Low-temperature fluorescence from single chlorosomes, photosynthetic antenna complexes of green filamentous and sulfur bacteria.

Fluorescence spectra of single chlorosomes isolated from a green filamentous bacterium (Chloroflexus (Cfl.) aurantiacus) and a green sulfur bacterium (Chlorobium (Cb.) tepidum) were measured by using a confocal laser microscope at 13 K. Chlorosomes were frozen either in a liquid solution (floating chlorosome) or on a quartz plate after being adsorbed (adsorbed chlorosome). Fluorescence peak wavelengths were shorter for the adsorbed single chlorosomes than for the floating ones. Single floating Cfl. chlorosomes showed a distribution of fluorescence peak positions having a center at 759.0 nm with a full width at half maximum of 6.3 nm. Single floating Cb. chlorosomes showed a 782.7 nm center with a full width at half-maximum of 3.4 nm. The distribution shifted to the blue and became wider with increasing temperature, especially in Cb. chlorosomes, suggesting a large excitonic density of states just above the lowest level. Energy transfer from BChl-c aggregates to BChl-a molecules in the baseplate proteins was observed in the floating chlorosomes but not in the adsorbed ones. A positive correlation was found between the peak wavelength of BChl-c fluorescence and the intensity of BChl-a fluorescence in single Cfl. chlorosomes. The results suggest that the BChl-c aggregates with longer wavelengths of the fluorescence peaks have a more efficient Förster-type energy transfer to the baseplate BChl-a.