Journal
JOURNAL OF PHYSICAL CHEMISTRY B
Volume 117, Issue 38, Pages 11162-11168Publisher
AMER CHEMICAL SOC
DOI: 10.1021/jp401195t
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Funding
- NWO Physical Sciences
- Netherlands Organization for Scientific Research (NWO)
- BioSolar Cells open innovation consortium
- Dutch Ministry of Economic Affairs, Agriculture and Innovation [C1.9]
- NWO-ECHO project [713.011.002]
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Using first-principles molecular dynamics (AIM])) 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 (B-A). 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 B-A. In this extended model, the LUMO is primarily localized on the acceptor B-A 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 center dot+) showing a weakening of the hydrogen bond between HisL168 and the 3(1)-acetyl of P-L. This work demonstrates how proton displacements are crucially coupled to the primary electron transfer and characterizes the reaction coordinate of the initial photoproduct formation.
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