Reversible O-O bond cleavage in copper-dioxygen isomers: impact of anion basicity

Low-temperature oxygenation of copper(I) complexes of N,N,N',N'-tetraethylpropane-1,3-diamine yields solutions containing both mu-eta2:eta2-peroxodicopper(II) (P) and bis(mu-oxo)dicopper(III) (O) valence isomers. The P/O equilibrium position depends on the nature of the counteranion; P is favored with more basic anions. Titration and EXAFS experiments as well as DFT calculations suggest that axial donation from a sulfonate anion to the copper centers imparts an electronic/electrostatic bias toward the P isomer.     

Reactivity of mononuclear alkylperoxo copper(II) complex. O-O bond cleavage and C-H bond activation

A detailed reactivity study has been carried out for the first time on a new mononuclear alkylperoxo copper(II) complex, which is generated by the reaction of copper(II) complex supported by the bis(pyridylmethyl)amine tridentate ligand containing a phenyl group at the 6-position of the pyridine donor groups and cumene hydroperoxide (CmOOH) in CH3CN. The cumylperoxo copper(II) complex thus obtained has been found to undergo homolytic cleavage of the O-O bond and induce C-H bond activation of exogenous substrates, providing important insights into the catalytic mechanism of copper monooxygenases.     

Decay of the peroxide intermediate in laccase: reductive cleavage of the O-O bond.

Laccase is a multicopper oxidase that contains four Cu ions, one type 1, one type 2, and a coupled binuclear type 3 Cu pair. The type 2 and type 3 centers form a trinuclear Cu cluster that is the active site for O(2) reduction to H(2)O. To examine the reaction between the type 2/type 3 trinuclear cluster and dioxygen, the type 1 Cu was removed and replaced with Hg(2+), producing the T1Hg derivative. When reduced T1Hg laccase is reacted with dioxygen, a peroxide intermediate (P) is formed. The present study examines the kinetics and mechanism of formation and decay of P in T1HgLc. The formation of P was found to be independent of pH and did not involve a kinetic solvent isotope effect, indicating that no proton is involved in the rate-determining step of formation of P. Alternatively, pH and isotope studies on the decay of P revealed that a proton enhances the rate of decay by 10-fold at low pH. This process shows an inverse k(H)/k(D) kinetic solvent isotope effect and involves protonation of a nearby residue that assists in catalysis, rather than direct protonation of the peroxide. Decay of P also involves a significant oxygen isotope effect (k(16)O(2)/k(18)O(2)) of 1.11 +/- 0.05, indicating that reductive cleavage of the O-O bond is the rate-determining step in the decay of P. The activation energy for this process was found to be approximately 9.0 kcal/mol. The exceptionally slow rate of decay of P is explained by the fact that this process involves a 1e(-) reductive cleavage of the O-O bond and there is a large Franck-Condon barrier associated with this process. Alternatively, the 2e(-) reductive cleavage of the O-O bond has a much larger driving force which minimizes this barrier and accelerates the rate of this reaction by approximately 10(7) in the native enzyme. This large difference in rate for the 2e(-) versus 1e(-) process supports a molecular mechanism for multicopper oxidases in which O(2) is reduced to H(2)O in two 2e(-) steps.     

O-O bond cleavage in dinuclear peroxo complexes of iron porphyrins: a quantum chemical study

To gain insight into the mechanisms of O2 activation and cleavage in metalloenzymes, biomimetic metal complexes have been constructed and experimentally characterized. One such model complex is the dinuclear peroxo complex of iron porphyrins observed at low temperature in a non-coordinating solvent. The present theoretical study examines the O-O bond cleavage in these complexes, experimentally observed to occur either at increased temperature or when a strongly coordinating base is added. Using hybrid density functional theory, it is shown that the O-O bond cleavage always occurs in a state where two low-spin irons (S = +/-1/2) are antiferromagnetically coupled to a diamagnetic state. This state is the ground state when the strong base is present and forms an axial ligand to the free iron positions. In contrast, without the axial ligands, the ground state of the dinuclear peroxo complex has two high-spin irons (S = +/-5/2) coupled antiferromagnetically. Thus, the activation barrier for O-O bond cleavage is higher without the base because it includes also the promotion energy from the ground state to the reacting state. It is further found that this excitation energy, going from 10 unpaired electrons in the high-spin case to 2 in the low-spin case, is unusually difficult to determine accurately from density functional theory because it is extremely sensitive to the amount of exact exchange included in the functional.     

