A chromium(III)-superoxo complex in oxygen atom transfer reactions as a chemical model of cysteine dioxygenase

Metal-superoxo species are believed to play key roles in oxygenation reactions by metalloenzymes. One example is cysteine dioxygenase (CDO) that catalyzes the oxidation of cysteine with O(2), and an iron(III)-superoxo species is proposed as an intermediate that effects the sulfoxidation reaction. We now report the first biomimetic example showing that a chromium(III)-superoxo complex bearing a macrocyclic TMC ligand, [Cr(III)(O(2))(TMC)(Cl)](+), is an active oxidant in oxygen atom transfer (OAT) reactions, such as the oxidation of phosphine and sulfides. The electrophilic character of the Cr(III)-superoxo complex is demonstrated unambiguously in the sulfoxidation of para-substituted thioanisoles. A Cr(IV)-oxo complex, [Cr(IV)(O)(TMC)(Cl)](+), formed in the OAT reactions by the chromium(III)-superoxo complex, is characterized by X-ray crystallography and various spectroscopic methods. The present results support the proposed oxidant and mechanism in CDO, such as an iron(III)-superoxo species is an active oxidant that attacks the sulfur atom of the cysteine ligand by the terminal oxygen atom of the superoxo group, followed by the formation of a sulfoxide and an iron(IV)-oxo species via an O-O bond cleavage.     

Multiplicity of Reaction Pathways in the Processes of Oxygen Transfer to Secondary Amines by Mo(VI) and W(VI) Peroxo Complexes

Oxidation of N,N-benzylmethylamine, N,N-benzylisopropylamine, and N,N-benzyl-tert-butylamine by both anionic and neutral Mo(VI) and W(VI) oxodiperoxo complexes yields the corresponding nitrones quantitatively. The oxidation reactions employing anionic oxidants were performed in CHCl(3) and follow second-order kinetics, first order with respect to the amine and to the oxidant. The data were rationalized on the basis of a rate-determining nucleophilic attack of the amine onto the peroxide oxygen of the oxidant, with a transition state in which N-O bond formation and O-O bond cleavage occur in a concerted way (electrophilic oxygen transfer mechanism). This attack yields the corresponding hydroxylamine, which then is furtherly oxidized to nitrone in a fast step. On the other hand, in the case of neutral oxidants (1)H-NMR data as well as kinetic data indicate that amine coordinates the metal center replacing the original ligand HMPA and yields a new peroxo complex. For N,N-benzyl-tert-butylamine such a complex was isolated and characterized. These new peroxo complexes can themselves behave as electrophilic oxidants, transferring oxygen to external amine molecules through the same pathway followed by anionic oxidants, or can yield the reaction product by intramolecular oxidation of the coordinate amine. Measurements of added HMPA effects on oxidation rate would seem more consistent with the electrophilic oxygen transfer mechanism.     

Substrate- and isoform-specific dioxygen complexes of nitric oxide synthase

Nitric oxide synthase (NOS) catalyzes the formation of NO via a consecutive two-step reaction. In the first step, L-arginine (Arg) is converted to N-hydroxy-L-arginine (NOHA). In the second step, NOHA is further converted to citrulline and nitric oxide (NO). To assess the mechanistic differences between the two steps of the reaction, we have used resonance Raman spectroscopy combined with a homemade continuous-flow rapid solution mixer to study the structural properties of the metastable dioxygen-bound complexes of the oxygenase domain of inducible NOS (iNOSoxy). We identified the O-O stretching frequency of the substrate-free enzyme at 1133 cm-1. This frequency is insensitive to the presence of tetrahydrobiopterin, but it shifts to 1126 cm-1 upon binding of Arg, which we attribute to H-bonding interactions to the terminal oxygen atom of the heme iron-bound dioxygen. In contrast, the addition of NOHA to the enzyme did not bring about a shift in the frequency of the O-O stretching mode, because, unlike Arg, there is no H-bond associated with the terminal oxygen atom of the dioxygen. The substrate-specific H-bonding interactions play a critical role in determining the fate of the key peroxy intermediate. In the first step of the reaction, the H-bonds facilitate the rupture of the O-O bond, leading to the formation of the active ferryl species, which is essential for the oxidation of the Arg. On the other hand, in the second step of the reaction, the absence of the H-bonds prevents the premature O-O bond cleavage, such that the peroxy intermediate can perform a nucleophilic addition reaction to the substrate, NOHA.     

