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ABSTRACT: A new catalyst hydroxylates phenols with O2 via a stable side-on peroxide complex, which is similar to the active site of tyrosinase in terms of the ligand environment and its spectroscopic properties. The catalytic oxidation of phenols to quinones proceeds at room temperature in the presence of NEt3 and even non-native substrates can be oxidized catalytically. The reaction mechanism is analogous to that of the enzyme-catalyzed reaction.
Angewandte Chemie International Edition 04/2013; · 13.45 Impact Factor
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ABSTRACT: The electronic structure of a doubly oxidized Ni salen complex NiSal(tBu) (Sal(tBu) = N,N'-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) has been investigated by both experimental and theoretical methods. The doubly oxidized product was probed by resonance Raman spectroscopy, UV-vis-NIR, and EPR to determine the locus of oxidation as well as the spectroscopic signature of the complex. It was determined that double oxidation of NiSal(tBu) affords a bis-ligand radical species in solution via the presence of phenoxyl radical bands at ν(7a) (1504 cm(-1)) and ν(8a) (1579 cm(-1)) in the Raman spectrum, and the loss of the intense NIR transition reported for the mono-radical complex (Angew. Chem., Int. Ed., 2007, 46, 5198). Spectroscopic experiments, complemented by DFT calculations, show that the two radical spins are predominantly localized on the phenolate moieties, in opposition to the extensive delocalization over the ligand framework observed for the mono-radical analogue.
Dalton Transactions 01/2013; · 3.84 Impact Factor
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ABSTRACT: The electronic structure of a doubly oxidized Ni salen complex NiSaltBu (SaltBu = N,N’-bis(3,5-di-tert-butylsalicylidene)-1,2-cyclohexane-(1R,2R)-diamine) has been investigated by both experimental and theoretical methods. The doubly oxidized product was probed by Raman spectroscopy, UV-vis-NIR, and EPR to determine the locus of oxidation as well as the spectroscopic signature of the complex. It was determined that double oxidation of NiSaltBu affords a bis-ligand radical species in solution via the presence of phenoxyl radical bands at n7a (1504 cm-1) and n8a (1579 cm-1) in the Raman spectrum, and the loss of the intense NIR transition reported for the mono-radical complex (Angew. Chem. Int. Ed. 2007, 46, 5198). Spectroscopic experiments, complemented by DFT calculations, show that the two radical spins are predominantly localized on the phenolate moieties, in opposition to the extensive delocalization over the ligand framework observed for the mono-radical analogue.
Dalton Transactions 01/2013; · 3.84 Impact Factor
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ABSTRACT: Nonsymmetric substitution of salen (1(R(1),R(2))) and reduced salen (2(R(1),R(2))) Cu(II)-phenoxyl complexes with a combination of -(t)Bu, -S(i)Pr, and -OMe substituents leads to dramatic differences in their redox and spectroscopic properties, providing insight into the influence of the cysteine-modified tyrosine cofactor in the enzyme galactose oxidase (GO). Using a modified Marcus-Hush analysis, the oxidized copper complexes are characterized as Class II mixed-valent due to the electronic differentiation between the two substituted phenolates. Sulfur K-edge X-ray absorption spectroscopy (XAS) assesses the degree of radical delocalization onto the single sulfur atom of nonsymmetric [1((t)Bu,SMe)](+) at 7%, consistent with other spectroscopic and electrochemical results that suggest preferential oxidation of the -SMe bearing phenolate. Estimates of the thermodynamic free-energy difference between the two localized states (ΔG(o)) and reorganizational energies (λ(R(1)R(2))) of [1(R(1),R(2))](+) and [2(R(1),R(2))](+) lead to accurate predictions of the spectroscopically observed IVCT transition energies. Application of the modified Marcus-Hush analysis to GO using parameters determined for [2(R(1),R(2))](+) predicts a ν(max) of ∼13600 cm(-1), well within the energy range of the broad Vis-NIR band displayed by the enzyme.