Photoreactions of cytochrome C oxidase

The photoreduction of oxidized bovine heart cytochrome c oxidase (CcO) by visible and UV radiation was investigated in the absence and presence of external reagents. In the former case, the quantum yields for direct photoreduction of heme A (heme a + heme a(3)) were 2.6 +/- 0.5 x 10(-3), 4 +/- 1 x 10(-4), and 4 +/- 2 x 10(-6) with pulsed laser irradiation at 266, 355 and 532 nm, respectively. Within experimental uncertainty, the quantum yields did not depend on pulse energy, implying that the mechanism is monophotonic. Irradiation with 355 nm light resulted in spectral changes similar to those produced independently by reduction with dithionite, whereby the low-spin heme a and Cu(A) are reduced first. Extended illumination at 355 and 532 nm yielded substantial amounts of reduced heme a(3). Heme decomposition was noted with 266 nm light. In the presence of formate and cyanide ions, which bind at the binuclear heme a(3)/copper center in CcO, irradiation at 355 nm caused selective reduction of only the low-spin heme a and Cu(A). The addition of ferrioxalate ion dramatically increased the efficiency of cytochrome c oxidase photoreduction. The quantum efficiency for heme A reduction was found to be near unity, significantly greater than for other known methods of photoreduction. The active reductant is most likely ferrous iron, and its reduction of the enzyme is thermodynamically driven by the reformation of ferrioxalate in the presence of excess oxalate ion. Other metalloenzymes with redox potentials similar to those of cytochrome c oxidase should be amenable to indirect photoreduction by this method.     

Proton pumping mechanism in cytochrome c oxidase

Two different issues, important for the pumping mechanism of cyctochrome c oxidase, have been addressed in the present study. One of them concerns the nature of two key proton transfer transition states. A simple electrostatic model is used to suggest that the transition state (TS) for transfer to the pump-site should be positively charged, while the one for transfer to the binuclear center should be charge-neutral. The character of the former TS will guarantee that the protons will be pumped to the outside and not return to the inside, while the neutral character of the latter one will allow transfer with a sufficiently low barrier. In the simple electrostatic analysis, leading to this qualitative picture of the pumping process, the results from the kinetic experiments are strictly followed, but it is at least as important to follow the fundamental requirements for pumping. In this perspective, the uncertainties in the quantitative analysis should be rather unimportant for the emerging qualitative picture of the pumping mechanism. The second problem addressed concerns the purpose of the K-channel. It is argued that the reason for the presence of the K-channel could be that protons cannot pass through the binuclear center at some stage of pumping. Barriers and water binding energies were computed using hybrid density functional theory (DFT) to investigate this question.     

Electrostatic study of the proton pumping mechanism in bovine heart cytochrome C oxidase

Cytochrome c oxidase (CcO) is the terminal enzyme of the cell respiratory chain in mitochondria and aerobic bacteria. It catalyzes the reduction of oxygen to water and utilizes the free energy of the reduction reaction for proton pumping across the inner-mitochondrial membrane, a process that results in a membrane electrochemical proton gradient. Although the structure of the enzyme has been solved for several organisms, the molecular mechanism of proton pumping remains unknown. In the present paper, continuum electrostatic calculations were employed to evaluate the electrostatic potential, energies, and protonation state of bovine heart cytochrome c oxidase for different redox states of the enzyme along its catalytic cycle. Three different computational models of the enzyme were employed to test the stability of the results. The energetics and pH dependence of the P-->F, F-->O, and O-->E steps of the cycle have been investigated. On the basis of electrostatic calculations, two possible schemes of redox-linked proton pumping are discussed. The first scheme involves His291 as a pump element, whereas the second scheme involves a group linked to propionate D of heme a(3). In both schemes, loading of the pump site is coupled to ET between the two hemes of the enzyme, while transfer of a chemical proton is accompanied by ejection of the pumped H(+). The two models, as well as the energetics results are compared with recent experimental kinetic data. The proton pumping across the membrane is an endergonic process, which requires a sufficient amount of energy to be provided by the chemical reaction in the active site. In our calculations, the conversion of OH(-) to H(2)O provides 520 meV of energy to displace pump protons from a loading site and overall about 635 meV for each electron passing through the system. Assuming that the two charges are translocated per electron against the membrane potential of 200 meV, the model predicts an overall efficiency of 63%.     

