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 sh ow for the LH2 complexes from Rhodobacter sphaeroides strains 2.4.1 and G1C at early times after light excitation a superposition of bacteriochlorophy ll and carotenoid triplet states. In LH1 complexes from Rhodospirillum rubr um and LH2 complexes from Chromatium purpuratum and Rhodopseudomonas acidop hila, 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 bacteriochlo rophyll triplet is accompanied by an apparently simultaneous rise of the ca rotenoid triplet. Transient optical absorption spectroscopy demonstrates id entical kinetics for the decay and rise of bacteriochlorophyll and caroteno id tripler states, respectively. The lifetime of the carotenoid triplet sta te shows only a minor temperature dependence in all species. The rate const ant for the rise of the carotenoid triplet state is strongly temperature an d species dependent. The triplet energy transfer in LH2 complexes from lib. sphaeroides 2.4.1 is about 1 order of magnitude slower than in Rps. acidop hila. Even the complexes from the Rb. sphaeroides strains 2.4.1 and G1C sho w differences. In Rb. sphaeroides G1C, a partial freezing out of the triple t energy transfer is observed which does not occur in any other investigate d complex. The observed temperature dependence of the triplet energy transf er from bacteriochlorophylls to carotenoids is discussed in terms of the di stance difference between the carotenoid and the two inequivalent bacterioc hlorophyll molecules.