Journal of the American Chemical Society 04/2012; 134(17):7367-77. · 9.91 Impact Factor
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ABSTRACT: The enzyme tyrosinase contains two Cu(I) centres, trigonally coordinated by imidazole nitrogens of six conserved histidine residues. The enzyme activates O(2) to form a µ-η(2):η(2)-peroxo-dicopper(II) core, which hydroxylates tyrosine to a catechol in the first committed step of melanin biosynthesis. Here, we report a family of synthetic peroxo complexes, with spectroscopic and chemical features consistent with those of oxygenated tyrosinase, formed through the self-assembly of monodentate imidazole ligands, Cu(I) and O(2) at -125 °C. An extensively studied complex reproduces the enzymatic electrophilic oxidation of exogenous phenolic substrates to catechols in good stoichiometric yields. The self-assembly and subsequent reactivity support the intrinsic stability of the Cu(2)O(2) core with imidazole ligation, in the absence of a polypeptide framework, and the innate capacity to effect hydroxylation of phenolic substrates. These observations suggest that a foundational role of the protein matrix is to facilitate expression of properties native to the core by bearing the entropic costs of assembly and precluding undesired oxidative degradation pathways.
Nature Chemistry 01/2012; 4(4):317-22. · 20.52 Impact Factor
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ABSTRACT: Integrating sulfanyl substituents into copper-bonded phenoxyls significantly alters their optical and redox properties and provides insight into the influence of cysteine modification of the tyrosine cofactor in the enzyme galactose oxidase. The model complexes [1(SR2)](+) are class II mixed-valent Cu(II)-phenoxyl-phenolate species that exhibit intervalence charge transfer bands and intense visible sulfur-aryl π → π* transitions in the energy range, which provides a greater spectroscopic fidelity to oxidized galactose oxidase than non-sulfur-bearing analogs. The potentials for phenolate-based oxidations of the sulfanyl-substituted 1(SR2) are lower than the alkyl-substituted analogs by up to ca. 150 mV and decrease following the steric trend: -S(t)Bu > -S(i) Pr > -SMe. Density functional theory calculations suggest that reducing the steric demands of the sulfanyl substituent accommodates an in-plane conformation of the alkylsulfanyl group with the aromatic ring, which stabilizes the phenoxyl hole by ca. 8 kcal mol(-1) (1 kcal = 4.18 kJ; 350 mV) through delocalization onto the sulfur atom. Sulfur K-edge X-ray absorption spectroscopy clearly indicates a contribution of ca. 8-13% to the hole from the sulfur atoms in [1(SR2)](+). The electrochemical results for the model complexes corroborate the ca. 350 mV (density functional theory) contribution of hole delocalization on to the cysteine-tyrosine cross-link to the stability of the phenoxyl radical in the enzyme, while highlighting the importance of the in-plane conformation observed in all crystal structures of the enzyme.
Proceedings of the National Academy of Sciences 11/2011; 108(46):18600-5. · 9.68 Impact Factor
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Chemistry 08/2010; 16(30):8980-3. · 5.93 Impact Factor
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ABSTRACT: A MerR family metalloregulatory protein CupR selectively responds to gold stress in Ralstonia metallidurans. A distorted trigonal geometry appears to be used by CupR to achieve the highly sensitive (K(d) approximately 10(-35) M) and selective recognition of gold(I).
Journal of the American Chemical Society 08/2009; 131(31):10869-71. · 9.91 Impact Factor
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ABSTRACT: A new hybrid permethylated-amine-guanidine ligand based on a 1,3-propanediamine backbone (2L) and its Cu-O2 chemistry is reported. [(2L)CuI(MeCN)]1+ complex readily oxygenates at low temperatures in polar aprotic solvents to form a bis(mu-oxo)dicopper(III) (O) species (2b), similar to the parent bis-guanidine ligand complex (1b) and permethylated-diamine ligand complex (3b). UV-vis and X-ray absorption spectroscopy experiments confirm this assignment of 2b as an O species, and full formation of the 2:1 Cu-O2 complex is demonstrated by an optical titration with ferrocene-monocarboxylic acid (FcCOOH). The UV-vis spectra of 1b and 2b with guanidine ligation show low-intensity visible features assigned as guanidine pi --> Cu2O2 core transitions by time-dependent density functional theory (TD-DFT) calculations. Comparison of the reactivity among the three related complexes (1b-3b) with phenolate at 195 K is particularly insightful as only 2b hydroxylates 2,4-di-tert-butylphenolate to yield 3,5-di-tert-butylcatecholate (>95% yield) with the oxygen atom derived from O2, reminiscent of tyrosinase reactivity. 1b is unreactive, while 3b yields the C-C radical-coupled bis-phenol product. Attenuated outer-sphere oxidative strength of the O complexes and increased phenolate accessibility to the Cu2O2 core are attributes that correlate with phenolate hydroxylation reactivity observed in 2b. The comparative low-temperature reactivity of 1b-3b with FcCOOH (O-H BDE 71 kcal mol(-1)) to form the two-electron, two-proton reduced bis(mu-hydroxo)dicopper(II,II) complex is quantitative and presumably precedes through two sequential proton-coupled electron transfer (PCET) steps. Optical titrations along with DFT calculations support that the reduced complexes formed in the first step are more powerful oxidants than the parent O complexes. These mechanistic insights aid in understanding the phenol to bis-phenol reactivity exhibited by 2b and 3b.