A density functional study of O-O bond cleavage for a biomimetic non-heme iron complex demonstrating an Fe(v)-intermediate

Density functional theory using the B3LYP hybrid functional has been employed to investigate the reactivity of Fe(TPA) complexes (TPA = tris(2-pyridylmethyl)amine), which are known to catalyze stereospecific hydrocarbon oxidation when H(2)O(2) is used as oxidant. The reaction pathway leading to O-O bond heterolysis in the active catalytic species Fe(III)(TPA)-OOH has been explored, and it is shown that a high-valent iron-oxo intermediate is formed, where an Fe(V) oxidation state is attained, in agreement with previous suggestions based on experiments. In contrast to the analogous intermediate [(Por.)Fe(IV)=O](+1) in P450, the TPA ligand is not oxidized, and the electrons are extracted almost exclusively from the mononuclear iron center. The corresponding homolytic O-O bond cleavage, yielding the two oxidants Fe(IV)=O and the OH. radical, has also been considered, and it is shown that this pathway is inaccessible in the hydrocarbon oxidation reaction with Fe(TPA) and hydrogen peroxide. Investigations have also been performed for the O-O cleavage in the Fe(III)(TPA)-alkylperoxide species. In this case, the barrier for O-O homolysis is found to be slightly lower, leading to loss of stereospecificity and supporting the experimental conclusion that this is the preferred pathway for alkylperoxide oxidants. The difference between hydroperoxide and alkylperoxide as oxidant derives from the higher O-O bond strength for hydrogen peroxide (by 8.0 kcal/mol).     

Unraveling the reactive species of a functional non-heme iron monooxygenase model using stopped-flow UV-vis spectroscopy

Low-temperature stopped-flow electronic spectroscopy was utilized to resolve the intermediates formed in the reaction of a diiron(II) compound, Fe2(H2Hbamb)2(N-MeIm)2 (H4HBamb = 2,3-bis(2-hydroxybenzamido)dimethylbutane), 1, with the oxygen atom donors 2,6-dimethyliodosylbenzene and p-cyanodimethylaniline N-oxide and the mechanistic probe hydroperoxide 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH). Previous studies showed that 1 is capable of catalytically oxidizing cyclohexane to cyclohexanol (300 turnovers) via a pathway involving the heterolytic cleavage of the O-O bond of MPPH (>98% peroxide utilization). We now report intimate details of the formation of the reactive intermediate and its subsequent decay in the absence of substrates. The reaction, which is independent of the nature of the oxidant, proceeds in three consecutive steps assigned as (i) oxygen-atom transfer to one of the iron centers of 1 to form an FeIV=O species, 2, (ii) ligand rearrangement to 3, and (iii) internal collapse of the terminal oxo group to generate a diferric, mu-oxo species, 4. Assignment of the second step as a ligand rearrangement was corroborated by stopped-flow spectroscopic studies of the one-electron oxidation of the starting diferrous 1, which is also known to undergo ligand rearrangement upon the formation of the [FeII, FeIII] mixed-valent complex. Observation of the reaction rates over a temperature range allowed for the determination of activation parameters for each of the three steps. The role of the ligand reorganization in the energetic profile for the formation of the catalytically competent intermediate is discussed, along with the potential biological significance of the internal conversion of the active oxidant to the inert, mu-oxo diiron(III) dimer, 4.     

