Temperature and Detection-Wavelength Dependence of the Electron Transfer Rates in Initial Stages of Photosynthesis

Abstract

Unusual temperature behavior, observed in the initial electron transfer stages in the photosynthetic reaction centers of the purple bacteria, and a strong probing pulse wavelength dependence of transfer rates, determined in transient absorption spectroscopy, can easily be explained on assuming that the transfer takes place from dynamically unrelaxed states of protein environment. The transitions from the primary special pair (P) to a single bacteriochlorophyll (B) and next to a bacteriopheophytin (H) are controlled by diffusion down the energy value of underdamped vibrational modes of frequency 200 K, probably determining distances between the succeeding cofactors. The subsequent transition to the quinone A (Q) is controlled by diffusion in the position value of an overdamped conformational mode, probably corresponding to the local polarization. From the fit of available experimental data to simple theoretical formulas, the important physical conclusion arises that the very electronic transitions are fast as compared to the relaxation processes and, in the first approximation, only the latter contribute to the overall times of the initial electron transfer stages in photosynthesis.