Microscopic and spectroscopic studies of untreated and hexanol-treated chlorosomes from Chloroflexus aurantiacus

When isolated chlorosomes from Chloroflexus aurantiacus are treated with 1-hexanol, the BChl cQ_y absorption band shifts from 740 to 670 nm, while the baseplate BChl a remains at 795 nm. The relative amount of BChl c in the 740 and 670 nm forms depends on the hexanol concentration. Atomic force microscopy was used to study the ultrastructure of native, hexanol-treated, and protein-free chlorosomes. Chlorosomes appeared to be larger and more rounded upon hexanol treatment and did not return to the original shape or size after 2-fold dilution. Therefore, the hexanol treatment is not completely reversible in terms of chlorosome structure. Untreated, hexanol-treated and and hexanol-treated and then diluted samples were investigated using steady-state and time-resolved fluorescence spectroscopy. For the sample treated with 68 mM hexanol, a 24 ps energy transfer from BChl c to a was observed in the picosecond fluorescence measurements. After 2-fold dilution, most of the kinetic properties of the untreated chlorosomes, characterized by a major energy transfer component of 15 ps from BChl c 740 to BChl a 795, were regained. Energy transfer from either BChI c 740 or BChl c 670 to baseplate BChl a is fast and relatively efficient in untreated chlorosomes. In hexanol-treated chlorosomes, the excited state lifetime is not very different from that in untreated samples, but the energy transfer efficiency is quite low. This may result from concentration quenching of the monomeric pigments in the hexanol-treated Chorosomes.