Carotenoid triplet states in reaction centers from Rhodopseudomonas sphaeroides and Rhodospirillum rubrum

Abstract

Purified photochemical reaction centers from three strains of Rhodopseudomonas sphaeroides and two of Rhodospirillum rubrum were reduced with Na 2S 2O 4 so as to block their photochemical electron transfer reactions. They then were excited with flashes lasting 5–30 ns. In all cases, absorbance measurements showed that the flash caused the immediate formation of a transient state (P F) which had been detected previously in reaction centers from Rps. sphaeroides strain R26. Previous work has shown that state P F is an intermediate in the photochemical electron transfer reaction in the reaction centers of that particular strain, and the present work generalizes that conclusion. In the reaction centers from two strains that lack carotenoids ( Rps. sphaeroides R26 and R. rubrum G9), the decay of P F yields a longer-lived state (P R) which is probably a triplet state of the bacteriochlorophyll of the reaction center. In the R26 preparation, the decay of P F was found to have a half-time of 10±2 ns. The decay kinetics rule out the identification of P F as the fluorescent excited singlet state of the reaction center. In the reaction centers from three strains that contain carotenoids ( Rps sphaeroides 2.4.1 and Ga, and R. rubrum S1), state P R was not detected, and the decay of P F generated triplet states of carotenoids. The efficiency of the coupling between the decay of P F and the formation of the carotenoid triplet appeared to be close to 100% at room temperature, but somewhat lower at 77 °K. Taken with previous results, this suggests that the coupling is direct and does not require the intermediate formation of state P R. This conclusion would be consistent with the view that P F is a biradical which can be triplet in character.