Mechanism by which a single water molecule affects primary charge separation kinetics in a bacterial photosynthetic reaction center of Rhodobacter sphaeroides

作者：Ivashin N V^*; Shchupak E E

来源：Optics and Spectroscopy, 2012, 113(5): 474-486.

DOI：10.1134/S0030400X1209007X

Using quantum-chemical methods, we have studied the role played by water molecules W-A and W-B that are bound by hydrogen bonds to accessory bacteriochlorophyll molecules B (A) and B (B) in the process of primary charge separation in the reaction center of Rhodobacter Sphaeroides. We have found that the occurrence of a rotational mode of the W-A molecule at 32 cm(-1) and/or its harmonics in stimulated emission of an electron donor P* and the dynamics of population of the states P+B (A) (-) and P+H (A) (-) may be related to the structural heterogeneity of the reaction center and the existence of a conformation in which the W-A molecule is predominantly involved in one hydrogen bond (with BA). Based on the calculated redox potentials B (A) and P, it has been shown that the appearance of the W-A molecule in the reaction center reduces the energy of the P+B (A) (-) state by 600 cm(-1). This is somewhat smaller than the influence of the amino-acid residue Tyr(M210) ( 870 cm(-1)) and correlates well with a substantial decrease in the electron transfer rate in mutant forms of reaction centers GM203L (which do not contain W-A molecules) and YM210F (in which Tyr(M210) is replaced with Phe). The data obtained allow us to suggest that rotation of the water molecule with a fixed position of its H atom that is involved in a hydrogen bond with the keto carbonyl group of B (A) is initiated due to the charge separation between the halves of special pair P and the formation of the state P (A) (+) P (B) (-) . The large effect of this rotation on the kinetics of population of the states P+B (A) (-) and P+H (A) (-) after the excitation of P is quite consistent with its influence on the energy of the state P+B (A) (-) .

• 出版日期2012-11