Influence of the energy of H-bond protons on the electron transfer rate in photosynthetic reaction centers

We present here a theoretical interpretation of the temperature dependence of the rate of dark recombination between a primary quinone (Q_A) and a bacteriochlorophyll dimer in the reaction center of Rhodobacter sphaeroides. We were able to describe qualitatively the nonmonotonous character of this dependence using the energy of interaction between an excess electron and H-bond protons. We considered a molecular model of Q_A and two reaction center fragments that make H-bonds with Q_A: His(M219) and Asn(M259)-Ala(M260). We used the two-center approach with regard for electron-phonon interaction in order to calculate the characteristic time of electron tunneling during the recombination reaction. The energy of the phonon emitted/ absorbed during the electron tunneling was determined by the relative shift of donor and acceptor energy levels, the detuning of levels. The detuning was shown to depend on temperature nonmonotonously for H-bonds with double-well potential energy surface. The characteristic time (or the reaction rate) depended on temperature parametrically. The computed dependence was in qualitative agreement with the experimental one.