Influence of electronic asymmetry on the spectroscopic and photodynamic properties of the primary electron donor in the photosynthetic reaction center

Resonance Raman and subpicosecond resolution transient absorption data are presented on the reaction center (RC) from the Rb. sphaeroides (M)H202L/(L)L131H double mutant. This RC contains a bacteriochlorophyll/bacteriopheophytin heterodimer primary electron donor (D), as a result of introducing a Leu at M subunit residue 202, as well as a hydrogen bond to the ring-V keto group of the BChl component, as a result of introducing a His at L subunit residue 131. The double mutant shows a longer lifetime (similar to 40 ps) for the lowest excited singlet state and a lower quantum yield of charge separation (similar to 25%) than observed previously for the heterodimer single mutants (M)H202L and (L)H173L. These changes are ascribed in large measure to a decrease in the contribution of the accessory pigment BChl(L) to the initial stage of charge separation. This perturbation in turn results from an upshift in the free energy of D(+)BChl(L)(-) relative to P(+)BChl(L)(-) (where P is the homodimer primary donor of wild-type RCs). This shift is due to the substantially elevated oxidation potential of the heterodimer in the (M)H202L/(L)L131H double mutant (ca. +765 meV) relative to that of the homodimer in wild-type RCs (ca. +500 meV). Both the rate of internal conversion of the excited primary donor and the characteristics of the near-infrared ground slate absorption of the dimer appear to be intermediate between those found for the (M)H202L single mutant and wild type RCs. Resonance Raman profiles obtained via excitation at several wavelengths across the heterodimer absorption band suggest that there is a change in the nature and perhaps ordering of the two lowest excited states of the heterodimer in the double mutant compared with the single mutant. Collectively, these data indicate that the addition of a hydrogen bond to the more easily oxidized (and less easily reduced) BChl component decreases the inherent electronic asymmetry of the heterodimer and partially restores characteristics of the wild-type homodimer.