A hybrid density functional study of O-O bond cleavage and phenyl ring hydroxylation for a biomimetic non-heme iron complex

Density functional calculations using the B3LYP functional have been used to study the reaction mechanism of [Fe(Tp(Ph2))BF] (Tp(Ph2) = hydrotris(3,5-diphenylpyrazol-1-yl)borate; BF = benzoylformate) with dioxygen. This mononuclear non-heme iron(II) complex was recently synthesized, and it proved to be the first biomimetic complex reproducing the dioxygenase activity of alpha-ketoglutarate-dependent enzymes. Moreover, the enthalpy and entropy of activation for this biologically interesting process were derived from kinetic experiments offering a unique possibility for direct comparison of theoretical and experimental data. The results reported here support a mechanism in which oxidative decarboxylation of the keto acid is the rate-limiting step. This oxygen activation process proceeds on the septet potential energy surface through a transition state for a concerted O-O and C-C bond cleavage. In the next step, a high-valent iron-oxo species performs electrophilic attack on the phenyl ring of the Tp(Ph2) ligand leading to an iron(III)-radical sigma-complex. Subsequent proton-coupled electron-transfer yields an iron(II)-phenol intermediate, which can bind dioxygen and reduce it to a superoxide radical. Finally, the protonated superoxide radical leaves the first coordination sphere of the iron(III)-phenolate complex and dismutates to dioxygen and hydrogen peroxide. The calculated activation barrier (enthalpy and entropy) and the overall reaction energy profile agree well with experimental data. A comparison to the enzymatic process, which is suggested to occur on the quintet surface, has been made.     

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.     

Kinetics and mechanism of O-O bond cleavage in the reaction of [RuIII(edta)(H2O)]- with hydroperoxides in aqueous solution

The reactions of [Ru(III)(edta)(H(2)O)](-) (1) (edta = ethylenediaminetetraacetate) with tert-butylhydroperoxide ((t)BuOOH) and potassium hydrogenpersulfate (KHSO(5)) were studied kinetically as a function of oxidant concentration and temperature (10-30 degrees C) at a fixed pH of 6.1 using stopped-flow techniques. Kinetic results were analyzed by using global kinetic analysis techniques. The reaction was found to consist of two steps involving the rapid formation of a [Ru(III)(edta)(OOR)](2-) intermediate, which subsequently undergoes heterolytic cleavage to form [(edta)Ru(V)=O](-). Since [(edta)Ru(V)=O](-) was produced almost quantitatively in the reaction of 1 with the hydroperoxides (t)BuOOH and KHSO(5), the common mechanism is one of heterolytic scission of the O-O bond. The water soluble and easy to oxidize substrate 2,2'-azobis(3-ethylbenzithiazoline-6-sulfonate (ABTS), was employed to substantiate the mechanistic proposal. Reactions were carried out under pseudo-first order conditions for [ABTS] > [hydroperoxide] > [1], and were monitored as a function of time for the formation of the one-electron oxidation product ABTS (*+). The detailed suggested mechanism is consistent with the reported rate and activation parameters, and discussed in reference to the results reported for the reaction of [Ru(II)(edta)(H(2)O)](-) with H(2)O(2).     

Electronic design criteria for O-O bond formation via metal-oxo complexes

Metal-oxos are critical intermediates for the management of oxygen and its activation. The reactivity of the metal-oxo is central to the formation of O-O bonds, which is the essential step for oxygen generation. Two basic strategies for the formation of O-O bonds at metal-oxo active sites are presented. The acid-base (AB) strategy involves the attack of a nucleophilic oxygen species (e.g., hydroxide) on an electrophilic metal-oxo. Here, active-site designs must incorporate the assembly of a hydroxide (or water) proximate to a high-valent metal-oxo of even d electron count. For the radical coupling (RC) strategy, two high-valent metal-oxos of an odd d electron count are needed to drive O-O coupling. This Forum Article focuses on the different electronic structures of terminal metal-oxos that support AB and RC strategies and the design of ligand scaffolds that engender these electronic structures.     

