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 triplet states of bacteriochlorophyll a, ³BChl a, and of the primary donor in reaction centers of Rhodobacter sphaeroides R-26, ³P₈₆₅, are investigated by pulsed EPR and ENDOR spectroscopy. In ³P₈₆₅ a splitting of ENDOR lines and reduction of corresponding positive and negative hyperfine couplings as compared with the monomeric ³BChl a is observed. This indicates an asymmetric distribution of the triplet excitation over the two BChl a moieties, P_L and P_M, forming ³P₈₆₅. 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 ³P₈₆₅.