Journal of the American Chemical Society 01/2009; 131(3):1154-69. · 9.91 Impact Factor
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ABSTRACT: The cytochrome P450 enzyme CYP119, its compound II derivative, and its nitrosyl complex were studied by iron K-edge x-ray absorption spectroscopy. The compound II derivative was prepared by reaction of the resting enzyme with peroxynitrite and had a lifetime of approximately 10 s at 23 degrees C. The CYP119 nitrosyl complex was prepared by reaction of the enzyme with nitrogen monoxide gas or with a nitrosyl donor and was stable at 23 degrees C for hours. Samples of CYP119 and its derivatives were studied by x-ray absorption spectroscopy at temperatures below 140 (K) at the Advanced Photon Source of Argonne National Laboratory. The x-ray absorption near-edge structure spectra displayed shifts in edge and pre-edge energies consistent with increasing effective positive charge on iron in the series native CYP119 < CYP119 nitrosyl complex < CYP119 compound II derivative. Extended x-ray absorption fine structure spectra were simulated with good fits for k = 12 A(-1) for native CYP119 and k = 13 A(-1) for both the nitrosyl complex and the compound II derivative. The important structural features for the compound II derivative were an iron-oxygen bond length of 1.82 A and an iron-sulfur bond length of 2.24 A, both of which indicate an iron-oxygen single bond in a ferryl-hydroxide, Fe(IV)OH, moiety.
Proceedings of the National Academy of Sciences 06/2008; 105(24):8179-84. · 9.68 Impact Factor
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ABSTRACT: The cytochrome P450 enzyme CYP119, its compound II derivative, and its
nitrosyl complex were studied by iron K-edge x-ray absorption
spectroscopy. The compound II derivative was prepared by reaction of the
resting enzyme with peroxynitrite and had a lifetime of ≈10 s at
23°C. The CYP119 nitrosyl complex was prepared by reaction of the
enzyme with nitrogen monoxide gas or with a nitrosyl donor and was
stable at 23°C for hours. Samples of CYP119 and its derivatives were
studied by x-ray absorption spectroscopy at temperatures below 140 (K)
at the Advanced Photon Source of Argonne National Laboratory. The x-ray
absorption near-edge structure spectra displayed shifts in edge and
pre-edge energies consistent with increasing effective positive charge
on iron in the series native CYP119 < CYP119 nitrosyl complex <
CYP119 compound II derivative. Extended x-ray absorption fine structure
spectra were simulated with good fits for k = 12
Å-1 for native CYP119 and k = 13
Å-1 for both the nitrosyl complex and the
compound II derivative. The important structural features for the
compound II derivative were an iron-oxygen bond length of 1.82 Å
and an iron-sulfur bond length of 2.24 Å, both of which indicate
an iron-oxygen single bond in a ferryl-hydroxide, FeIVOH,
moiety.
