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 (PSI) 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 pK(a)'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 pK(a)'s of terminal water ligands in [Mn(II/III)(H2O)(6)](2+/3+)(1), [Mn(III)(P)(H2O)2](3-)(2, P = 5,10,15,20-tetrakis (2,6-dichloro-3-sulfonatophenyl)porphyrinato), [Mn-2(IV,IV)(mu-0)(2)(terpy)(2)(H2O)(2))(4+) (3, terpy = 2,2':6',2 ''-terpyridine), and [Mn-3(IV,IV,IV)(mu-O)(4)(Phen)(4)(H2O)(2)](4+) (4, phen = 1,10-phenanthroline) and the pK(a)'s of mu-oxo bridges and Mn E-m's in [Mn-2(mu-O)(2)(bpy)(4)] (5, bpy = 2,2'-bipyridyl), [Mn-2(mu-O)(2)(salpn)(2)] (6, salpn = N,N'-bis(salicylidene)-1,3-propanediamine), [Mn-2(mu-O)(2)(3,5-di(Cl)-salpn)(2)] (7), and [Mn-2(mu-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 pK(a) 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.
- QUANTUM MECHANICS/MOLECULAR MECHANICS
- DENSITY-FUNCTIONAL THERMOCHEMISTRY
- O BOND FORMATION
- O-2-EVOLVING COMPLEX
- WATER OXIDATION
- MOLECULAR CALCULATIONS
- STRUCTURAL MODELS