FEMTOSECOND DYNAMICS IN ISOLATED LH2 COMPLEXES OF VARIOUS SPECIES OF PURPLE BACTERIA

Polarized and magic angle two-color femtosecond spectroscopy was used to study B800-850 antenna complexes of the photosynthetic purple bacte ria Rhodopseudomonas acidophila (Rps. acidophila), Rhodobacter sphaero ides (Rb, sphaeroides), and Chromatium tepidum (Chr. tepidum) and the B800-820 complex of Chromatium vinosum at room temperature. As was ear lier found for Chr. tepidum, in the B800-850 complexes of Rps. acidoph ila and Rb. sphaeroides the bleaching signal of B850 was found to be s everal times larger than that of B800, indicating strong exciton inter actions between the bacteriochlorophylls (BChls) in the B850 aggregate . Depolarization of the B850 excited state was found to occur within o ur time resolution of 80 fs. In all species, B800 to B850 or B820 tran sfer took place with a time constant of 0.7 to 0.9 ps. Depolarization studies indicated a transfer time of 1.5 ps between B800 molecules in Rps. acidophila. In Chr. tepidum, B800 depolarizes 2 to 4 times slower , dependent on the wavelength of excitation. Our results indicate that the double band structure of B800 of the latter organism is due to tw o separate pools of BChls, rather than dimeric exciton interaction. Up on excitation of the B800-820 complex of Chr. vinosum at 795 nm, the B 820 absorbance difference spectrum shifted with time to the red by 20 nm, indicating that B820 is spectrally very heterogeneous. A 2 ps down hill energy transfer process within the B820 band is assigned to energ y transfer between aggregated B800-820 complexes. Assuming that the B8 00-820 complex is similar to B800-850, we propose that the large spect ral heterogeneity of B820 does not occur within individual B800-820 co mplexes.