Proceedings of the National Academy of Sciences 05/2008; 105:8179-8184. · 9.68 Impact Factor
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ABSTRACT: The excited-state structural dynamics of nickel(II)tetrakis(2,4,6-trimethylphenyl)porphyrin (NiTMP) and nickel(II)tetrakis(tridec-7-yl)porphyrin (NiSWTP) in a toluene solution were investigated via ultrafast transient optical absorption spectroscopy. An ultrashort stimulated emission between 620 and 670 nm from the S1 state was observed in both nickel porphyrins only when this state was directly generated via Q-band excitation, whereas such a stimulated emission was absent under B (Soret)-band excitation. Because the stimulated emission in the spectral region occurs only from the S1 state, this photoexcitation-wavelength-dependent behavior of Ni(II) porphyrins is attributed to a faster intersystem crossing from the S2 state than the internal conversion S2 --> S1. The dynamics of the excited-state pathways involving the (pi, pi*) and (d, d) states varies with the meso-substituted peripheral groups, which is attributed to the nickel porphyrin macrocycle distortion from a planar configuration. Evidence for intramolecular vibrational relaxation within 2 ps and vibrational cooling in 6-20 ps of a (d, d) excited state has been established for NiTMP and NiSWTP. Finally, the lifetimes of the vibrationally relaxed (d, d) state also depend on the nature of the peripheral groups, decreasing from 200 ps for NiTMP to 100 ps for the bulkier NiSWTP.
The Journal of Physical Chemistry A 12/2007; 111(46):11736-42. · 2.95 Impact Factor
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Journal of the American Chemical Society 11/2007; 129(41):12350-1. · 9.91 Impact Factor
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ABSTRACT: Simultaneously tracking electronic and molecular structures of a photoexcited metalloporphyrin, present for only 200 ps in a dilute solution, has been realized using X-ray transient absorption spectroscopy (XTA). Using laser pulses as excitation sources and delayed X-ray pulses as probes, we were able to identify the excited state electronic configuration of a nickel porphyrin as singly occupied 3dx2-y2 and 3dz2 molecular orbitals (MOs) with an energy gap of 2.2 eV, and energy shifts 4pz MOs to 1.5 eV higher relative to that of the ground state, and an expanded porphyrin ring characterized by lengthening of Ni−N and Ni−C bonds. Moreover, kinetic XTA signals at different X-ray photon energies demonstrate the capability for acquiring the correlation and coherence between different optically excited states with the same technique. These results provide guidance for theoretical calculations as well as insightful understanding of optically excited states that play important roles in photochemical processes.
Journal of the American Chemical Society 09/2007; 129(31):9616-8. · 9.91 Impact Factor
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Angewandte Chemie International Edition 02/2007; 46(27):5198-201. · 13.45 Impact Factor
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ABSTRACT: Hemes (iron porphyrins) are involved in a range of functions in biology, including electron transfer, small-molecule binding and transport, and O2 activation. The delocalization of the Fe d-electrons into the porphyrin ring and its effect on the redox chemistry and reactivity of these systems has been difficult to study by optical spectroscopies due to the dominant porphyrin pi-->pi(*) transitions, which obscure the metal center. Recently, we have developed a methodology that allows for the interpretation of the multiplet structure of Fe L-edges in terms of differential orbital covalency (i.e., differences in mixing of the d-orbitals with ligand orbitals) using a valence bond configuration interaction (VBCI) model. Applied to low-spin heme systems, this methodology allows experimental determination of the delocalization of the Fe d-electrons into the porphyrin (P) ring in terms of both P-->Fe sigma and pi-donation and Fe-->P pi back-bonding. We find that pi-donation to Fe(III) is much larger than pi back-bonding from Fe(II), indicating that a hole superexchange pathway dominates electron transfer. The implications of the results are also discussed in terms of the differences between heme and non-heme oxygen activation chemistry.
Journal of the American Chemical Society 02/2007; 129(1):113-25. · 9.91 Impact Factor
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ABSTRACT: Distinct spectral features at the Fe L-edge of the two compounds K3[Fe(CN)6] and K4[Fe(CN)6] have been identified and characterized as arising from contributions of the ligand pi orbitals due to metal-to-ligand back-bonding. In addition, the L-edge energy shifts and total intensities allow changes in the ligand field and effective nuclear charge to be determined. It is found that the ligand field term dominates the edge energy shift. The results of the experimental analysis were compared to BP86 DFT calculations. The overall agreement between the calculations and experiment is good; however, a larger difference in the amount of pi back-donation between Fe(II) and Fe(III) is found experimentally. The analysis of L-edge spectral shape, energy shift, and total intensity demonstrates that Fe L-edge X-ray absorption spectroscopy provides a direct probe of metal-to-ligand back-bonding.
