REORGANIZATION ENERGY OF THE INITIAL ELECTRON-TRANSFER STEP IN PHOTOSYNTHETIC BACTERIAL REACTION CENTERS

The reorganization energy (lambda) for electron transfer from the prim ary electron donor (P) to the adjacent bacteriochlorophyll (B) in pho tosynthetic bacterial reaction centers is explored by molecular-dynami cs simulations. Relatively long (40 ps) molecular-dynamics trajectorie s are used, rather than free energy perturbation techniques. When the surroundings of the reaction center are modeled as a membrane, lambda for PB --> P+B- is found to be similar to 1.6 kcal/mol. The results a re not sensitive to the treatment of the protein's ionizable groups, b ut surrounding the reaction center with water gives higher values of l ambda (similar to 6.5 kcal/mol). In light of the evidence that P+B- li es slightly below P in energy, the small lambda obtained with the mem brane model is consistent with the speed and temperature independence of photochemical charge separation. The calculated reorganization ener gy is smaller than would be expected if the molecular dynamics traject ories had sampled the full conformational space of the system. Because the system does not relax completely on the time scale of electron tr ansfer, the lambda obtained here probably is more pertinent than the l arger value that would be obtained for a fully equilibrated system.