Vibrational Dephasing of Long- and Short-Lived Primary Donor Excited States in Mutant Reaction Centers of Rhodobacter sphaeroides

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Abstract
Femtosecond spectroscopy was used to study vibrational dynamics in the first singlet excited state (P*) of the primary donor of bacterial reaction centers (RC)in which primary electron transfer dynamics have been altered by single amino acid modifications. We studied intracytoplasmic RC-only membranes containing Rhodobacter sphaeroides wild-type RCs and RCs bearing mutations in the vicinity of P, where Tyr M210 was modified to His, Leu, and Trp and where Phe L181 was modified to Tyr. These mutations do not change the frequencies of the main low-frequency activated modes, which is consistent with a description in which these modes involve extended regions of the protein. Electron transfer in FL181Y, YM210H, and wild-type RCs at 10 K occurs in approximately 1 ps or less, and damping of the coherences occurs simultaneously with the decay of the P* excited state. These results, and a comparison with YM210L RCs, show that in wild-type RCs the damping is primarily determined by the depletion of P* and not by vibrational dephasing induced by interactions with the bath or nonharmonic coupling. In the YM210L and W mutants, electron transfer occurs on a time scale of hundreds of picoseconds at 10 K. Analysis of the longer-lasting vibrational dynamics in these mutants sets a new lower limit for the intrinsic vibrational dephasing time of 1.2 ps for some modes, but of approximately 2 ps for most activated modes.