Protein Medium\nFacilitates Electron Transfer in Photosynthetic\nHeliobacterial Reaction Center

This computational study addresses the question of how\nlarge membrane-bound\nproteins of electron transport chains facilitate fast vector-based\ncharge transport. We study electron transfer reactions following ultrafast\ninitial charge separation induced by absorption of light by P<sub>800</sub> primary pair and leading to the electron localization at\nthe A<sub>0</sub> cofactor. Two subsequent, much slower reactions,\nelectron transfer to the iron–sulfur cluster F<sub><i>x</i></sub> and reduction of the menaquinone (MQ) cofactor,\nare studied by combining molecular dynamics simulations, electronic\nstructure calculations, and theoretical modeling. The low value of\nthe electronic coupling between A<sub>0</sub> and F<sub><i>x</i></sub> brings this reaction to the microsecond time scale even at\nthe zero activation barrier. In contrast, A<sub>0</sub>-MQ electron\ntransfer occurs on a subnanosecond time scale and might become the\npreferred route for charge transport. We elucidate mechanistic properties\nof the protein medium allowing fast, vectorial charge transfer. The\nelectric field is high and inhomogeneous inside the protein and is\ncoupled to high polarizabilities of cofactors to significantly lower\nthe reaction barrier. The A<sub>0</sub>-MQ separation puts this reaction\nat the edge between the plateau characterizing the reaction dynamical\ncontrol and exponential falloff due to electronic tunneling. A strong\nseparation in relaxation times of the medium dynamics for the forward\nand backward reactions promotes vectorial charge transfer.