Coupling of nuclear wavepacket motion and charge separation in bacterial reaction centers

Abstract

The mechanism of the charge separation and stabilization of separated charges was studied using the femtosecond absorption spectroscopy. It was found that nuclear wavepacket motions on potential energy surface of the excited state of the primary electron donor P* leads to a coherent formation of the charge separated states P +B A −, P +H A − and P +H B − (where B A, H B and H A are the primary and secondary electron acceptors, respectively) in native, pheophytin-modified and mutant reaction centers (RCs) of Rhodobacter sphaeroides R-26 and in Chloroflexus aurantiacus RCs. The processes were studied by measurements of coherent oscillations in kinetics at 890 and 935 nm (the stimulated emission bands of P*), at 800 nm (the absorption band of B A) and at 1020 nm (the absorption band of B A −) as well as at 760 nm (the absorption band of H A) and at 750 nm (the absorption band of H B). It was found that wavepacket motion on the 130–150 cm −1 potential surface of P* is accompanied by approaches to the intercrossing region between P* and P +B A − surfaces at 120 and 380 fs delays emitting light at 935 nm (P*) and absorbing light at 1020 nm (P +B A −). In the presence of Tyr M210 ( Rb. sphaeroides) or M195 ( C. aurantiacus) the stabilization of P +B A − is observed within a few picosseconds in contrast to YM210W. At even earlier delay (∼40 fs) the emission at 895 nm and bleaching at 748 nm are observed in C. aurantiacus RCs showing the wavepacket approach to the intercrossing between the P* and P +H B − surfaces at that time. The 32 cm −1 rotation mode of HOH was found to modulate the electron transfer rate probably due to including of this molecule in polar chain connecting P B and B A and participating in the charge separation. The mechanism of the charge separation and stabilization of separated charges is discussed in terms of the role of nuclear motions, of polar groups connecting P and acceptors and of proton of OH group of TyrM210.