Electronic energy transfer involving carotenoid pigments in chlorosomes oftwo green bacteria: Chlorobium tepidum and Chloroflexus aurantiacus

Electronic energy transfer processes in chlorosomes isolated from the green sulphur bacterium Chlorobium tepidum and from the green filamentous bacter ium Chloroflexus aurantiacus have been investigated. Steady-state fluoresce nce excitation spectra and time-resolved triplet-minus-singlet (TmS) spectr a, recorded at ambient temperature and under non-reducing or reducing condi tions, are reported. The carotenoid (Car) pigments in both species transfer their singlet excitation to bacteriochlorophyll c (BChlc) with an efficien cy which is high (between 0.5 and 0.8) but smaller than unity; BChlc and ba cteriochlorophyll a (BChla) transfer their tripler excitation to the Car's with nearly 100% efficiency. The lifetime of the Car tripler states is appr oximately 3 mu s, appreciably shorter than that of the Car triplets in the light-harvesting complex II (LHCII) in green plants and in other antenna sy stems. In both types of chlorosomes the yield of BChlc triplets (as judged from the yield of the Car triplets) remains insensitive to the redox condit ions. 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 triplet s originate either from a large BChlc pool which is essentially non-fluores cent and non-responsive to changes in the redox conditions, or as a result of a process which quenches BChlc singlet excitation and becomes more effic ient under non-reducing conditions. In chlorosomes from Cfx. aurantiacus wh ose Car content is lowered, by hexane extraction, to 10% of the original va lue, 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 mig ration of tripler excitation; the BChlc triplets which escape rapid static quenching contribute a depletion signal at the long-wavelength edge of the Q(y) absorption band, indicating the existence of at least two pools of BCh lc. (C) 2000 Elsevier Science B.V. All rights reserved.