Kinetic study of PF and CarT states in the LM subunit purified from the wild-type Rhodobacter sphaeroides reaction centers

Abstract

The kinetics of absorbance changes related to the charge-separated state, P(F), and to the formation and decay of the carotenoid triplet state (Car(T)) were studied in the LM reaction center subunit isolated from a wild-type strain of the purple bacterium Rhodobacter sphaeroides (strain Y). The P(F) lifetime is lengthened (20±1.5 ns) in the LM complex as compared to the intact reaction centers (11±1 ns). The yield of the carotenoid triplet formation is higher (0.28±0.01) in the LM complex than in native reaction centers. We interpret our results in terms of perturbations of a first-order reaction connecting the singlet and the triplet state of the radical-pair state. Our results, together with those of a recent work (Agalidis, I., Nuijs, A.M. and Reiss-Husson, F. (1987) Biochim. Biophys. Acta (in press)) are consistent with a high I to QA electron transfer rate in this LM subunit, which is metal-depleted.The LM complex is considerably more sensitive than the reaction centers to photooxidative damage in the presence of oxygen. This is not readily accounted for simply by the higher carotenoid triplet yield, and may suggest a greater accessibility of the internal structures in the absence of the H-subunit.The lifetime of the carotenoid triplet decay (6.4±0.3 μs) in the LM subunit is unchanged compared to the native reaction centers.