Internal conversion and energy transfer dynamics of spheroidene in solution and in the LH-1 and LH-2 light-harvesting complexes

Femtosecond time resolved fluorescence upconversion was applied to determine directly the lifetime of the 1B~ + state of the carotenoid spheroidene in vitro in various polar and nonpolar solvents and in the LH-1 and LH-2 light-harvesting complexes of Rhodobacter sphaeroides at room temperature. The spheroidene IB + ~ 2A~internal conversion time varies from 150 to 250 fs in the solvents studied and depends on the polarizability of the surrounding environment. The internal conversion time in the protein environment is estimated based on the solution measurements. There is a dramatic shortening for the spheroidene 1B + fluorescence lifetime seen inside LH-1 and LH-2 (60 fs and 80 fs, respectively) over this expected internal conversion time. 1. I n t r o d u c t i o n the pigments found in the purple bacter ium