Pressure-Induced Red Shift and Broadening of the Qy Absorption of Main Light-Harvesting Antennae Chlorosomes from Green Photosynthetic Bacteria and Their Dependency upon Alkyl Substituents of the Composite Bacteriochlorophylls

Abstract

When pressure was applied to the main light-harvesting apparatus (chlorosomes) isolated from several green photosynthetic bacteria (up to 128 MPa), the Qy-absorption band in an aqueous solution was shifted to longer wavelengths. The shift, deltav, was completely reversible for (de)compression and also showed a linear relation as a function of the applied pressure. The pressure-sensitivity in the deltav was dependent upon the bacterial species. The pressure coefficient, deltav/deltaP, was -565 to -535 cm(-1) GPa(-1) for the chlorosomes from several green sulfur bacteria (Chlorobium species), which have several bacteriochlorophyll(BChl) homologues at the 8- and 12-positions as the antenna pigments. In contrast, a smaller value (-445 cm(-1) GPa(-1)) was estimated for the chlorosomes from the green nonsulfur bacterium (Chloroflexus aurantiacus), which has a single homologue with 8-ethyl and 12-methyl groups. These results were confirmed by the similar pressure dependency of in vitro self-aggregates of isolated BChls-c having various alkyl substituents at the 8- and 12-positions. The present pressurization study enables us to discuss a physiological meaning of a variety of antenna pigments in green photosynthetic bacteria.