Quantum-chemical calculations of O-O bond strengths in organic hydrotrioxides

The O-O bond strengths in ten organic hydrotrioxides have been calculated by semiempirical MNDO and AMI methods. The RO-OOH bond strength is independent of the nature of substituent R and is equal to 20.4±1.1 kcal mol^–1 (AM1). The influence of the inductive effect of substituent R on the value ofD(ROO-OH) has been established.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 5, pp. 1129–1131, May, 1996.     

Theoretical studies of O-O bond formation in photosystem II

The most critical part of dioxygen evolution in photosystem II is the O-O bond formation step. In order to reach an efficient mechanism, nature uses a unique oxygen-evolving complex (OEC) having four manganese and one calcium center. Even though the structure of the OEC has become much more clear during recent years, it has still been difficult to find a transition state (TS) for O-O bond formation with a sufficiently low barrier. However, about a year ago, a quite surprising type of TS was found. With the latest X-ray ligand assignment, the local barrier for this TS is only 5.1 kcal/mol. It can be described as an attack by an oxygen radical, held by a dangling manganese, on a bridging oxo ligand in the Mn3Ca cube. In the present short Article, energy diagrams describing the entire process of dioxygen formation will be presented. An important conclusion drawn from these diagrams is that the major features of dioxygen formation remain the same irrespective of which one of the experimentally suggested structures the diagram is built on. Compared to earlier presentations of the same type, a slightly different approach has been used for setting up the diagrams. Results from a recent experimental study of the pressure dependence of oxygen release have been used to define the final energy levels. The loss of energy in the electron transfer from Tyrz to P680 has also been incorporated into the diagrams. A good agreement with experimental observations is demonstrated.     

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).     

Hydroxyl radical formation by O-O bond homolysis in peroxynitrous acid

Peroxynitrite decay in weakly alkaline media occurs by two concurrent sets of pathways which are distinguished by their reaction products. One set leads to net isomerization to NO(3)(-) and the other set to net decomposition to O(2) plus NO(2)(-). At sufficiently high peroxynitrite concentrations, the decay half-time becomes concentration-independent and approaches a limiting value predicted by a mechanism in which reaction is initiated by unimolecular homolysis of the peroxo O-O bond, i.e., the following reaction: ONOOH --> (*)OH + (*)NO(2). This dynamical behavior excludes alternative postulated mechanisms that ascribe decomposition to bond rearrangement within bimolecular adducts. Nitrate and nitrite product distributions measured at very low peroxynitrite concentrations also correspond to predictions of the homolysis model, contrary to a recent report from another laboratory. Additionally, (1) the rate constant for the reaction ONOO(-) --> (*)NO + (*)O(2)(-), which is critical to the kinetic model, has been confirmed, (2) the apparent volume of activation for ONOOH decay (DeltaV() = 9.7 +/- 1.4 cm(3)/mol) has been shown to be independent of the concentration of added nitrite and identical to most other reported values, and (3) complex patterns of inhibition of O(2) formation by radical scavengers, which are impossible to rationalize by alternative proposed reaction schemes, are shown to be quantitatively in accord with the homolysis model. These observations resolve major disputes over experimental data existing in the literature; despite extensive investigation of these reactions, no verifiable experimental evidence has been advanced that contradicts the homolysis model.     

Stereochemistry and mechanism of the cleavage of the cobalt-tin bond

Summary The experimental results obtained for cleavage of cobalt-tin bonds by means of NaFe(CO)₂Cp, LiBHEt₃ and Ph₃SnLi can be explained by a one-electron transfer mechanism leading to a triorganostannyl radical, which can invert before reacting with another radical to give the reaction products.This paper is Part 75 of the series Organometallic Compounds. For Part 74, see ref. (1).     

