Photooxidation and triplet formation of the primary electron donor of the green photosynthetic bacterium Prosthecochloris aestuarii observed with ESR spectroscopy

Relatively little is known about the photochemical reaction in green photosynthetic bacteria. The high antenna bacteriochlorophyll to reaction center ratio of 1000-2000 BChl c + a per reaction center [ 1,2] and the difficulty of obtaining purified reaction center preparations has impeded the study of the electron transport by means of optical and ESR spectroscopy. In [3], we reported the isolation of photochemically active preparations ofl’rosthecochloris aestuarii. Starting from the membrane vesicle preparation complex I [2,4,5], the photosystem pigment (PP) complex and the reaction center pigment-protein (RCPP) complex were isolated, with antenna BChl a to reaction center ratios of -75 and 35, respectively [3]. At room temperature both complexes show photooxidation of P-840, the primary electron donor, and of 1 cytochrome c-553/reaction center [3]. Upon flash excitation the formation of the triplet state of P-840 was observed with optical difference spectroscopy in both complexes [6]. At <120 K the amount of P-840 triplet formed in a saturating flash was 0.4405 in the PP complex and close to 1 /reaction center in the RCPP complex. Here, we report on the kinetics of photooxidized P-840 and its triplet state monitored at 5 K with ESR and optical spectroscopy. Depending on the redox potential, the photooxidation of P-840 is reversible in the PP complex, yielding a dimer type BChl (I free radical ESR signal at g = 2.0025. The triplet of P-840 is found to be spin polarized; the zero-field splitting parameters and the decay times of the different triplet levels are calculated. An average decay time of 280 ~.ts was found. This, and the value of the zero field param-