A Systematic Account on Aromatic Hydroxylation by a Cytochrome P450 Model Compound I: A Low‐Pressure Mass Spectrometry and Computational Study

Cytochrome P450 enzymes are heme‐containing mono‐oxygenases that mainly react through oxygen‐atom transfer. Specific features of substrate and oxidant that determine the reaction rate constant for oxygen atom transfer are still poorly understood and therefore, we did a systematic gas‐phase study on reactions by iron(IV)‐oxo porphyrin cation radical structures with arenes. We present herein the first results obtained by using Fourier transform‐ion cyclotron resonance mass spectrometry and provide rate constants and product distributions for the assayed reactions. Product distributions and kinetic isotope effect studies implicate a rate‐determining aromatic hydroxylation reaction that correlates with the ionization energy of the substrate and no evidence of aliphatic hydroxylation products is observed. To further understand the details of the reaction mechanism, a computational study on a model complex was performed. These studies confirm the experimental hypothesis of dominant aromatic over aliphatic hydroxylation and show that the lack of an axial ligand affects the aliphatic pathways. Moreover, a two‐parabola valence bond model is used to rationalize the rate constant and identify key properties of the oxidant and substrate that drive the reaction. In particular, the work shows that aromatic hydroxylation rates correlate with the ionization energy of the substrate as well as with the electron affinity of the oxidant.     

Iron(III) [bond] Salen complexes as enzyme models: mechanistic study of oxo(salen)iron complexes oxygenation of organic sulfides

The oxidation of a series of para-substituted phenyl methyl sulfides was carried out with several oxo(salen)iron (salen = N,N'-bis(salicylidine)ethylenediaminato) complexes in acetonitrile. The oxo complex [O=Fe(IV)(salen)](*+), generated from an iron(III) [bond] salen complex and iodosylbenzene, effectively oxidizes the organic sulfides to the corresponding sulfoxides. The formation of [O [double bond] Fe(IV)(salen)](*+) as the active oxidant is supported by resonance Raman studies. The kinetic data indicate that the reaction is first-order in the oxidant and fractional-order with respect to sulfide. The observed saturation kinetics of the reaction and spectral data indicate that the substrate binds to the oxidant before the rate-controlling step. The rate constant (k) values for the product formation step determined using Michaelis-Menten kinetics correlate well with Hammett sigma constants, giving reaction constant (rho) values in the range of -0.65 to -1.54 for different oxo(salen)iron complexes. The log k values observed in the oxidation of each aryl methyl sulfide by substituted oxo(salen)iron complexes also correlate with Hammett sigma constants, giving positive rho values. The substituent effect, UV-vis absorption, and EPR spectral studies indicate oxygen atom transfer from the oxidant to the substrate in the rate-determining step.     

Ethylene biosynthesis by 1-aminocyclopropane-1-carboxylic acid oxidase: a DFT study

The reaction catalyzed by the plant enzyme 1-aminocyclopropane-1-carboxylic acid oxidase (ACCO) was investigated by using hybrid density functional theory. ACCO belongs to the non-heme iron(II) enzyme superfamily and carries out the bicarbonate-dependent two-electron oxidation of its substrate ACC (1-aminocyclopropane-1-carboxylic acid) concomitant with the reduction of dioxygen and oxidation of a reducing agent probably ascorbate. The reaction gives ethylene, CO(2), cyanide and two water molecules. A model including the mononuclear iron complex with ACC in the first coordination sphere was used to study the details of O-O bond cleavage and cyclopropane ring opening. Calculations imply that this unusual and complex reaction is triggered by a hydrogen atom abstraction step generating a radical on the amino nitrogen of ACC. Subsequently, cyclopropane ring opening followed by O-O bond heterolysis leads to a very reactive iron(IV)-oxo intermediate, which decomposes to ethylene and cyanoformate with very low energy barriers. The reaction is assisted by bicarbonate located in the second coordination sphere of the metal.     