Journal of the American Chemical Society 09/2006; 128(32):10442-51. · 9.91 Impact Factor
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ABSTRACT: Full multiple scattering calculations of the Fe K-edge X-ray absorption near edge structure of bleomycin have been performed. Structural insight is based on the comparison between experimental and theoretical data calculated for different active site models coming from NMR-informed molecular dynamic simulations. In all models considered, the equatorial ligands (secondary amine in beta-aminoalanine, pyrimidine and imidazole rings and the beta-hydroxyhistidine) were left unchanged. Seven models with two axial ligands (the primary amine in beta-aminoalanine and the carbomoyl group of the mannose or a solvent molecule) were tested. The best agreement between theoretical and experimental spectra is achieved for the model of bleomycin with the primary amine and the oxygen of the mannose sugar occupying the axial positions. The coordination environment is characterized by serious distortions of the Fe octahedron, including the presence of one ligand with a very short bond length and significant angular distortions.
Inorganic Chemistry 04/2004; 43(6):1825-7. · 4.60 Impact Factor
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ABSTRACT: X-ray absorption spectroscopy has been utilized to obtain the L-edge multiplet spectra for a series of non-heme ferric and ferrous complexes. Using these data, a methodology for determining the total covalency and the differential orbital covalency (DOC), that is, differences in covalency in the different symmetry sets of the d orbitals, has been developed. The integrated L-edge intensity is proportional to the number of one-electron transition pathways to the unoccupied molecular orbitals as well as to the covalency of the iron site, which reduces the total L-edge intensity and redistributes intensity, producing shake-up satellites. Furthermore, differential orbital covalency leads to differences in intensity for the different symmetry sets of orbitals and, thus, further modifies the experimental spectra. The ligand field multiplet model commonly used to simulate L-edge spectra does not adequately reproduce the spectral features, especially the charge transfer satellites. The inclusion of charge transfer states with differences in covalency gives excellent fits to the data and experimental estimates of the different contributions of charge transfer shake-up pathways to the t(2g) and e(g) symmetry orbitals. The resulting experimentally determined DOC is compared to values calculated from density functional theory and used to understand chemical trends in high- and low-spin ferrous and ferric complexes with different covalent environments. The utility of this method toward problems in bioinorganic chemistry is discussed.
Journal of the American Chemical Society 11/2003; 125(42):12894-906. · 9.91 Impact Factor
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ABSTRACT: The extradiol dioxygenase, 2,3-dihydroxybiphenyl 1,2-dioxygenase (DHBD, EC 1.13.11.39), has been studied using magnetic circular dichroism (MCD), variable-temperature variable-field (VTVH) MCD, X-ray absorption (XAS) pre-edge, and extended X-ray absorption fine structure (EXAFS) spectroscopies, which are analogous to methods used in earlier studies on the extradiol dioxygenase catechol 2,3-dioxygenase [Mabrouk et al. J. Am. Chem Soc. 1991, 113, 4053-4061]. For DHBD, the spectroscopic data can be correlated to the results of crystallography and with the results from density functional calculations to obtain detailed geometric and electronic structure descriptions of the resting and substrate (DHB) bound forms of the enzyme. The geometry of the active site of the resting enzyme, square pyramidal with a strong Fe-glutamate bond in the equatorial plane, localizes the redox active orbital in an orientation appropriate for O(2) binding. However, the O(2) reaction is not favorable, as it would produce a ferric superoxide intermediate with a weak Fe-O bond. Substrate binding leads to a new square pyramidal structure with the strong Fe-glutamate bond in the axial direction as indicated by a decrease in the (5)E(g) and increase in the (5)T(2g) splitting. Electronic structure calculations provide insight into the relative lack of dioxygen reactivity for the resting enzyme and its activation upon substrate binding.
Journal of the American Chemical Society 10/2003; 125(37):11214-27. · 9.91 Impact Factor
