Optical and optically detected magnetic resonance investigation on purple photosynthetic bacterial antenna complexes Original Research Article

Low temperature (10K) absorption, fluoroscence emission, fluorescence excitation, absorption detected magnetic resonance, and microwave induced absorption spectra were recorded together with temperature dependent light induced absorption transients of carotenoid triplet formation and decay in the B800–850 antenna complexes of Rb. sphaeroides G1C, Rb. sphaeroides 2.4.1, Rps. acidophila 10050, Rps. palustris I, and Rps. palustris II (low-light), in B800–820 antenna complexes of Rps. acidophila 7050, and Rps. cryptolactis, and in the B830 complex of Chr. purpuratum. From the spectra the zero field splitting parameters |D| and |E| were extracted as well as the energies of the main carotenoid triplet-triplet and singlet-singlet transitions. They are linearly dependent on the inverse of the number of conjugated double bonds in the polyene chain. The rates of triplet carotenoid formation correlate with the relative intensity of the carotenoid-bacteriochlorophyll interaction bands observed in the 800–850 nm region in the microwave induced absorption spectra. Both, the triplet energy transfer rate and the interaction bands depend on the exchange interaction between the excited and ground state wavefunctions of the two pigments.