Oxovanadium(IV)‐Salen Ion Catalyzed H2O2 Oxidation of Tertiary Amines to N‐Oxides– Critical Role of Acetate Ion as External Axial Ligand

The oxovanadium(IV)‐salen ion catalyzed H₂O₂ oxidation of N,N‐dimethylaniline forms N‐oxide as the product of the reaction. The reaction follows Michaelis–Menten kinetics and the rate of the reaction is accelerated by electron donating groups present in the substrate as well as in the salen ligand. This peculiar substituent effect is accounted for in terms of rate determining bond formation between peroxo bond of the oxidant and the N‐atom of the substrate in the transition state. Trichloroacetic acid (TCA) shifts the λ_max value of the oxidant to the red region and catalyzes reaction enormously. The cleavage of N? O bond by vanadium complex leads to moderate yield of the product. But the percentage yield of the product becomes excellent in the presence of TCA.     

Fe2+-catalyzed heterolytic RO-OH bond cleavage and substrate oxidation: a functional synthetic non-heme iron monooxygenase system

The reaction of [Fe22+(H2Hbamb)2(N-MeIm)2], [1], a binuclear, non-heme iron complex, with 2-methyl-1-phenylprop-2-yl hydroperoxide (MPPH) shows that [1] induces heterolytic cleavage of the peroxy O-O bond. Catalytic atom transfer reactions (1:MPPH:PhSMe 1:596:6011) resulted in the highly efficient (99 +/- 1%), catalytic oxidation of phenyl methyl sulfide to phenyl methyl sulfoxide/sulfone (T.N. = 500/11 respectively) and cyclohexane to cyclohexanol/cyclohexanone (T.N. = 230/5 respectively) showing the highly efficient, catalytic capacity of [1] to carry out oxygen insertion chemistry.     

On the mechanisms of oxidation of organic sulfides by H2O2 in aqueous solutions

The mechanism of oxidation of organic sulfides in aqueous solutions by hydrogen peroxide was investigated via ab initio calculations. Specifically, two reactions, hydrogen transfer of hydrogen peroxide to form water oxide and the oxidation of dimethyl sulfide (DMS) by hydrogen peroxide to form dimethyl sulfoxide, were studied as models of these processes in general. Solvent effects are included both via including explicitly water molecules and via the polarizable continuum model. The former was found to have a much more significant effect than the latter. When explicit water molecules are included, a mechanism different from those proposed in the literature was found. Specific interactions including hydrogen bonding with 2-3 water molecules can provide enough stabilization for the charge separation of the activation complex. The energy barrier of the oxidation of DMS by hydrogen peroxide was estimated to be 12.7 kcal/mol, within the experimental range of the oxidation of analogous compounds (10-20 kcal/mol). The major reaction coordinates of the reaction are the breaking of the O-O bond of H2O2 and the formation of the S-O bond, the transfer of hydrogen to the distal oxygen of hydrogen peroxide occurring after the system has passed the transition state. Reaction barriers of the hydrogen transfer of H2O2 are an average of 10 kcal/mol or higher than the reaction barriers of the oxidation of DMS. Therefore, a two-step oxidation mechanism in which, first, the transfer of a hydrogen atom occurs to form water oxide and, second, the transfer of oxygen to the substrate occurs is unlikely to be correct. Our proposed oxidation mechanism does not suggest a pH dependence of oxidation rate within a moderate range around neutral pH (i.e., under conditions in which hydronium and hydroxide ions do not participate directly in the reaction), and it agrees with experimental observations over moderate pH values. Also, without including a protonated solvent molecule, it has activation energies that correspond to measured activation energies.     

Characterization of a high-spin non-heme Fe(III)-OOH intermediate and its quantitative conversion to an Fe(IV)═O complex

We have generated a high-spin Fe(III)-OOH complex supported by tetramethylcyclam via protonation of its conjugate base and characterized it in detail using various spectroscopic methods. This Fe(III)-OOH species can be converted quantitatively to an Fe(IV)═O complex via O-O bond cleavage; this is the first example of such a conversion. This conversion is promoted by two factors: the strong Fe(III)-OOH bond, which inhibits Fe-O bond lysis, and the addition of protons, which facilitates O-O bond cleavage. This example provides a synthetic precedent for how O-O bond cleavage of high-spin Fe(III)-peroxo intermediates of non-heme iron enzymes may be promoted.     

