Article ID | Journal | Published Year | Pages | File Type |
---|---|---|---|---|
9575160 | Chemical Physics | 2005 | 11 Pages |
Abstract
Coherent components in the dynamics of decay of stimulated emission from the primary electron donor excited state P*, and of population of the product charge-separated states P+BA- and P+HA-, were studied in GM203L mutant reaction centers (RCs) of Rhodobacter (Rb.) sphaeroides by measuring oscillations in the kinetics of absorbance changes at 940Â nm (P* stimulated emission region), 1020Â nm (BA- absorption region) and 760Â nm (HA bleaching region). Absorbance changes were induced by excitation of P (870Â nm) with 18Â fs pulses at 90Â K. In the GM203L mutant, replacement of Gly M203 by Leu results in exclusion of the crystallographically defined water molecule (HOH55) located close to the oxygen of the 131-keto carbonyl group of BA and to His M202, which provides the axial ligand to the Mg of the PB bacteriochlorophyll. The results of femtosecond measurements were compared with those obtained with Rb. sphaeroides R-26 RCs containing an intact water HOH55. The main consequences of the GM203L mutation were found to be as follows: (i) a low-frequency oscillation at 32Â cmâ1, which is characteristic of the HOH55-containing RCs, disappears from the kinetics of absorbance changes at 1020 and 760Â nm in the mutant RC; (ii) electron transfer from P* to BA in the wild type RC was characterized by two time constants of 1.1Â ps (80%) and 4.3Â ps (20%), but in the GM203L mutant was characterized by a single time constant of 4.3Â ps, demonstrating a slowing of primary charge separation. The previously postulated rotation of water HOH55 with a fundamental frequency of 32Â cmâ1, triggered by electron transfer from P* to BA, was confirmed by observation of an isotopic shift of the 32Â cmâ1 oscillation in the kinetics of P+BA- population in deuterated, pheophytin-modified RCs of Rb. sphaeroides R-26, by a factor of 1.6. These data are discussed in terms of the influence of water HOH55 on the energetics of the PââP+BA- reaction, and protein dynamic events that occur on the time scale of this reaction.
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Physical Sciences and Engineering
Chemistry
Physical and Theoretical Chemistry
Authors
Andrey G. Yakovlev, Michael R. Jones, Jane A. Potter, Paul K. Fyfe, Lyudmila G. Vasilieva, Anatoli Ya. Shkuropatov, Vladimir A. Shuvalov,