Pulsed ENDOR of the photoexcited triplet states of bacteriochlorophyll a and of the primary donor P865 in reaction centers of Rhodobacter sphaeroides R-26

The photoexcited tripler states of bacteriochlorophyll a, (3)BChl a, and of the primary donor in reaction centers of Rhodobacter sphaeroides R-26, P-3(865), are investigated by pulsed EPR and ENDOR spectroscopy. In P-3(865) a splitting of ENDOR lines and reduction of corresponding positive and negative hyperfine couplings as compared with the monomeric (3)BChl a is observed. This indicates an asymmetric distribution of the triplet excitation over the two BChl a moieties, P-L and P-M forming P-3(865). Based on the signs of the hyperfine couplings and on a comparison with the cation and anion radical of BChl a an assignment to nuclei in the different dimer halves is proposed. This yields an estimate for the extent of delocalization of the triplet excitation over P-L and P-M and for the charge transfer contribution of P-3(865).