ABSORPTION AND ADMR STUDIES ON BACTERIAL PHOTOSYNTHETIC REACTION CENTERS WITH MODIFIED PIGMENTS

[3-Vinyl]-13(2)-OH-bacteriochlorophyll a has been selectively exchange d against native bacteriochlorophyll a in the monomer binding sites at the A- and B-branch of the photosynthetic reaction centre from Rhodob acter sphaeroides. Optical absorption and absorption-detected magnetic resonance experiments were performed on these samples at 8 K. The sit e-specific interaction of only pigment B-B (at the electron transfer-i nactive branch) with the triplet state on the neighbouring carotenoid, spheroidene, clearly demonstrates the selective pigment exchange. The absorption spectra allow a distinction of the two monomer bacteriochl orophyll pigments (native as well as modified bacteriochlorophylls) by their absorption band maximum positions in native reaction centres: B -A absorbs at 803 nm, B-B at 812 nm. The upper exciton component of th e primary donor was observed around 807 nm. The microwave induced abso rption spectra of the primary donor triplet in the modified preparatio ns are quite distinct from those of native reaction centres revealing specific pigment-pigment interactions between the monomers and the pri mary donor triplet state. These interactions appear as band shifts of the Q(y)-transitions of the accessory monomers, and are in agreement w ith a primary donor triplet that is delocalized over both dimer parts.