Chemoselective alkane oxidation by superoxo-vanadium(V) in vanadosilicate molecular sieves

Electron paramagnetic resonance (EPR) spectroscopy of reactive superoxo-vanadium(V) species in vanadosilicate molecular sieves (microporous VS-1 and mesoporous V-MCM-41) generated on contact with H2O2, tert-butyl hydroperoxide (TBHP), or (H2+O2) is reported for the first time. By suitable choice of the silicate structure, solvent, and oxidant, we could control the vanadium-(O2-*) bond (i.e., the V-O bond) covalency, the mode of O-O cleavage (in the superoxo species), and, therefore, chemoselectivity in the oxidation of n-hexane: Oxidation by TBHP over V-MCM-41, for example, yielded 27.2% of (n-hexanol+n-hexanal+n-hexanoic acid), among the highest chemoselectivities for oxidation of the terminal -CH3 in a linear paraffin reported to date. Over these vanadosilicates, oxidation of the primary C-H bond occurs only via a homolytic O-O bond cleavage; the secondary C-H bond oxidations may proceed via both the homo- and heterolytic O-O cleavage mechanisms.     

Mechanism of selective oxidation of organic sulfides with Oxo(salen)chromium(V) complexes

The selective oxidation of organic sulfides to sulfoxides by oxo(salen)chromium(V) complexes in acetonitrile is overall second-order, first-order each in the oxidant and the substrate. The rate constant, k(2), values of several para-substituted phenyl methyl sulfides correlate linearly with Hammett sigma constants and the rho values are in the range of -1.3 to -2.7 with different substituted oxo(salen)chromium(V) complexes. The reactivity of different alkyl sulfides is in accordance with Taft's steric substituent constant, E(S). A mechanism involving direct oxygen atom transfer from the oxidant to the substrate rather than electron transfer is envisaged. Correlation analyses show the presence of an inverse relationship between reactivity and selectivity in the reaction of various sulfides with a given oxo(salen)chromium(V) complex and vice versa. Mathematical treatment of the results shows that this redox system falls under strong reactivity-selectivity principle (RSP).     

Explanation for main features of structure-genotoxicity relationships of aromatic amines by theoretical studies of their activation pathways in CYP1A2

Aromatic and heteroaromatic amines (ArNH(2)) represent a class of potential mutagens that after being metabolically activated covalently modify DNA. Activation of ArNH(2) in many cases starts with N-hydroxylation by P450 enzymes, primarily CYP1A2. Poor understanding of structure-mutagenicity relationships of ArNH(2) limits their use in drug discovery programs. Key factors that facilitate activation of ArNH(2) are revealed by exploring their reaction intermediates in CYP1A2 using DFT calculations. On the basis of these calculations and extensive analysis of structure-mutagenicity data, we suggest that mutagenic metabolites are generated by ferric peroxo intermediate, (CYP1A2)Fe(III)-OO(-), in a three-step heterolytic mechanism. First, the distal oxygen of the oxidant abstracts proton from H-bonded ArNH(2). The subsequent proximal protonation of the resulting (CYP1A2)Fe(III)-OOH weakens both the O-O and the O-H bonds of the oxidant. Heterolytic cleavage of the O-O bond leads to N-hydroxylation of ArNH(-) via S(N)2 mechanism, whereas cleavage of the O-H bond results in release of hydroperoxy radical. Thus, our proposed reaction offers a mechanistic explanation for previous observations that metabolism of aromatic amines could cause oxidative stress. The primary drivers for mutagenic potency of ArNH(2) are (i) binding affinity of ArNH(2) in the productive binding mode within the CYP1A2 substrate cavity, (ii) resonance stabilization of the anionic forms of ArNH(2), and (iii) exothermicity of proton-assisted heterolytic cleavage of N-O bonds of hydroxylamines and their bioconjugates. This leads to a strategy for designing mutagenicity free ArNH(2): Structural alterations in ArNH(2), which disrupt geometric compatibility with CYP1A2, hinder proton abstraction, or strongly destabilize the nitrenium ion, in this order of priority, prevent genotoxicity.     

