory (DFT) calculations, to evaluate the sensitivity of the spectral features to the ligand environment. While the low-energy Kβ_2,5 emission feature reflects the ionization energy of ligand np valence orbitals, the high-energy Kβ_2,5 emission feature corresponds to transitions from molecular orbitals (MOs) having mainly Cu 3d character with the intensities determined by ligand-mediated d-p mixing. A Kβ XES spectrum of the Cu(I) site in preprocessed galactose oxidase (GO_pre) supports the 1Tyr/2His structural model that was determined by our previous X-ray absorption spectroscopy and DFT study. The high-energy Kβ_2,5 emission feature in the Cu(I)-GO_pre data has information about the MO containing mostly Cu 3d_x²-y² character that is the frontier molecular orbital (FMO) for O₂ activation, which shows the potential of Kβ XES in probing the Cu(I) FMO associated with small-molecule activation in metalloproteins.

Formation of Monofluorinated Radical Cofactor in Galactose Oxidase through Copper-Mediated C-F Bond Scission

Galactose oxidase (GAO) contains a Cu(II)-ligand radical cofactor. The cofactor, which is autocatalytically generated through the oxidation of the copper, consists of a cysteine-tyrosine radical (Cys-Tyr^•) as a copper ligand. The formation of the cross-linked thioether bond is accompanied by a C-H bond scission on Tyr272 with few details known thus far. Here, we report the genetic incorporation of 3,5-dichlorotyrosine (Cl₂-Tyr) and 3,5-difluorotyrosine (F₂-Tyr) to replace Tyr272 in the GAO^V previously optimized for expression through directed evolution. The proteins with an unnatural tyrosine residue are catalytically competent. We determined the high-resolution crystal structures of the GAO^V, Cl₂-Tyr272, and F₂-Tyr272 incorporated variants at 1.48, 1.23, and 1.80 Å resolution, respectively. The structural data showed only one halogen remained in the cofactor, indicating that an oxidative carbon-chlorine/fluorine bond scission has occurred during the autocatalytic process of cofactor biogenesis. Using hydroxyurea as a radical scavenger, the spin-coupled hidden Cu(II) was observed by EPR spectroscopy. Thus, the structurally defined catalytic center with genetic unnatural tyrosine substitution is in the radical containing form as in the wild-type, i.e., Cu(II)-(Cl-Tyr^•-Cys) or Cu(II)-(F-Tyr^•-Cys). These findings illustrate a previously unobserved C-F/C-Cl bond cleavage in biology mediated by a mononuclear copper center.

Free radicals for cancer theranostics

Free radicals were generally regarded as highly reactive, transient and harmful species. In fact, some of the free radicals can also be inactive, long-lived and beneficial for our health. These properties of free radicals provide future possibilities for their application in various fields. Owning to their open-shell electronic structure, free radicals exhibit unique advantages in biomedical applications, such as high reactivity, photoacoustic and photothermal conversion ability, molecular magnetic. In this review, recent progress on free radicals and their applications in cancer theranostics are presented. Typical materials that exhibit controlled generation of free radicals and their applications for photodynamic therapy (PDT), chemodynamic therapy (CDT), sonodynamic therapy (SDT), gas therapy, hypoxic cancer treatment, photothermal therapy (PTT), photoacoustic imaging (PAI) and magnetic resonance imaging (MRI) are summarized and discussed.

Distorted copper(ii) radicals with sterically hindered salens: electronic structure and aerobic oxidation of alcohols

