The formation of excited states and energy transfer in chlorosomes of the green photosynthetic bacteria Chlorobium limicola and Chloroflexus aurantiacus were studied by measurements of flash-induced absorbance changes and fluorescence. Upon excitation with 35 ps, 532 nm flashes, large absorbance decreases around 750 nm were observed that were due to the disappearance of ground state absorption of the main pigment, bacteriochlorophyll (BChl) c. The absorbance changes decayed after the flash with a time constant of approx. 1 ns, together with faster components. Absorbance changes that could be ascribed to formation of excited BChl a were much smaller than those of BChl c. The yields of BChl c and BChl a fluorescence were measured as a function of the energy density of the exciting flash. At high energy a strong quenching occurred caused by annihilation of singlet excited states. An analysis of the results shows that energy transfer between BChl c molecules is very efficient and that in C. limicola excitations can probably move freely through the entire chlorosome (which contains about 10 000 BChls c). The chlorosome thus serves as a common antenna for several reaction centres. The small amounts of BChl a present in the chlorosomes of both species form clusters of only a few molecules. Upon cooling to 4 K the sizes of the domains of BChl c for energy transfer decreased considerably. The results are discussed in relation to recently suggested models for the pigment organization within chlorosomes.
Excitation transfer in chlorosomes of green photosynthetic bacteria
GEROLA, PAOLO;
1987-01-01
Abstract
The formation of excited states and energy transfer in chlorosomes of the green photosynthetic bacteria Chlorobium limicola and Chloroflexus aurantiacus were studied by measurements of flash-induced absorbance changes and fluorescence. Upon excitation with 35 ps, 532 nm flashes, large absorbance decreases around 750 nm were observed that were due to the disappearance of ground state absorption of the main pigment, bacteriochlorophyll (BChl) c. The absorbance changes decayed after the flash with a time constant of approx. 1 ns, together with faster components. Absorbance changes that could be ascribed to formation of excited BChl a were much smaller than those of BChl c. The yields of BChl c and BChl a fluorescence were measured as a function of the energy density of the exciting flash. At high energy a strong quenching occurred caused by annihilation of singlet excited states. An analysis of the results shows that energy transfer between BChl c molecules is very efficient and that in C. limicola excitations can probably move freely through the entire chlorosome (which contains about 10 000 BChls c). The chlorosome thus serves as a common antenna for several reaction centres. The small amounts of BChl a present in the chlorosomes of both species form clusters of only a few molecules. Upon cooling to 4 K the sizes of the domains of BChl c for energy transfer decreased considerably. The results are discussed in relation to recently suggested models for the pigment organization within chlorosomes.
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