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

Mechanism of charge separation and stabilization of separated charges in native and mutant reaction centers of Rhodobacter sphaeroides excited by 20-fs pulses at 90 K. Andrey Yakovlev¹, Ludmila Vasilieva², Anatoly Shkuropatov², Michael Jones³, Vladimir Shuvalov^{*,1, 2}, ¹ Department of Photobiophysics, Belozersky Institute of Chemical and Physical of Moscow State University, Moscow, Russia² Institute of Basic Biological Problems, Russian Academy of Sciences, Pushchino, Moscow region, Russia³ Dept. of Biochemistry, School of Medical Sciences, Univ. of Bristol, Bristol, UK

ABSTRACT- The nuclear wavepacket formed by 20-fs excitation on the P* potential energy surface in native and mutant (YM210L and GM203L) reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides RCs was found to be reversibly transferred to the P+BA- surface at 120 femtosecond (fs) delays (monitored by measurements of BA- absorption at 1020 nm). The YM210L mutant RCs show the most simple pattern of fs oscillations with a period of 230 fs in stimulated emission from P* and with the initial amplitude comparable to that in plant pheophytin a (Pheo)-modified Rb. sphaeroides RCs. Similar reversible oscillations are observed in the 1020-nm band of the mutants, the initial amplitude of which is similar for GM203L and smaller by a factor of 10 for YM210L with respect to Pheo-modified Rb. sphaeroides RCs. In contrast to native and Pheo-modified Rb. sphaeroides RCs, irreversible quasi-exponential stabilization of P+BA- is considerably suppressed in the mutant YM210L RCs in the picosecond (ps) time domain. The stabilization time is delayed in the GM203L mutant by a factor of 3.9 due to the absence of the water molecule HOH55 in RCs. The water rotational mode with a frequency of 32 cm^-1 and its overtones, described earlier (Yakovlev et al., Biochemistry, 2002, 41, 14019), are decreased in the YM210L mutants and strongly suppressed in the GM203L mutant RCs. The mechanism of the electron transfer pathway between P* and BA, including the polar atoms and HOH55 between P and BA, and of the stabilization of the state P+BA- in bacterial RCs are discussed.

KEY WORDS: primary charge separation, femtosecond nuclear wavepacket, bacterial reaction centers