The sterically hindered salen ligands featuring biphenyl and tetramethyl putrescine linkers were synthesized and chelated to copper. The resulting complexes CuL^bp,tBu, CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe were structurally characterized, showing a significanty tetrahedrally distorted metal center. The complexes show two reversible oxidation waves in the range 0.2 to 0.8 V vs. Fc⁺/Fc. A further reduction wave is detected in the range -1.4 to -1.7 V vs. Fc⁺/Fc. It is reversible for CuL^bp,tBu and CuL^bp,OMe and assigned to the Cu^II/Cu^I redox couple. One-electron oxidation of CuL^bp,OMe, CuL^pu,tBu and CuL^pu,OMe was performed chemically and electrochemically. It is accompanied by a quenching of the EPR resonances. Phenoxyl radical formation was established by X-Ray diffraction on the cations [CuL^bp,OMe]⁺ and [CuL^pu,OMe]⁺, whereby the coordination sphere is elongated upon oxidation with quinoidal distributions of bond distances. The cations exhibit a NIR band of moderate intensity in their optical spectrum, supporting their classification as class II mixed-valent radical species according to the Robin Day classification. The proposed electronic structures are supported by DFT calculations. The cations [CuL^bp,OMe]⁺, [CuL^pu,tBu]⁺ and [CuL^pu,OMe]⁺ were active towards aerobic oxidation of the unactivated alcohol 2-phenylethanol, with TON numbers up to 58 within 3 h.

Assigning Ligand Redox Levels in Complexes of 2-Aminophenolates: Structural Signatures

The purpose of this Viewpoint is to provide a broad-ranging update of advances in the coordination chemistry of redox-active (noninnocent) 2-aminophenolates, with emphasis on two ligand backbone structural parameters, the average of C-O and C-N (C-O/N) bond distances and Δa values, signifying the degree of bond-length alternation in the six-membered ring, in order to identify the redox level of the coordinated ligands. In the absence of magnetic, spectroscopic, and redox results, it has been established that it is possible to assign the electronic ground state of a coordination complex of 2-aminophenolates with consideration of charge, metal-ligand bond distances, and two very promising ligand backbone structural parameters. From a closer look at the sensitive ligand backbone metrical parameters of a diversified group of about 120 transition-metal complexes, a few very useful generalizations have been made.

Biocatalytic Oxidation of Alcohols

Enzymatic methods for the oxidation of alcohols are critically reviewed. Dehydrogenases and oxidases are the most prominent biocatalysts, enabling the selective oxidation of primary alcohols into aldehydes or acids. In the case of secondary alcohols, region and/or enantioselective oxidation is possible. In this contribution, we outline the current state-of-the-art and discuss current limitations and promising solutions.

Nickel(II), Copper(II) and Palladium(II) Complexes with Bis-Semicarbazide Hexaazamacrocycles: Redox-Noninnocent Behavior and Catalytic Activity in Oxidation and C-C Coupling Reactions

Nickel(II), copper(II), and palladium(II) complexes ML^H, where M = Ni (1), Cu (2), Pd (3), and ML^OMe, where M = Ni (4), Cu (5), Pd (6), have been prepared by reactions of NiCl₂·6H₂O, Cu(OAc)₂·H₂O, and PdCl₂(MeCN)₂ with 14-membered bis-semicarbazide hexaazamacrocycles H₂L^H and H₂L^OMe in dimethylformamide (DMF). The compounds were characterized by elemental analysis, ESI mass spectrometry, IR, UV-vis, and 1D (¹H, ¹³C) and 2D (¹H-¹H COSY, ¹H-¹H TOCSY, ¹H-¹H NOESY, ¹H-¹³C HSQC, ¹H-¹³C HMBC) NMR spectra (1, 3, 4, and 6), and X-ray diffraction (2, 4-6). The complexes with M^IIN₄ coordination environment have S = 0, 1/2, 0 ground states for Ni, Cu, and Pd, respectively. The electrochemical behavior of 1-6 was investigated in detail. The electronic structures of 1e-oxidized species were studied by EPR, UV-vis-NIR spectroelectrochemistry, and DFT calculations, indicating the redox-noninnocent behavior of the ligands. Compounds 1-6 were tested in the oxidation of styrene and C-C coupling (Henry and Knoevenagel condensations). Compounds 2 and 5 selectively catalyze the microwave-assisted oxidation of neat styrene to benzaldehyde (up to 88% yield), whereas the 1 and 4 catalytic systems afforded up to 99% β-nitroethanol yield with an appreciable diastereoselectivity toward the formation of the anti isomer.

