Energetics of the Electron Transfer from Bacteriopheophytin to Ubiquinone in the Photosynthetic Reaction Center of Rhodopseudomonas Viridis: Theoretical Study

Abstract

The energetics of electron transfer in the photosynthetic reaction center of Rhodopseudomonas viridis was studied using the density functional theory (DFT). By examining the basis set-dependence and the accuracy of the DFT for calculating adiabatic electron affinity, single-point calculations with 6-31+G(d) basis sets, at the geometry optimized with 6-31G(d) basis sets, were found to be almost independent of the basis set. In gas-phase calculations, bacteriopheophytin (H) had the greatest electron affinity among the three chromophores: H, menaquinone (MQ), and ubiquinone (UQ). However, the order of the electron affinity was reversed to be UQ > MQ > H by including residues that interacted with the chromophores through hydrogen bonding. Based on the QM/MM optimized geometries, cluster models for the binding sites were constructed. The computed reaction energy was comparable to values obtained experimentally. The reaction energy can be decomposed into a vertical electron affinity term and a relaxation energ...