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 bacteria Rhodopseudomonas acidophila (Rps. acidophila), Rhodobacter sphaeroides (Rb, sphaeroides), and Chromatium tepidum (Chr. tepidum) and the B800-820 complex of Chromatium vinosum at room temperature. As was earlier found for Chr. tepidum, in the B800-850 complexes of Rps. acidophila and Rb. sphaeroides the bleaching signal of B850 was found to be several times larger than that of B800, indicating strong exciton interactions between the bacteriochlorophylls (BChls) in the B850 aggregate. Depolarization of the B850 excited state was found to occur within our time resolution of 80 fs. In all species, B800 to B850 or B820 transfer 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 two separate pools of BChls, rather than dimeric exciton interaction. Upon excitation of the B800-820 complex of Chr. vinosum at 795 nm, the B820 absorbance difference spectrum shifted with time to the red by 20 nm, indicating that B820 is spectrally very heterogeneous. A 2 ps downhill energy transfer process within the B820 band is assigned to energy transfer between aggregated B800-820 complexes. Assuming that the B800-820 complex is similar to B800-850, we propose that the large spectral heterogeneity of B820 does not occur within individual B800-820 complexes.