Excited-state structure and dynamics in FMO antenna complexes from photosynthetic green sulfur bacteria

Absorption difference spectra for the singlet excited states of the Fenna-M atthews-Olson (FMO) complex from the green sulfur bacteria Prosthecochloris aestuarii and Chlorobium tepidum were simulated by exciton theory. The sam e assumptions and parameters were used as applied earlier (Louwe, R. J. W.; Vrieze, J.; Hoff, A. J.; Aartsma, T. J. J. Phys. Chem. B 1997, 101, 11280. Vulto, S. I. E.; de Baat, M. A.; Louwe, R. T. W.; Permentier, H. P.; Neef, T.; Miller, M.; van Amerongen, H.; Aartsma, T. J. J. Phys. Chem. B 1998, 1 02, 9577). The difference spectra show a bleaching near the wavelength of e xcitation, due to ground-state bleaching and stimulated emission. Additiona l negative and positive bands reflect changes in interaction with other bac teriochlorophylls than the one that is mainly excited at the transition fre quency. Simulated spectra were compared with experimental difference spectr a obtained by pump-probe experiments in the femto- and picosecond time regi on with excitation pulses in the spectral range 800-828 nm. In general, goo d agreement was obtained. Various difference spectra developed during the f irst 1 to 2 ps, but in all cases the system relaxed to the lowest energy st ate, which was largely completed in 10 ps.