Quantum chemical studies of dioxygen activation by mononuclear non-heme iron enzymes with the 2-His-1-carboxylate facial triad

Density functional theory with the B3LYP hybrid functional has been used to study the mechanisms for dioxygen activation by four families of mononuclear non-heme iron enzymes: alpha-ketoacid-dependent dioxygenases, tetrahydrobiopterin-dependent hydroxylases, extradiol dioxygenases, and Rieske dioxygenases. These enzymes have a common active site with a ferrous ion coordinated to two histidines and one carboxylate group (aspartate or glutamate). In contrast to the heme case, this type of weak field environment always leads to a high-spin ground state. With the exception of the Rieske dioxygenases, which have an electron source outside the active site, the dioxygen activation process passes through the formation of a bridging-peroxide species, which then undergoes O-O bond cleavage finally leading to the four electron reduction of O(2). In the case of tetrahydrobiopterin- and alpha-ketoacid-dependent enzymes, the O-O heterolysis yields a high-valent iron-oxo species, which is capable of performing a two-electron oxidation chemistry on various organic substrates. For the other two families of enzymes (extradiol dioxygenases and Rieske dioxygenases) the substrate oxidation and the O-O bond cleavage are found to be coupled. In the extradiol dioxygenases the product of the O-O bond cleavage is a ferric iron with an oxy-substrate with a mixture of radical and anionic character, which is essential for the selectivity of the catechol cleavage.     

Insights into the structure and reactivity of acylperoxo complexes in the Kochi-Jacobsen-Katsuki catalytic system. A density functional study

Structural properties of the acylperoxo complexes [(Salen)Mn(III)RCO(3)] (2) and [(Salen)Mn(IV)RCO(3)] (3), the critical intermediates in the Kochi-Jacobsen-Katsuki reaction utilizing organic peracids or O(2)/aldehydes as oxygen source, have been studied with the density functional theory. Four distinct isomers, cis(O,N), cis(N,O), cis(N,N), and trans, of these complexes have been located. The isomer 2-cis(O,N) in its quintet ground state, and nearly degenerate isomers 3-cis(O,N) and 3-cis(N,O) in their quartet ground states are found to be the lowest in energy among the other isomers. The O-O bond cleavage in the cis(O,N), cis(N,O), and trans isomers of 2 and 3 has been elucidated. In complex 3, the O-O bond is inert. On the contrary, in complex 2, the O-O bond cleaves via two distinct pathways. The first pathway occurs exclusively on the quintet potential energy surface (PES) and corresponds to heterolytic O-O bond scission coupled with insertion of an oxygen atom into an Mn-N(Salen) bond to form 2-N-oxo species; this pathway has the lowest barrier of 14.9 kcal/mol and is 15.6 kcal/mol exothermic. The second pathway is tentatively a spin crossover pathway. In particular, for 2-cis(O,N) and 2-cis(N,O) the second pathway proceeds through a crucial minimum on the seam of crossing (MSX) between the quintet and triplet PESs followed by heterolytic O-O cleavage on the triplet PES, and produces unusual triplet 2-cis(O,N)- and 2-cis(N,O)-oxo ([(Salen)Mn(V)(O)RCO(2)]) species; this pathway requires 12.8 kcal/mol and is 1.4 kcal/mol endothermic. In contrast, for the 2-trans isomer, spin crossing is less crucial and the O-O cleavage proceeds homolytically to generate 2-trans-oxo [(Salen)Mn(IV)(O)] species with RCO(2) radical; this pathway, however, cannot compete with that in 2-cis because it needs 21.9 kcal/mol for activation and is 15.3 kcal/mol endothermic. In summary, the O-O cleavage occurs predominantly in the 2-cis complexes, and may proceed either through pure high spin or spin crossover heterolytic pathway to produce 2-cis-oxo and 2-N-oxo species.     

