Femtosecond dynamics of carotenoid-to-bacteriochlorophyll a energy transfer in the light-harvesting antenna complexes from the purple bacterium Chromatium purpuratum Original Research Article

The energy transfer from the carotenoid okenone to bacteriochlorophyll a (Bchl a) in the light harvesting complex B800–830 and chromatophores of Chromatium purpuratum was studied by steady-state fluorescence and fermtosecond transient absorption spectroscopy. By comparing the fluorescence excitation and the absorption spectra of the B800–830 antenna complex the total energy transfer efficiency of the okenone-to-Bchl a transfer was determined to 95 ± 5%. A fast (<200 fs) transfer from (at least) one carotenoid was observed to the B830 chlorophylls. This transfer was found by fs anisotropy measurements to be a transfer from the okenone S2 state to the Bchl a Qx state, probably caused by the dipole-dipole (Förster) mechanism. Later the low-lying okenone S1 state, which has a lifetime of ∼8ps in solution, was observed transferring energy to the B830 pool in ∼3.8 ps, while a second carotenoid transferred its energy also from the S1 state to B800 in ∼0.5 ps. The results are discussed in relation to the crystal structure of the B800–850 complex of Rhodopseudomonas acidophila [McDermott et al. Nature 374 (1995)].