Triplet energy transfer between bacteriochlorophyll and carotenoids in B850 light-harvesting complexes ofRhodobacter sphaeroides R-26.1.

The build-up and decay of bacteriochlorophyll (BChl) and carotenoid triplet states were studied by flash absorption spectroscopy in (a) the B800-850 antenna complex ofRhodobacter (Rb.)sphaeroides wild type strain 2.4.1, (b) theRb. sphaeroides R-26.1 B850 light-harvesting complex incorporated with spheroidene, (c) the B850 complex incorporated with 3,4-dihydrospheroidene, (d) the B850 complex incorporated with 3,4,5,6-tetrahydrospheroidene and (e) theRb. sphaeroides R-26.1 B850 complex lacking carotenoids. Steady state absorption and circular dichroism spectroscopy were used to evaluate the structural integrity of the complexes. The transient data were fit according to either single or double exponential rate expressions. The triplet lifetimes of the carotenoids were observed to be 7.0±0.1 μs for the B800-850 complex, 14±2 μs for the B850 complex incorporated with spheroidene, and 19±2 μs for the B850 complex incorporated with 3,4-dihydrospheroidene. The BChl triplet lifetime in the B850 complex was 80±5 μs. No quenching of BChl triplet states was seen in the B850 complex incorporated with 3,4,5,6-tetrahydrospheroidene. For the B850 complex incorporated with spheroidene and with 3,4-dihydrospheroidene, the percentage of BChl quenched by carotenoids was found to be related to the percentage of carotenoid incorporation. The triplet energy transfer efficiencies are compared to the values for singlet energy transfer measured previously (Frank et al. (1993) Photochem. Photobiol. 57: 49-55) on the same samples. These studies provide a systematic approach to exploring the effects of state energies and lifetimes on energy transfer between BChls and carotenoids in vivo.