Electronic energy transfer involving carotenoid pigments in chlorosomes of two green bacteria: Chlorobium tepidum and Cholroflexus aurantiacus.

Electronic energy transfer processes in chlorosomes isolated from the green sulphur bacterium Chlorobium tepidum and from the green filamentous bacterium Chloroflexus aurantiacus have been investigated. Steady-state fluorescence excitation spectra and time-resolved triplet-minus-singlet (TmS) spectra, recorded at ambient temperature and under non-reducing or reducing conditions, are reported. The carotenoid (Car) pigments in both species transfer their singlet excitation to bacteriochlorophyll c (BChlc) with an efficiency which is high (between 0.5 and 0.8) but smaller than unity; BChlc and bacteriochlorophyll a (BChla) transfer their triplet excitation to the Car's with nearly 100% efficiency. The lifetime of the Car triplet states is approximately 3 micros, appreciably shorter than that of the Car triplets in the light-harvesting complex II (LHCII) in green plants and in other antenna systems. In both types of chlorosomes the yield of BChlc triplets (as judged from the yield of the Car triplets) remains insensitive to the redox conditions. In notable contrast the yield of BChlc singlet emission falls, upon a change from reducing to non-reducing conditions, by factors of 4 and 35 in Cfx. aurantiacus and Cb. tepidum, respectively. It is possible to account for these observations if one postulates that the bulk of the BChlc triplets originate either from a large BChlc pool which is essentially non-fluorescent and non-responsive to changes in the redox conditions, or as a result of a process which quenches BChlc singlet excitation and becomes more efficient under non-reducing conditions. In chlorosomes from Cfx. aurantiacus whose Car content is lowered, by hexane extraction, to 10% of the original value, nearly one-third of the photogenerated BChlc triplets still end up on the residual Car pigments, which is taken as evidence of BChlc-to-BChlc migration of triplet excitation; the BChlc triplets which escape rapid static quenching contribute a depletion signal at the long-wavelength edge of the Qy absorption band, indicating the existence of at least two pools of BChlc.