Abstract :
A steady-state and ultrafast time-resolved spectroscopic examination is presented at room and cryogenic temperatures of four different LH2 pigment-protein complexes containing the Cars, neurosporene, spheroidene, or rhodopin glucoside. This study has resolved the question regarding the cause of the precipitous drop in energy transfer efficiency when the extent of [pi]-electron conjugation of the carotenoid is extended from ten to eleven conjugated carbon-carbon double bonds in LH2 complexes from purple photosynthetic bacteria.