Transient optical spectroscopy of single crystals of the reaction center from Rhodobacter sphaeroides wild-type 2.4.1

The photoactivity of the crystallized reaction centers from Rhodobacter sphaeroides wild-type strain 2.4.1 has been examined by light-induced absorption spectral changes associated with charge separation and triplet state formation in the reaction center. Upon excitation of a crystal at ambient redox potential, the primary donor 865 nm band bleaches reversibly. The kinetics of its recovery were found to be biphasic with rate constants 11.5 +/- 1.3 s-1 and 0.9 +/- 0.4 s-1 which correspond to lifetimes of 87.0 +/- 9.0 ms and 1.0 +/- 0.7 s, respectively. The ratio of the fast-to-slow component preexponential terms was 3.5 +/- 1.1 suggesting that the majority (78.9 +/- 13.0%) of the reaction centers in the crystals lack the secondary quinone, QB. The addition of sodium ascorbate to the crystals attenuates the 865 nm absorption change, and gives rise to strong carotenoid triplet-triplet absorption changes at 547 nm. These data indicate that the reaction center-bound carotenoid in the crystals is capable of accepting triplet energy from the primary donor triplet.