Intriguing roles of reactive intermediates in dissociation chemistry of N-phenylcinnamides

In mass spectrometry of protonated N-phenylcinnamides, the carbonyl oxygen is the thermodynamically most favorable protonation site and the added proton is initially localized on it. Upon collisional activation, the proton transfers from the carbonyl oxygen to the dissociative protonation site at the amide nitrogen atom or the α-carbon atom, leading to the formation of important reactive intermediates. When the amide nitrogen atom is protonated, the amide bond is facile to rupture to form ion/neutral complex 1, [RC(6)H(4)CH[double bond, length as m-dash]CHCO(+)/aniline]. Besides the dissociation of the complex, proton transfer reaction from the α-carbon atom to the nitrogen atom within the complex takes place, leading to the formation of protonated aniline. The presence of electron-withdrawing groups favored the proton transfer reaction, whereas electron-donating groups strongly favored the dissociation (aniline loss). When the proton transfers from the carbonyl oxygen to the α-carbon atom, the cleavage of the C(α)-CONHPh bond results in another ion/neutral complex 2, [PhNHCO(+)/RC(6)H(4)CH[double bond, length as m-dash]CH(2)]. However, in this case, electron-donating groups expedited the proton transfer reaction from the charged to the neutral partner to eliminate phenyl isocyanate. Besides the cleavage of the C(α)-CONHPh bond, intramolecular nucleophilic substitution (a nucleophilic attack of the nitrogen atom at the β-carbon) and stepwise proton transfer reactions (two 1,2-H shifts) also take place when the α-carbon atom is protonated, resulting in the loss of ketene and RC(6)H(5), respectively. In addition, the H/D exchanges between the external deuterium and the amide hydrogen, vinyl hydrogens and the hydrogens of the phenyl rings were discovered by D-labeling experiments. Density functional theory-based (DFT) calculations were performed to shed light on the mechanisms for these reactions.     

Oxygen-oxygen bond homolysis in a novel titanium(IV) alkylperoxide complex,Cp2Ti(OOtBu)Cl

Cp2TiCl2 reacts with NaOOtBu to form the new titanium peroxide complex, Cp2Ti(OOtBu)Cl (1), which has been characterized both in solution and in the solid state. This complex is surprisingly unreactive towards olefins and phosphines, as it does not directly transfer an oxygen atom. Instead, decomposition occurs via initial homolysis of the oxygen-oxygen bond, yielding a tert-butoxyl radical. Decomposition of 1 in the presence of phosphines yields either phosphine oxides (e.g., OPPh3) or phosphinites (e.g., tBuOPEt2), products that result from tBuO* + PR3. O-O bond homolysis is surprising because the Ti(IV) center is d0 and cannot be oxidized, where all previous clear examples of homolytic cleavage of metal peroxide complexes are facilitated by oxidation of the metal center.     

Spectroscopic and kinetic studies of perturbed trinuclear copper clusters: the role of protons in reductive cleavage of the O-O bond in the multicopper oxidase Fet3p

The multicopper oxidase Fet3p couples four 1e(-) oxidations of substrate to the 4e(-) reduction of O2 to H2O. Fet3p uses four Cu atoms to accomplish this reaction: the type 1, type 2, and coupled binuclear type 3 sites. The type 2 and type 3 sites together form a trinuclear Cu cluster (TNC) which is the site of O2 reduction. This study focuses on mutants of two residues, E487 and D94, which lie in the second coordination sphere of the TNC and defines the role that each plays in the structural integrity of the TNC, its reactivity with O2, and in the directional movement of protons during reductive cleavage of the O-O bond. The E487D, E487A, and D94E mutants have been studied in the holo and type 1 depleted (T1D) forms. Residue E487, located near the T3 center, is found to be responsible for donation of a proton during the reductive cleavage of the O-O bond in the peroxide intermediate and an inverse kinetic solvent isotope effect, which indicates that this proton is already transferred when the O-O bond is cleaved. Residue D94, near the T2 site, plays a key role in the reaction of the reduced TNC with O2 and drives electron transfer from the T2 Cu to cleave the O-O bond by deprotonating the T2 Cu water ligand. A mechanism is developed where these second sphere residues participate in the proton assisted reductive cleavage of the O-O bond at the TNC.