Energetics of Proton-Coupled Electron Transfer in High-Valent Mn<sub>2</sub>(μ-O)<sub>2</sub> Systems:  Models for Water Oxidation by the Oxygen-Evolving Complex of Photosystem II

Baldwin, Michael J.; Pecoraro, Vincent L.

doi:10.1021/ja9626906

Cited by 124 publications

(118 citation statements)

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“…We adopted this approach to calculate solution BDEs for inorganic compounds, including manganese hydroxide and iron bi-imidazoline complexes (Mayer 1998). Pecoraro et al and our group both found that a number of manganese hydroxide complexes have BDEs slightly below that of tyrosine, as required for the HAT model of the S-state transitions (Baldwin and Pecoraro 1996;Caudle and Pecoraro 1997;Wang and Mayer 1997). In general, the reactivity of metal complexes toward HAT reactions -such as abstraction of H from a C-H or an O-H bond -correlates well with their BDEs (Mayer 1998;Mayer 2004).…”

Section: ðEmentioning

confidence: 99%

Models for Proton-coupled Electron Transfer in Photosystem II

et al. 2006

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The coupling of proton and electron transfers is a key part of the chemistry of photosynthesis. The oxidative side of photosystem II (PS II) in particular seems to involve a number of proton-coupled electron transfer (PCET) steps in the S-state transitions. This mini-review presents an overview of recent studies of PCET model systems in the authors' laboratory. PCET is defined as a chemical reaction involving concerted transfer of one electron and one proton. These are thus distinguished from stepwise pathways involving initial electron transfer (ET) or initial proton transfer (PT). Hydrogen atom transfer (HAT) reactions are one class of PCET, in which H(+) and e (-) are transferred from one reagent to another: AH + B --> A + BH, roughly along the same path. Rate constants for many HAT reactions are found to be well predicted by the thermochemistry of hydrogen transfer and by Marcus Theory. This includes organic HAT reactions and reactions of iron-tris(alpha-diimine) and manganese-(mu-oxo) complexes. In PS II, HAT has been proposed as the mechanism by which the tyrosine Z radical (Y(Z)*) oxidizes the manganese cluster (the oxygen evolving complex, OEC). Another class of PCET reactions involves transfer of H(+) and e (-) in different directions, for instance when the proton and electron acceptors are different reagents, as in AH-B + C(+) --> A-HB(+) + C. The oxidation of Y(Z) by the chlorophyll P680 + has been suggested to occur by this mechanism. Models for this process - the oxidation of phenols with a pendent base - are described. The oxidation of the OEC by Y(Z)* could also occur by this second class of PCET reactions, involving an Mn-O-H fragment of the OEC. Initial attempts to model such a process using ruthenium-aquo complexes are described.

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Section: ðEmentioning

confidence: 99%

Models for Proton-coupled Electron Transfer in Photosystem II

et al. 2006

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“…On B fi C an additional l-oxo bridge is formed preventing a potential increase for the subsequent oxidation step. (Baldwin and Pecoraro 1996). The given value of 0.8 V for the difference in the midpoint potential of B fi E in comparison A fi B represents the mean value of (E m III,IV/IV,IV ) E m III,III/III,IV ) of 10 di-l-oxo bridged Mn 2 -complexes; the individual values range from 0.6 to 1 V (Goodson et al 1990; see also increased E m for B fi C in comparison to A fi B in Figure 2).…”

Section: The Acceptor-base Problemmentioning

confidence: 99%

“…The given value of 0.8 V for the difference in the midpoint potential of B fi E in comparison A fi B represents the mean value of (E m III,IV/IV,IV ) E m III,III/III,IV ) of 10 di-l-oxo bridged Mn 2 -complexes; the individual values range from 0.6 to 1 V (Goodson et al 1990; see also increased E m for B fi C in comparison to A fi B in Figure 2). The difference between the pK-values of B and A of 10 units also represents a mean value (three complexes; Baldwin and Pecoraro 1996). and $20 units, respectively.…”

Section: The Acceptor-base Problemmentioning

confidence: 99%

“…In the present work, the focus is on the interrelation between proton removal and two mechanistic 'problems' of water oxidation (both have been discussed in the past; e.g., in Krishtalik 1989;Baldwin and Pecoraro 1996): (I) How can the Mn 4 Ca complex be oxidized in four successive steps without significant increase in its redox potential? (II) How are the proton accepting bases provided that are most likely needed in the OAO bond formation step?…”

Section: Introductionmentioning

confidence: 99%

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Considerations on the mechanism of photosynthetic water oxidation – dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water

Dau

Haumann

2005

Photosynth Res

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Two mechanistic problems of photosynthetic water oxidation at the Mn complex of Photosystem II (PS II) are considered. (I) In the four Mn-oxidizing transitions, any pure Mn oxidation is predicted to cause an increase in redox potential that renders subsequent oxidation steps impossible (redox-potential problem). Formation of unprotonated oxo-bridges may counteract the potential increase. (II) The O-O formation step without any high-pK bases acting as proton acceptors is energetically unfavorable (acceptor-base problem). The pK of oxides in a bridging position between Mn ions may increase drastically upon reduction of Mn in the water-oxidation step (>10 units), thus rendering them favorable proton acceptors. It is proposed that in PS II, in the course of the four oxidizing transitions at least two unprotonated oxo-bridges are formed. Thereby (i) a redox potential increase is prevented and (ii) proton acceptors are prepared for the O-O formation step. Water oxidation in the O-O bond formation step is facilitated by simultaneous Mn reduction and proton transfer to bridging oxides amounting to hydrogen atom or hydride transfer from substrate water to the Mn-oxo core of the Mn complex of PS II.

