Department of Physics, City College of New York, New York, New York 10031, United States.
J Phys Chem B. 2013 May 23;117(20):6217-26. doi: 10.1021/jp403321b. Epub 2013 May 15.
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's and oxidation midpoint potentials (E(m)'s) 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's of terminal water ligands in Mn(II/III)(H2O)6 (1), Mn(III)(P)(H2O)2 (2, P = 5,10,15,20-tetrakis(2,6-dichloro-3-sulfonatophenyl)porphyrinato), Mn2(IV,IV)(μ-O)2(terpy)2(H2O)2 (3, terpy = 2,2':6',2″-terpyridine), and Mn3(IV,IV,IV)(μ-O)4(phen)4(H2O)2 (4, phen = 1,10-phenanthroline) and the pKa's of μ-oxo bridges and Mn E(m)'s in [Mn2(μ-O)2(bpy)4] (5, bpy = 2,2'-bipyridyl), [Mn2(μ-O)2(salpn)2] (6, salpn = N,N'-bis(salicylidene)-1,3-propanediamine), [Mn2(μ-O)2(3,5-di(Cl)-salpn)2] (7), and [Mn2(μ-O)2(3,5-di(NO2)-salpn)2] (8). The analysis of complexes 6-8 highlights the strong coupling between electron and proton transfers, with any Mn oxidation lowering the pKa of an oxo bridge by 10.5 ± 0.9 pH units. The model also accounts for changes in the E(m)'s by ligand substituents, such as found in complexes 6-8, 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.
受光合作用系统 II (PSII) 中氧析出复合物 (OEC) 启发，模拟的氧合锰配合物中静电相互作用对质子偶联电子转移自由能的影响。本研究提出了一种改进的多构象连续静电 (MCCE) 模型，该模型在密度泛函理论 (DFT) 水平上进行参数化，采用经典价键模型模拟氧合锰核。所计算的 pKa 值和氧化中点电位 (E(m)') 与八个配合物的实验值匹配，表明纯粹的静电贡献解释了观察到的去质子化和氧化态跃迁之间的大部分偶联。我们关注的是 Mn(II/III)(H2O)6 (1)、Mn(III)(P)(H2O)2 (2, P = 5,10,15,20-四(2,6-二氯-3-磺酰基苯基)卟啉)、Mn2(IV,IV)(μ-O)2(terpy)2(H2O)2 (3,terpy = 2,2':6',2″-三联吡啶)、Mn3(IV,IV,IV)(μ-O)4(phen)4(H2O)2 (4, phen = 1,10-菲咯啉) 中末端水分子配体的 pKa 值以及 μ-氧桥和 Mn E(m) 在 [Mn2(μ-O)2(bpy)4] (5, bpy = 2,2'-联吡啶)、[Mn2(μ-O)2(salpn)2] (6, salpn = N,N'-双(水杨醛基)-1,3-丙二胺)、[Mn2(μ-O)2(3,5-二(Cl)-salpn)2] (7) 和 [Mn2(μ-O)2(3,5-二(NO2)-salpn)2] (8) 中的 pKa 值。对配合物 6-8 的分析突出了电子和质子转移之间的强耦合，任何 Mn 氧化都会使氧桥的 pKa 值降低 10.5 ± 0.9 pH 单位。该模型还解释了由于吸电子 Cl (7) 和 NO2 (8) 导致的配体取代对 E(m) '的影响，如在配合物 6-8 中发现的那样。本研究为分析其他氧合锰配合物和金属酶中的静电效应提供了基础，在这些配合物和金属酶中，质子偶联电子转移在氧化还原平衡机制中起着至关重要的作用。