Structure, Spectroscopy, and Reactivity of a Mononuclear Copper Hydroxide Complex in Three Molecular Oxidation States

Structural, spectroscopic, and reactivity studies are presented for an electron transfer series of copper hydroxide complexes supported by a tridentate redox-active ligand. Single crystal X-ray crystallography shows that the mononuclear [CuOH]¹⁺ core is stabilized via intramolecular H-bonds between the H-donors of the ligand and the hydroxide anion when the ligand is in its trianionic form. This complex undergoes two reversible oxidation processes that produce two metastable "high-valent" CuOH species, which can be generated by addition of stoichiometric amounts of 1e^- oxidants. These CuOH species are characterized by an array of spectroscopic techniques including UV-vis absorption, electron paramagnetic resonance (EPR), and X-ray absorption spectroscopies (XAS), which together indicate that all redox couples are ligand-localized. The reactivity of the complexes in their higher oxidation states toward substrates with modest O-H bond dissociation energies (e.g., 4-substitued-2,6-di-tert-butylphenols) indicates that these complexes act as 2H⁺/2e^- oxidants, differing from the 1H⁺/1e^- reactivity of well-studied [CuOH]²⁺ systems.

Recent Advances in One-Electron-Oxidized CuII -Diphenoxide Complexes as Models of Galactose Oxidase: Importance of the Structural Flexibility in the Active Site

The phenoxyl radical plays important roles in biological systems as cofactors in some metalloenzymes, such as galactose oxidase (GO) catalyzing oxidation of primary alcohols to give the corresponding aldehydes. Many metal(II)-phenoxyl radical complexes have hitherto been studied for understanding the detailed properties and reactivities of GO, and thus the nature of GO has gradually become clearer. However, the effects of the subtle geometric and electronic structural changes at the active site of GO, especially the structural change in the catalytic cycle and the effect of the second coordination sphere, have not been fully discussed yet. In this Review, we focus on further details of the model studies of GO and discuss the importance of the structural change at the active site of GO.

Multivariate Analysis of Coupled Operando EPR/XANES/EXAFS/UV-Vis/ATR-IR Spectroscopy: A New Dimension for Mechanistic Studies of Catalytic Gas-Liquid Phase Reactions

Operando EPR, XANES/EXAFS, UV‐Vis and ATR‐IR spectroscopic methods have been coupled for the first time in the same experimental setup for investigation of unclear mechanistic aspects of selective aerobic oxidation of benzyl alcohol by a Cu/TEMPO catalytic system (TEMPO=2,2,6,6‐tetramethylpiperidinyloxyl). By multivariate curve resolution with alternating least‐squares fitting (MCR‐ALS) of simultaneously recorded XAS and UV‐Vis data sets, it was found that an initially formed (bpy)(NMI)Cu^I‐ complex (bpy=2,2′‐bipyridine, NMI=N‐methylimidazole ) is converted to two different Cu^II species, a mononuclear (bpy)(NMI)(CH₃CN)Cu^II‐OOH species detectable by EPR and ESI‐MS, and an EPR‐silent dinuclear (CH₃CN)(bpy)(NMI)Cu^II(μ‐OH)₂⋅Cu^II (bpy)(NMI) complex. The latter is cleaved in the further course of reaction into (bpy)(NMI)(HOO)Cu^II‐TEMPO monomers that are also EPR‐silent due to dipolar interaction with bound TEMPO. Both Cu monomers and the Cu dimer are catalytically active in the initial phase of the reaction, yet the dimer is definitely not a major active species nor a resting state since it is irreversibly cleaved in the course of the reaction while catalytic activity is maintained. Gradual formation of non‐reducible Cu^II leads to slight deactivation at extended reaction times.

Harnessing the active site triad: merging hemilability, proton responsivity, and ligand-based redox-activity

Metalloenzymes catalyze many important reactions by managing the proton and electron flux at the enzyme active site. The motifs utilized to facilitate these transformations include hemilabile, redox-active, and so called proton responsive sites. Given the importance of incorporating and understanding these motifs in the area of coordination chemistry and catalysis, we highlight recent milestones in the field. Work incorporating the triad of hemilability, redox-activity, and proton responsivity into single ligand scaffolds will be described.

A four-parameter system for rationalising the electronic properties of transition metal–radical ligand complexes

A heuristic four-parameter scheme captures and predicts the electronic properties of radical-ligand transition metal compounds, overcoming ligand specific descriptions.

