Photochemical trapping of a bacteriopheophytin anion in site‐specific reaction‐center mutants from the photosynthetic bacterium Rhodobacter sphaeroides

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Abstract
The mutant YY in the rection center of Rhodobacter sphaeroide, in which Phe181 on the L chain has been replaced by Tyr, and the double mutant FY, With Tyr210 on th M chain replaced by Phe and Phe181 on the L chain replaced by Tyr, have been constructed by site‐directed mutagenesis. The studies described here were performed to complement a previous mutational analysis of mutant FF with Tyr210 replaced by Phe. Both new strains grow photohererotrophically. The optical absorption spectra of reaction centers isolated from these mutants have band shifts attributable to the monomer bacteriochlorophylls in the vicinity of the substitutions. Photochemical trapping of the bacteriopheophytin anion (I−) indicates that the bacteriopheophytin on the B branch is reduced to a much greater extent in FF and FY as compared to YY and wild‐type YF. Low temperature (77K) absorption spectra clearly show that in the wild‐type (YF) and YY rection centers only the 545‐nm‐absorbing bacteriopheophytin is reduced while in the FF and FY reaction centers both the 535‐nm and 545‐nm‐absorbing bacteriopheophytins are reduced. A simple kinetic analysis is used to ecplain these results. This analysis suggests that, in order for the observed trapping results to occur, a decrease in the ‘cycling’ time must take place, that is changes in the rate(s) of charge recombination must accompany the already known decrease in the forward election transfer rate.