Proton Displacements Coupled to Primary Electron Transfer in the Rhodobacter Sphaeroides Reaction Center

Abstract

Using first-principles molecular dynamics (AIMD) and constrained density functional theory (CDFT) we identify the pathway of primary electron transfer in the R. Sphaeroides reaction center from the special pair excited state (P*) to the accessory bacteriochlorophyll (BA). Previous AIMD simulations on the special pair (PLPM) predicted a charge-transfer intermediate formation through the excited-state relaxation along a reaction coordinate characterized by the rotation of an axial histidine (HisM202). To account for the full electron transfer we extend the model to include the primary acceptor BA. In this extended model, the LUMO is primarily localized on the acceptor BA and extends over an interstitial water (water A) that is known to influence the rate of electron transfer (Potter et al. Biochemistry 2005 280, 27155–27164). A vibrational analysis of the dynamical trajectories gives a frequency of 30–35 cm–1 for a molecular motion involving the hydrogen-bond network around water A, in good agreement with experimental findings (Yakovlev et al. Biochemistry, 2003, 68, 603–610). In its binding pocket water A can act as a switch by breaking and forming hydrogen bonds. With CDFT we calculate the energy required to the formation of the charge-separated state and find it to decrease along the predicted anisotropic reaction coordinate. Furthermore, we observe an increased coupling between the ground and charge-separated state. Water A adapts its hydrogen-bonding network along this reaction coordinate and weakens the hydrogen bond with HisM202. We also present AIMD simulations on the radical cation (P•+) showing a weakening of the hydrogen bond between HisL168 and the 31-acetyl of PL. This work demonstrates how proton displacements are crucially coupled to the primary electron transfer and characterizes the reaction coordinate of the initial photoproduct formation.