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“…In an alternate description of reaction sequences, a strong pre-equilibrium binding of NO Precedences of such differences in basicities of the oxo bridges in di-or multinuclear higher-valent Mn complexes are well-established [33], and these differences are a key feature in redox steps of the Kok cycle in PS II [34]. The possibility of protonation at the oxo bridge of 1 is ruled out as during its synthesis from Fe(NO 3 ) 3 · 9H 2 O and 1,10-phenanthroline in a 1 : 2 ratio, a pH as low as ca.…”

mentioning

confidence: 99%

Mechanistic Studies on the Oxidation of Nitrite by a μ‐Oxodiiron(III,III) Complex in Aqueous Acidic Media

Bhattacharyya

Mukhopadhyay

2005

Helvetica Chimica Acta

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. The order of reactivity of the oxidant species is 1 > 2 > 3, in agreement with the progressive cationic charge reduction. The reactions appear to be inner-sphere where the initial one-electron proton-coupled redox (1e À , 1H + ; electroprotic) seems to be rate-determining.Introduction. -Proteins containing nonheme, nonsulfur, carboxylato-or oxobridged diiron sites have not escaped the apparently irresistible tendency of chemists and biochemists to classify natural phenomenon (quoted from [1]). Most of these proteins react with dioxygen as a part of their functional processes, which are perhaps the mostly studied among many of their biological processes. Among these studies, exploration of the structures of several diiron(II) sites and insights into the high-valent oxodiiron intermediates are included. Synthetic chemistry has already suggested many novel structures that are structural and sometimes also functional models for these high-valent iron species as well as for diiron(II) sites [2].However, reactivity studies of these model di-or polyiron species appear to be occasional [3], and mechanistic studies are extremely rare [4] outside a protein environment, whereas quite a lot of reactivity and kinetic studies on addition reactions to iron centers and subsequent redox changes of diiron sites in ribonucleotide reductase (RNR) [5], hemerythrin (Hr), semimetHr and metHr [6] with a broad spectrum of small ligands as well as reducing agents are known. The lack of information on reactivity of model oxodi-or oxopolyiron complexes is possibly due to the high stability of the m-oxodiiron(III,III) unit that translates to inertness under a variety of conditions and is, in fact, one likely reason for the availability of quite a large number of synthetic complexes with the FeÀOÀFe unit.The complex salt [Fe 2 (m-O)(phen) 4 (H 2 O) 2 ](NO 3 ) 4 · 5H 2 O (phen = 1,10-phenanthroline; 1; Fig. 1) selected for the present investigation, is an attractive Raman-spectroscopy model for the binuclear iron site in the RNR and metHr, the oxidized form of the oxygen-transport protein hemerythrin [7]. Its conjugate base, [Fe 2 3+ (2) is also a possible functional model for the purple acid phosphatase [2a] [8]. The dinuclear complex 1 is soluble in H 2 O, and the solution is fairly stable towards self-decomposition in a wide pH range (3.0 -7.0) in presence of added 1,10-

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Energetics of Proton-Coupled Electron Transfer in High-Valent Mn₂(μ-O)₂ Systems: Models for Water Oxidation by the Oxygen-Evolving Complex of Photosystem II

Cited by 124 publications

References 23 publications

Models for Proton-coupled Electron Transfer in Photosystem II

Models for Proton-coupled Electron Transfer in Photosystem II

Considerations on the mechanism of photosynthetic water oxidation – dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water

Mechanistic Studies on the Oxidation of Nitrite by a μ‐Oxodiiron(III,III) Complex in Aqueous Acidic Media

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Energetics of Proton-Coupled Electron Transfer in High-Valent Mn2(μ-O)2 Systems: Models for Water Oxidation by the Oxygen-Evolving Complex of Photosystem II

Cited by 124 publications

References 23 publications

Models for Proton-coupled Electron Transfer in Photosystem II

Models for Proton-coupled Electron Transfer in Photosystem II

Considerations on the mechanism of photosynthetic water oxidation – dual role of oxo-bridges between Mn ions in (i) redox-potential maintenance and (ii) proton abstraction from substrate water

Mechanistic Studies on the Oxidation of Nitrite by a μ‐Oxodiiron(III,III) Complex in Aqueous Acidic Media

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Energetics of Proton-Coupled Electron Transfer in High-Valent Mn₂(μ-O)₂ Systems: Models for Water Oxidation by the Oxygen-Evolving Complex of Photosystem II