Tuning of the redox potential and catalytic activity of a new Cu(ii) complex byo-iminobenzosemiquinone as an electron-reservoir ligand

The synthesis and characterization of a new Cu(ii) complex, LNIS2Cu^II(L^NIS=o-iminobenzosemiquinone), are reported.

A biradical oxo-molybdenum complex containing semiquinone and o-aminophenol benzoxazole-based ligands

We report a new mononuclear molybdenum(iv) complex, MoOL^BISL^SQ, in which L^SQ (2,4-di-tert-butyl o-semibenzoquinone ligand) has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, L^BIS, and MoO₂(acac)₂. The complex was characterized by X-ray crystallography, elemental analysis, IR and UV-vis spectroscopy and magnetic susceptibility measurements. The crystal structure of MoOL^BISL^SQ revealed a distorted octahedral geometry around the metal centre, surrounded by one O and two N atoms of L^BIS and two O atoms of L^SQ. The effective magnetic moment (μ_eff) of MoOL^BISL^SQ decreased from 2.36 to 0.2 μ_B in the temperature range of 290 to 2 K, indicating a singlet ground state caused by antiferromagnetic coupling between the metal and ligand centred unpaired electrons. Also, the latter led to the EPR silence of the complex. Cyclic voltammetry (CV) studies indicate both ligand and metal-centered redox processes. MoOL^BISL^SQ was applied as a catalyst for the oxidative cleavage of cyclohexene to adipic acid and selective oxidation of sulfides to sulfones with aqueous hydrogen peroxide.

Biradical molybdenum(iv) complex, MoOL^BISL^SQ, has been prepared from the reaction of the o-iminosemibenzoquinone form of a tridentate non-innocent benzoxazole ligand, L^BIS, and MoO₂(acac)₂ and used as catalyst in oxidation reaction..

Determination of biocatalytic parameters of a copper radical oxidase using real-time reaction progress monitoring

VTNA is applied to reaction progress curves to glean key kinetic and mechanistic details for a copper radical oxidase.

Hydroxo Iron(III) Sites in a Metal-Organic Framework: Proton-Coupled Electron Transfer and Catalytic Oxidation of Alcohol with Molecular Oxygen

Metalloenzymes are powerful biocatalysts that can catalyze particular chemical reactions with high activity, selectivity, and specificity under mild conditions. Metal-organic frameworks (MOFs) composed of metal ions or metal clusters and organic ligands with defined cavities have the potential to impart enzyme-like catalytic activity and mimic metalloenzymes. Here, a new metal-organic framework implanted with hydroxo iron(III) sites with the structural and reactivity characteristics of iron-containing lipoxygenases is reported. Similar to lipoxygenases, the hydrogen atoms and electrons of the substrate can transfer to the hydroxo iron(III) sites, showing typical proton-coupled electron transfer behavior. In the reactivity mimicking biology system, similar to alcohol oxidase, the material also catalyses the oxidation of alcohol into aldehyde by using O₂ with a high yield and 100% selectivity under mild conditions, without the use of a radical cocatalyst or photoexcitation. These results provide strong evidence for the high structural fidelity of enzymatically active sites in MOF materials, verifying that MOFs provide an ideal platform for designing biomimetic heterogeneous catalysts with high conversion efficiency and product selectivity.

Formation of the CuII -Phenoxyl Radical by Reaction of O2 with a CuII -Phenolate Complex via the CuI -Phenoxyl Radical

