MOLECULAR-ORGANIZATION OF BACTERIOCHLOROPHYLL IN CHLOROSOMES OF THE GREEN PHOTOSYNTHETIC BACTERIUM CHLOROFLEXUS-AURANTIACUS - STUDIES OF FLUORESCENCE DEPOLARIZATION ACCOMPANIED BY ENERGY-TRANSFER PROCESSES

Examination was made of changes in fluorescence polarization plane by energy transfer in the chlorosomes of the green photosynthetic bacteri um, Chloroflexus aurantiacus. Fluorescence anisotropy in the picosecon d (ps) time region was analyzed using chlorosomes suspended in solutio n as well as those oriented in a polyacrylamide gel. When the main com ponent of BChl c was preferentially excited, the decay of fluorescence anisotropy was found to depend on wavelength. In the chlorosome suspe nsion, the anisotropy ratio of BChl c changed from 0.31 to 0.24 within 100 ps following excitation. In the baseplate BChl a region, this rat io decreased to a negative value (-0.09) from the initial 0.14. In ori ented samples, the degree of polarization remained at 0.68 for BChl c, and changed from 0.25 to -0.40 for the baseplate BChl a by excitation light whose electric vector was parallel to the longest axis of chlor osomes. In the latter case, there was a shift from 0.30 to -0.55 by ex citation perpendicular to the longest axis. Time-resolved fluorescence polarization spectra clearly indicated extensive changes in polarizat ion plane accompanied by energy transfer. The directions of polarizati on plane of emission from oriented samples were mostly dependent on ch lorosome orientation in the gel but not on that of the polarization pl ane of excitation light. Orientations of the dipole moment of fluoresc ence components was consistent with that of absorption components as d etermined by the linear dichroism (Matsuura et al. (1993) Photochem. P hotobiol. 57: 92-97). A model for molecular organization of BChl c and a in chlorosomes is proposed based on anisotropic optical properties.