CHLOROSOMES OF GREEN SULFUR BACTERIA - PIGMENT COMPOSITION AND ENERGY-TRANSFER

The pigment composition and energy transfer pathways in isolated chlor osomes of Chlorobium phaeovibrioides and Chlorobium vibrioforme were s tudied by means of high performance liquid chromatography (HPLC) and p icosecond absorbance difference spectroscopy. Analysis of pigment extr acts of the chlorosomes revealed that they contain small amounts of ba cteriochlorophyll (BChl) a esterified with phytol, whereas the BChls c , d and e are predominantly esterified with farnesol. The chlorosomal BChl a content in C. phaeovibrioides and C. vibrioforme was found to b e 1.5% and 0.9%, respectively. The time resolved absorbance difference spectra showed a bleaching shifted to longer wavelengths as compared to the Qy absorption maxima and in chlorosomes of C. vibrioforme also an absorbance increase at shorter wavelengths was observed. These spec tral features were ascribed to excitation of oligomers of BChl e and B Chl c/d, respectively. 'One-color' and 'two-color' pump-probe kinetics of C. phaeovibrioides showed rapid energy transfer to long-wavelength absorbing BChl e oligomers, followed by trapping of excitations by BC hl a with a time constant of about 60 ps. Time resolved anisotropy mea surements in C. vibrioforme showed randomization of excitations among BChl a molecules with a time constant of about 20 ps, indicating that BChl a in the baseplate is organized in clusters. One-color and two-co lor pump-probe measurements in C. vibrioforme showed rapid energy tran sfer from short-wavelength to long-wavelength absorbing oligomers with a time constant of about 11 ps. Trapping of excitations by BChl a in this species could not be resolved unambiguously due to annihilation p rocesses in the BChl a clusters, but may occur with time constants of 15, 70 and 200 ps.