Reaction of Cu(ClO₄ )₂ ⋅6 H₂ O with a tripodal 2N2O ligand, H₂ Me₂ NL, having a p-(dimethylamino)phenol moiety, in CH₂ Cl₂ /MeOH (1:1 v/v) under basic conditions under an inert gas atmosphere gave [Cu(Me₂ NL)(H₂ O)] (1). The same reaction carried out under aerobic conditions gave [Cu(Me₂ NL)(MeOH)]ClO₄ (2), which could be obtained also from the isolated complex 1 by reaction with O₂ in CH₂ Cl₂ /MeOH. The X-ray crystal structures of 1 and 2 revealed similar square-pyramidal structures, but 2 showed the (dimethylamino)phenoxyl radical features. Complex 1 exhibits characteristic Cu^II EPR signals of the d x 2 - y 2 ground state in CH₂ Cl₂ /MeOH at 77 K, whereas 2 is EPR-silent. The EPR and X-ray absorption fine structure (XAFS) results suggest that 2 is assigned to the Cu^II -(dimethylamino)phenoxyl radical. However, complex 1 showed different features in the absence of MeOH. The EPR spectrum of the CH₂ Cl₂ solution of 1 exhibits distortion from the d x 2 - y 2 ground state and a temperature-dependent equilibrium between the Cu^II -(dimethylamino)phenolate and the Cu^I -(dimethylamino)phenoxyl radical. From these results, Cu^II -phenoxyl radical complex 2 is concluded to be formed by the reaction of 1 with O₂ via the Cu^I -phenoxyl radical species.

Theory Demonstrated a "Coupled" Mechanism for O2 Activation and Substrate Hydroxylation by Binuclear Copper Monooxygenases

Multiscale simulations have been performed to address the longstanding issue of "dioxygen activation" by the binuclear copper monooxygenases (PHM and DβM), which have been traditionally classified as "noncoupled" binuclear copper enzymes. Our QM/MM calculations rule out that Cu_M(II)-O₂^• is an active species for H-abstraction from the substrate. In contrast, Cu_M(II)-O₂^• would abstract an H atom from the cosubstrate ascorbate to form a Cu_M(II)-OOH intermediate in PHM and DβM. Consistent with the recently reported structural features of DβM, the umbrella sampling shows that the "open" conformation of the Cu_M(II)-OOH intermediate could readily transform into the "closed" conformation in PHM, in which we located a mixed-valent μ-hydroperoxodicopper(I,II) intermediate, (μ-OOH)Cu(I)Cu(II). The subsequent O-O cleavage and OH moiety migration to Cu_H generate the unexpected species (μ-O^•)(μ-OH)Cu(II)Cu(II), which is revealed to be the reactive intermediate responsible for substrate hydroxylation. We also demonstrate that the flexible Met ligand is favorable for O-O cleavage reactions, while the replacement of Met with the strongly bound His ligand would inhibit the O-O cleavage reactivity. As such, the study not only demonstrates a "coupled" mechanism for O₂ activation by binuclear copper monooxygenases but also deciphers the full catalytic cycle of PHM and DβM in accord with the available experimental data. These findings of O₂ activation and substrate hydroxylation by binuclear copper monooxygenases could expand our understanding of the reactivities of the synthetic monocopper complexes.

His-Cys and Trp-Cys cross-links generated by post-translational chemical modification

Galactose oxidase and amine oxidase contain a cofactor which is generated by post-translational chemical modification to the corresponding amino acid side chains near the copper active center. Such cofactors provide proteins unusual catalytic ability that canonical amino acids cannot exert as well as their structural stability, and thereby are called as protein-derived cofactors. These cofactors and modifications are mostly derived from aromatic amino acid residues, especially Tyr, Trp, and His. Current information about unusual cofactors derived from two of those, heteroaromatic residues (Trp and His) is summarized, especially chemical properties and maturation process of the cross-links between cysteine and heteroaromatic amino acids (His-Cys and Trp-Cys cross-links).Abbreviations: FMN: flavin mononucleotide; FAD: flavin adenine nucleotide; RNA: ribonucleic acid; PDC: protein-derived cofactor; GFP: green fluorescent protein; MIO: 3,5-dihydro-5-methylidene-4-imidazol-4-one; LTQ: lysyl tyrosylquinone; CTQ: cysteine tryptophylquinone; TTQ: tryptophan tryptophylquinone; E.coli: Escherichia coli; WT: wild type.

