Abstract
In the photosynthetic bacteria, 15-cis carotenoid bound to the reaction center quenches triplet bacteriochlorophyll which is generated when the excess energy is applied. Our group determined that the carotenoid is twisted upon triplet excitation and that the rate constants of intersystem crossing of cis-carotenoids are almost the same as those of isomerization, and proposed a hypothesis of efficient triplet-energy dissipation by the structural change of the 15-cis carotenoid bound to the reaction center. In the present investigation, we measured time-resolved ESR spectra of the reaction center from a purple bacterium Rb. sphaeroides 2.4.1 at 20 - 200 K. Two ESR signals appeared in sequence, after appearing signal of triplet bacteriochlorophyll, could be assigned to the carotenoid having the same structure as in the ground state (3Car(I)) and the carotenoid after twisting (3Car(II)). The data matrix of whole time and temperature range could be explained in terms of the above model.
