Chlorosomes of green sulfur bacteria: Pigment composition and energy transfer

The pigment composition and energy transfer pathways in isolated chlorosomes ofChlorobium phaeovibrioides andChlorobium vibrioforme were studied by means of high performance liquid chromatography (HPLC) and picosecond absorbance difference spectroscopy. Analysis of pigment extracts of the chlorosomes revealed that they contain small amounts of bacteriochlorophyll (BChl)a esterified with phytol, whereas the BChlsc, d ande are predominantly esterified with farnesol. The chlorosomal BChla content inC. phaeovibrioides andC. vibrioforme was found to be 1.5% and 0.9%, respectively. The time resolved absorbance difference spectra showed a bleaching shifted to longer wavelengths as compared to the Q_y absorption maxima and in chlorosomes ofC. vibrioforme also an absorbance increase at shorter wavelengths was observed. These spectral features were ascribed to excitation of oligomers of BChle and BChlc/d, respectively. One-color and two-color pump-probe kinetics ofC. phaeovibrioides showed rapid energy transfer to long-wavelength absorbing BChle oligomers, followed by trapping of excitations by BChla with a time constant of about 60 ps. Time resolved anisotropy measurements inC. vibrioforme showed randomization of excitations among BChla molecules with a time constant of about 20 ps, indicating that BChla in the baseplate is organized in clusters. One-color and two-color pump-probe measurements inC. vibrioforme showed rapid energy transfer from short-wavelength to long-wavelength absorbing oligomers with a time constant of about 11 ps. Trapping of excitations by BChla in this species could not be resolved unambiguously due to annihilation processes in the BChla clusters, but may occur with time constants of 15, 70 and 200 ps.