Enzyme-catalyzed C–F bond formation and cleavage

Organofluorines are widely used in a variety of applications, ranging from pharmaceuticals to pesticides and advanced materials. The widespread use of organofluorines also leads to its accumulation in the environment, and two major questions arise: how to synthesize and how to degrade this type of compound effectively? In contrast to a considerable number of easy-access chemical methods, milder and more effective enzymatic methods remain to be developed. In this review, we present recent progress on enzyme-catalyzed C–F bond formation and cleavage, focused on describing C–F bond formation enabled by fluorinase and C–F bond cleavage catalyzed by oxidase, reductase, deaminase, and dehalogenase.

The effect of π-π stacking interaction of the indole ring with the coordinated phenoxyl radical in a nickel(ii)-salen type complex. Comparison with the corresponding Cu(ii) complex

In order to gain new insights into the effect of the π-π stacking interaction of the indole ring with the phenoxyl radical moiety as seen in the active form of galactose oxidase, we have prepared Ni(ii) complexes of a methoxy substituted salen-type ligand containing a pendent indole ring on the dinitrogen chelate backbone and characterized their one-electron oxidized forms. The X-ray crystal structure analysis and the other physicochemical experiments of the Ni(ii) complex revealed no significant intramolecular interaction of the indole ring with the coordination plane. On the other hand, the X-ray crystal structures of the oxidized Ni(ii) complex exhibited the π-π stacking interaction of the indole ring mainly with one of the two phenolate moieties. While the phenoxyl radical electron was delocalized on the two phenolate moieties in the Ni(ii)-salen coordination plane, the phenolate moiety in close contact with the indole moiety was considered to be the initial oxidation locus, indicating that the indole ring interacted with the phenoxyl radical by π-π stacking. The UV-vis-NIR spectrum of the oxidized Ni(ii) complex with the pendent indole ring was different from that of the complex without the side chain indole ring, but the differences were rather small in comparison with the oxidized Cu(ii)-salen complexes with the π-π stacking interaction of the indole ring. Such differences are due to the electronic structure difference, the localized radical electron on one of the phenolate moieties in the oxidized Cu(ii) complexes being more favorable for the π-π stacking interaction.

Stable M(II)-Radicals and Nickel(III) Complexes of a Bis(phenol) N-Heterocyclic Carbene Chelated to Group 10 Metal Ions

The tetradentate ligand based on (1-imidazolium-3,5-di tert-butylphenol) units was prepared and chelated to group 10 metal ions (Ni(II), Pd(II), and Pt(II)), affording complexes 1, 2, and 3, respectively. The X-ray crystal structures of 1-3 show a square planar metal ion coordinated to two N-heterocyclic carbenes and two phenolate moieties. The cyclic voltammetry curves of complexes 1-3 show two reversible oxidation waves in the range 0.11-0.21 V ( E_1/2¹) and 0.55-0.65 V ( E_1/2²) vs Fc⁺/Fc, which are assigned to the successive oxidations of the phenolate moieties. One-electron oxidation affords mononuclear ( S = 1/2) systems. Complex 1⁺·SbF₆^- was remarkably stable, and its structure was characterized. The coordination sphere is slightly dissymmetric, while the typical patterns of phenoxyl radicals were observed within the ligand framework. Complex 1⁺ exhibits a rhombic signal at g = 2.087, 2.016, and 1.992, confirming its predominant phenoxyl radical character. The g-values are slightly smaller for 2⁺ (2.021, 2.008, and 1.983) and larger for 3⁺ (2.140, 1.999, and 1.885) yet consistent with phenoxyl radical species. The electronic spectra of 1⁺-3⁺ display an intervalence charge-transfer (IVCT) transition at 2396, 2600, and 2294 nm, respectively. Its intensity supports the description of cations 1⁺ and 3⁺ as mixed-valent (Class II/III) compounds according to the Robin Day classification. Complex 2⁺ behaves as a mixed-valent class II radical compound. In the presence of pyridine, radical species 1⁺ is successively converted into stable mono and bis(adducts), which are both Ni(III) complexes. Dications 1⁺²-3⁺² were prepared electrochemically. They are electron paramagnetic resonance (EPR)-silent and do not show IVCT transition in their NIR spectra, consistent with a bis(radical) formulation. The proposed electronic structures are fully supported by density functional theory calculations.

