Triplet EPR spectra of the primary electron donor in bacterial photosynthesis at temperatures between 15 and 296 K

Abstract Spin polarized triplet EPR spectra of reaction centers of the photosynthetic bacterium Rhodopseudomonas sphaeroides 2.4.1 and its carotenoidless mutant R-26 have been measured from 15 to 296 K with direct detection flash-EPR. For the mutant, the zero-field parameter | D | increases appreciably with temperature making it unlikely that at higher temperatures a donor-acceptor charge-transfer state is mixed in the triplet state of the primary electron donor, 3 D. g -values along the canonical field directions have been measured, and spin-lattice relaxation dominates over molecular decay at temperatures > 133 K. For the wild type, the temperature dependence of the rate of triplet transfer from 3 D to reaction center carotenoid has been measured, and it is shown that the difficulty in observing 3 Car above 200 K is due to fast spin-lattice relaxation.