Transient EPR and Absorption Studies of Carotenoid Triplet Formation in Purple Bacterial Antenna Complexes

The dynamics of triplet excited states in light harvesting complexes from a range of different purple bacteria have been investigated by time-resolved EPR and optical absorption spectroscopy. The spin-polarized EPR spectra show for the LH2 complexes from Rhodobacter sphaeroides strains 2.4.1 and G1C at early times after light excitation a superposition of bacteriochlorophyll and carotenoid triplet states. In LH1 complexes from Rhodospirillum rubrum and LH2 complexes from Chromatium purpuratum and Rhodopseudomonas acidophila, only a carotenoid triplet spectrum is observed. In all complexes from every species, a time-dependent change in the polarization pattern of the carotenoid triplet is seen. In those complexes that show a superposition of bacteriochlorophyll and carotenoid triplets, the decay of the bacteriochlorophyll triplet is accompanied by an apparently simultaneous rise of the carotenoid triplet. Transient optical absorption spectroscopy demonstrates identical kinetics for the decay and rise of bacteriochlorophyll and carotenoid triplet states, respectively. The lifetime of the carotenoid triplet state shows only a minor temperature dependence in all species. The rate constant for the rise of the carotenoid triplet state is strongly temperature and species dependent. The triplet energy transfer in LH2 complexes from Rb. sphaeroides 2.4.1 is about 1 order of magnitude slower than in Rps. acidophila . Even the complexes from the Rb. sphaeroides strains 2.4.1 and G1C show differences. In Rb. sphaeroides G1C, a partial freezing out of the triplet energy transfer is observed which does not occur in any other investigated complex. The observed temperature dependence of the triplet energy transfer from bacteriochlorophylls to carotenoids is discussed in terms of the distance difference between the carotenoid and the two inequivalent bacteriochlorophyll molecules.