π-π Stacking Interaction in an Oxidized CuII -Salen Complex with a Side-Chain Indole Ring: An Approach to the Function of the Tryptophan in the Active Site of Galactose Oxidase

In order to gain new insights into the effect of the π-π stacking interaction of the indole ring with the Cu^II -phenoxyl radical as seen in the active form of galactose oxidase, we have prepared a Cu^II complex of a methoxy-substituted salen-type ligand, containing a pendent indole ring on the dinitrogen chelate backbone, and characterized its one-electron-oxidized forms. The X-ray crystal structures of the oxidized Cu^II complex exhibited the π-π stacking interaction of the indole ring mainly with one of the two phenolate moieties. The phenolate moiety in close contact with the indole moiety showed the characteristic phenoxyl radical structural features, indicating that the indole ring favors the π-π stacking interaction with the phenoxyl radical. The UV/Vis/NIR spectra of the oxidized Cu^II complex with the pendent indole ring was significantly different from those of the complex without the side-chain indole ring, and the absorption and CD spectra exhibited a solvent dependence, which is in line with the phenoxyl radical-indole stacking interaction in solution. The other physicochemical results and theoretical calculations strongly support that the indole ring, as an electron donor, stabilizes the phenoxyl radical by the π-π stacking interaction.

Biomimetic Cu/Nitroxyl Catalyst Systems for Selective Alcohol Oxidation

The oxidation of alcohols to the corresponding carbonyl products is an important organic transformation and the products are used in a variety of applications. The development of catalytic methods for selective alcohol oxidation have garnered significant attention in an attempt to find a more sustainable method without any limitations. Copper, in combination with 2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO) and supported by organic ligands, have emerged as the most effective catalysts for selective alcohol oxidation and these catalyst systems are frequently compared to galactose oxidase (GOase). The efficiency of GOase has led to extensive research to mimic the active sites of these enzymes, leading to a variety of Cu/TEMPO· catalyst systems being reported over the years. The mechanistic pathway by which Cu/TEMPO· catalyst systems operate has been investigated by several research groups, which led to partially contradicting mechanistic description. Due to the disadvantages and limitations of employing TEMPO· as co-catalyst, alternative nitroxyl radicals or in situ formed radicals, as co-catalysts, have been successfully evaluated in alcohol oxidation. Herein we discuss the development and mechanistic elucidation of Cu/TEMPO· catalyst systems as biomimetic alcohol oxidation catalysts.

Probing the Cys-Tyr Cofactor Biogenesis in Cysteine Dioxygenase by the Genetic Incorporation of Fluorotyrosine

Cysteine dioxygenase (CDO) is a nonheme iron enzyme that adds two oxygen atoms from dioxygen to the sulfur atom of l-cysteine. Adjacent to the iron site of mammalian CDO, there is a post-translationally generated Cys-Tyr cofactor, whose presence substantially enhances the oxygenase activity. The formation of the Cys-Tyr cofactor in CDO is an autocatalytic process, and it is challenging to study by traditional techniques because the cross-linking reaction is a side, uncoupled, single-turnover oxidation buried among multiple turnovers of l-cysteine oxygenation. Here, we take advantage of our recent success in obtaining a purely uncross-linked human CDO due to site-specific incorporation of 3,5-difluoro-l-tyrosine (F₂-Tyr) at the cross-linking site through the genetic code expansion strategy. Using EPR spectroscopy, we show that nitric oxide (^•NO), an oxygen surrogate, similarly binds to uncross-linked F₂-Tyr157 CDO as in wild-type human CDO. We determined X-ray crystal structures of uncross-linked F₂-Tyr157 CDO and mature wild-type CDO in complex with both l-cysteine and ^•NO. These structural data reveal that the active site cysteine (Cys93 in the human enzyme), rather than the generally expected tyrosine (i.e., Tyr157), is well-aligned to be oxidized should the normal oxidation reaction uncouple. This structure-based understanding is further supported by a computational study with models built on the uncross-linked ternary complex structure. Together, these results strongly suggest that the first target to oxidize during the iron-assisted Cys-Tyr cofactor biogenesis is Cys93. Based on these data, a plausible reaction mechanism implementing a cysteine radical involved in the cross-link formation is proposed.