A "push-pull" mechanism for heterolytic o-o bond cleavage in hydroperoxo manganese porphyrins

A water-soluble manganese porphyrin, 5,10,15,20-tetrakis-(1,3-dimethylimidazolium-2-yl)porphyrinatomanganese(III) (Mn(III)TDMImP) is shown to react with H(2)O(2) to generate a relatively stable dioxomanganese(V) porphyrin complex (a compound I analog). Stopped-flow kinetic studies revealed Michaelis Menton-type saturation kinetics for H(2)O(2). The visible spectrum of a compound 0 type intermediate, assigned as Mn(III)(OH)(OOH)TDMImP, can be directly observed under saturating H(2)O(2) conditions (Soret band at 428 nm and Q bands at 545 and 578 nm). The rate-determining O-O heterolysis step was found to have a very small activation enthalpy (ΔH(≠) = 4.2 ± 0.2 kcal mol(-1)) and a large, negative activation entropy (ΔS(≠) = -36 ± 1 cal mol(-1) K(-1)). The O-O bond cleavage reaction was pH independent at 8.8 < pH < 10.4 with a first-order rate constant of 66 ± 12 s(-1). These observations indicate that the O-O bond in Mn(III)(OH)(OOH)TDMImP is cleaved via a concerted "push-pull" mechanism. In the transition state, the axial (proximal) (-)OH is partially deprotonated ("push"), while the terminal oxygen in (-)OOH is partially protonated ("pull") as a water molecule is released to the medium. This mechanism is reminiscent of O-O bond cleavage in heme enzymes, such as peroxidases and cytochrome P450, and similar to the fast, reversible O-Br bond breaking and forming reaction mediated by similar manganese porphyrins. The small enthalpy of activation suggests that this O-O bond cleavage could also be made reversible.     

Spectroscopic properties and electronic structure of low-spin Fe(III)-alkylperoxo complexes: homolytic cleavage of the O-O bond

The spectroscopic properties, electronic structure, and reactivity of the low-spin Fe(III)-alkylperoxo model complex [Fe(TPA)(OH(x))(OO(t)Bu)](x+) (1; TPA = tris(2-pyridylmethyl)amine, (t)Bu = tert-butyl, x = 1 or 2) are explored. The vibrational spectra of 1 show three peaks that are assigned to the O-O stretch (796 cm(-1)), the Fe-O stretch (696 cm(-)(1)), and a combined O-C-C/C-C-C bending mode (490 cm(-1)) that is mixed with upsilon(FeO). The corresponding force constants have been determined to be 2.92 mdyn/A for the O-O bond which is small and 3.53 mdyn/A for the Fe-O bond which is large. Complex 1 is characterized by a broad absorption band around 600 nm that is assigned to a charge-transfer (CT) transition from the alkylperoxo pi*(upsilon) to a t(2g) d orbital of Fe(III). This metal-ligand pi bond is probed by MCD and resonance Raman spectroscopies which show that the CT state is mixed with a ligand field state (t(2g) --> e(g)) by configuration interaction. This gives rise to two intense transitions under the broad 600 nm envelope with CT character which are manifested by a pseudo-A term in the MCD spectrum and by the shapes of the resonance Raman profiles of the 796, 696, and 490 cm(-1) vibrations. Additional contributions to the Fe-O bond arise from sigma interactions between mainly O-O bonding donor orbitals of the alkylperoxo ligand and an e(g) d orbital of Fe(III), which explains the observed O-O and Fe-O force constants. The observed homolytic cleavage of the O-O bond of 1 is explored with experimentally calibrated density functional (DFT) calculations. The O-O bond homolysis is found to be endothermic by only 15 to 20 kcal/mol due to the fact that the Fe(IV)=O species formed is highly stabilized (for spin states S = 1 and 2) by two strong pi and a strong sigma bond between Fe(IV) and the oxo ligand. This low endothermicity is compensated by the entropy gain upon splitting the O-O bond. In comparison, Cu(II)-alkylperoxo complexes studied before [Chen, P.; Fujisawa, K.; Solomon, E. I. J. Am. Chem. Soc. 2000, 122, 10177] are much less suited for O-O bond homolysis, because the resulting Cu(III)=O species is less stable. This difference in metal-oxo intermediate stability enables the O-O homolysis in the case of iron but directs the copper complex toward alternative reaction channels.