Theoretical understanding of the interprotein electron transfer between cytochrome c2 and the photosynthetic reaction center

Interprotein electron transfer (ET) reactions play a central role in the early steps in bioenergetic processes such as photosynthesis and respiration. This paper describes initial advances in the theoretical understanding of the interprotein electron transfer between cytochrome c, and the photosynthetic reaction center in Rhodobactersphaeroides from calculations based on the recently determined X-ray crystal structure.(1) Dynamical effects at the protein-protein interface of the docked complex, which mediates the ET, are investigated by calculating the protein and solvent (low-frequency) contributions to the reorganization energy. Since this interface contains almost no solvent, the low-frequency contributions to the reorganization energy are small (a few tenths of an electronvolt). The small value of the reorganization energy is consistent with experimental results if high-frequency (quantum) modes due to cofactor vibrations are taken into account. The results indicate that the mutational effects on the Franck-Condon factor are likely to be small. Influences on the tunneling matrix element have also been investigated. Utilizing an approach that integrates molecular dynamics and the Pathways method, we have observed that the ensemble dominant tunneling pathways in this reaction go though the tyrosine L162 and can be water mediated.