Electrostatic Effects on Proton Coupled Electron Transfer in Oxomanganese Complexes Inspired by the Oxygen-Evolving Complex of Photosystem II
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- 作者：Muhamed Amin ; Leslie Vogt ; Serguei Vassiliev ; Ivan Rivalta ; Mohammad M. Sultan ; Doug Bruce ; Gary W. Brudvig ; Victor S. Batista ; M. R. Gunner
- 刊名：The Journal of Physical Chemistry B
- 出版年：2013
- 出版时间：May 23, 2013
- 年：2013
- 卷：117
- 期：20
- 页码：6217-6226
- 全文大小：461K
- 年卷期：v.117,no.20(May 23, 2013)
- ISSN：1520-5207
文摘
The influence of electrostatic interactions on the free energy of proton coupled electron transfer in biomimetic oxomanganese complexes inspired by the oxygen-evolving complex (OEC) of photosystem II (PSII) are investigated. The reported study introduces an enhanced multiconformer continuum electrostatics (MCCE) model, parametrized at the density functional theory (DFT) level with a classical valence model for the oxomanganese core. The calculated pKa鈥檚 and oxidation midpoint potentials (Em鈥檚) match experimental values for eight complexes, indicating that purely electrostatic contributions account for most of the observed couplings between deprotonation and oxidation state transitions. We focus on pKa鈥檚 of terminal water ligands in [Mn(II/III)(H2O)6]2+/3+ (1), [Mn(III)(P)(H2O)2]3鈥?/sup> (2, P = 5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrinato), [Mn2(IV,IV)(渭-O)2(terpy)2(H2O)2]4+ (3, terpy = 2,2鈥?6鈥?2鈥?terpyridine), and [Mn3(IV,IV,IV)(渭-O)4(phen)4(H2O)2]4+ (4, phen = 1,10-phenanthroline) and the pKa鈥檚 of 渭-oxo bridges and Mn Em鈥檚 in [Mn2(渭-O)2(bpy)4] (5, bpy = 2,2鈥?bipyridyl), [Mn2(渭-O)2(salpn)2] (6, salpn = N,N鈥?/i>-bis(salicylidene)-1,3-propanediamine), [Mn2(渭-O)2(3,5-di(Cl)-salpn)2] (7), and [Mn2(渭-O)2(3,5-di(NO2)-salpn)2] (8). The analysis of complexes 6鈥?b>8 highlights the strong coupling between electron and proton transfers, with any Mn oxidation lowering the pKa of an oxo bridge by 10.5 卤 0.9 pH units. The model also accounts for changes in the Em鈥檚 by ligand substituents, such as found in complexes 6鈥?b>8, due to the electron withdrawing Cl (7) and NO2 (8). The reported study provides the foundation for analysis of electrostatic effects in other oxomanganese complexes and metalloenzymes, where proton coupled electron transfer plays a fundamental role in redox-leveling mechanisms.