PARENT SESSION
Posters P6A Type II reaction centres: Excited state dynamics and donor side. Abstracts (313-346)

Prediction of inelastic electron tunneling mechanism from bacteriopheophytin to the primary quinone in modified bacterial photosynthetic reaction centers. Hirotaka Nishioka^*,1, Tsutomu Kawatsu², Akihiro Kimura¹, Takahisa Yamato¹, Toshiaki Kakitani³, ¹ Department of Physics, Nagoya, Aichi, Japan² Department of Chemistry, Durham, North Carolina, USA³ Faculty of Science and Technology, Nagoya, Aichi, Japan

ABSTRACT- We conducted a computational study for the electron transfer from bacteriopheophytin (Bph) to the primary quinone (Q_A) in the reaction center of Rodobacter sphaeroides, taking into account the effects of the thermal fluctuation of protein conformation on the electron transfer matrix element T_DA. Molecular dynamics (MD) simulations are performed for whole protein of reaction center to reproduce thermal fluctuation of protein conformation. The value of T_DA and the electron tunneling pathways are calculated by the quantum chemistry and analyzed by the method of inter-atomic tunneling currents for each conformation. We observed that very rapid ( ∼1 fs) and large ( ∼two orders of magnitude ) fluctuations of T_DA owing to the thermal fluctuation of protein conformation happen. We also found that two main tunneling routes that alternate with time exist. One is the route through the Trp(M252), the other is the route through the Met(M218). These remarkable phenomena show that the conventional electron transfer theory using the Condon approximation, in which T_DA is assumed to be independent of the nuclear dynamics, is not valid. In order to overcome the Condon approximation, we advanced a new analytical method that we take Fourier transform of the product of autocorrelation function as to T_DA fluctuations and the decoherence function due to the nuclear Franck-Condon factor. By arbitrarily changing the energy gap for the electron transfer reaction, we obtained a new energy gap dependence of the electron transfer rate for the reaction centers. Thus obtained energy gap dependence in the normal region and around the maximum region is similar to the conventional one (Marucs energy gap law). However, the electron transfer rate remained almost the same value as a function of the energy gap in the inverted region, in contrast to the conventional one in which the electron transfer rate drops very rapidly in the inverted region. This fact indicates that the inelastic electron tunneling mechanism can work significantly in the system with a large energy gap, which could be prepared by modifying the reaction centers.

KEY WORDS: energy gap law, inelastic electron tunneling, reaction center, fluctuating electronic factor