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

[3-Vinyl]-13(2)-OH-bacteriochlorophyll a has been selectively exchanged against native bacteriochlorophyll a in the monomer binding sites at the A- and B-branch of the photosynthetic reaction centre from Rhodobacter sphaeroides. Optical absorption and absorption-detected magnetic resonance experiments were performed on these samples at 8 K. The site-specific interaction of only pigment B-B (at the electron transfer-inactive 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 bacteriochlorophyll 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 the primary donor was observed around 807 nm. The microwave induced absorption spectra of the primary donor triplet in the modified preparations are quite distinct from those of native reaction centres revealing specific pigment-pigment interactions between the monomers and the primary donor triplet state. These interactions appear as band shifts of the Q(y)-transitions of the accessory monomers, and are in agreement with a primary donor triplet that is delocalized over both dimer parts.