2Institute of Basic Biological Problems, Russian Academy of Sciences, 142290 Pushchino, Moscow Region, Russia; fax: (496) 779-0532; E-mail: firstname.lastname@example.org
* To whom correspondence should be addressed.
Received March 12, 2009; Revision received May 18, 2009
Difference femtosecond absorption spectroscopy with 20-fsec temporal resolution was applied to study a primary stage of charge separation and transfer processes in reaction centers of YM210L and YM210L/FM197Y site-directed mutants of the purple bacterium Rhodobacter sphaeroides at 90 K. Photoexcitation was tuned to the absorption band of the primary electron donor P at 880 nm. Coherent oscillations in the kinetics of stimulated emission of P* excited state at 940 nm and of anion absorption of monomeric bacteriochlorophyll BA– at 1020 nm were monitored. The absence of tyrosine YM210 in RCs of both mutants leads to strong slowing of the primary reaction P* → P+BA– and to the absence of stabilization of separated charges in the state P+BA–. Mutation FM197Y increases effective mass of an acetyl group of pyrrole ring I in the bacteriochlorophyll molecule PB of the double mutant YM210L/FM197Y by a hydrogen bond with OH-TyrM197 group that leads to a decrease in the frequency of coherent nuclear motions from 150 cm–1 in the single mutant YM210L to ~100 cm–1 in the double mutant. Oscillations with 100-150 cm–1 frequencies in the dynamics of the P* stimulated emission and in the kinetics of the reversible formation of P+BA– state of both mutants reflect a motion of the PB molecule relatively to PA in the area of mutual overlapping of their pyrrole rings I. In the double mutant YM210L/FM197Y the oscillations in the P* emission band and the BA– absorption band are conserved within a shorter time ~0.5 psec (1.5 psec in the YM210L mutant), which may be a consequence of an increase in the number of nuclei forming a wave packet by adding a supplementary mass to the dimer P.
KEY WORDS: photosynthesis, charge separation, reaction center, wave packet, electron transfer