TRIPLET ENERGY-TRANSFER BETWEEN BACTERIOCHLOROPHYLL AND CAROTENOIDS IN B850 LIGHT-HARVESTING COMPLEXES OF RHODOBACTER-SPHAEROIDES R-26.1

The build-up and decay of bacteriochlorophyll (BChl) and carotenoid tr iplet states were studied by flash absorption spectroscopy in (a) the B800-850 antenna complex of Rhodobacter (Rb.) sphaeroides wild type st rain 2.4.1, (b) the Rb. sphaeroides R-26.1 B850 light-harvesting compl ex incorporated with spheroidene, (c) the B850 complex incorporated wi th 3,4-dihydrospheroidene, (d) the B850 complex incorporated with 3,4, 5,6-tetrahydrospheroidene and (e) the Rb. sphaeroides R-26.1 B850 comp lex lacking carotenoids. Steady state absorption and circular dichrois m spectroscopy were used to evaluate the structural integrity of the c omplexes. The transient data were fit according to either single or do uble exponential rate expressions. The triplet lifetimes of the carote noids were observed to be 7.0 +/- 0.1 mu s for the B800-850 complex, 1 4 +/- 2 mu s for the B850 complex incorporated with spheroidene, and 1 9 +/- 2 mu s for the B850 complex incorporated with 3,4-dihydrospheroi dene. The BChl triplet lifetime in the B850 complex was 80 +/- 5 mu s. No quenching of BChl triplet states was seen in the B850 complex inco rporated with 3,4,5,6-tetrahydrospheroidene. For the B850 complex inco rporated with spheroidene and with 3,4-dihydrospheroidene, the percent age of BChl quenched by carotenoids was found to be related to the per centage of carotenoid incorporation. The triplet energy transfer effic iencies are compared to the values for singlet energy transfer measure d previously (Frank et al. (1993) Photochem. Photobiol. 57: 49-55) on the same samples. These studies provide a systematic approach to explo ring the effects of state energies and lifetimes on energy transfer be tween BChls and carotenoids in vivo.