The mutation of carotenoids affects the energy transfer in LH2 light harvesting complexes from Rhodobacter sphaeroides 601 at room temperature

Energy transfer in two kinds of peripheral antenna complexes LH2 from Rhodobacter sphaeroides 601 was studied by absorption, fluorescence emission, time-resolved fluorescence and femtosecond transient absorption spectroscopy at room temperature. These two complexes are LH2 (RS601) and green carotenoid mutated LH2 (GM309). The obtained results demonstrate that: (a) compared with spheroidenes, which have ten carbon-carbon double bonds in native RS601, carotenoids in GM309 were identified as containing neurosporenes with nine carbon-carbon double bonds, which show a significant blue shift of ∼20 nm in the three absorption peaks because of the higher energy levels of neurosporene than those of spheroidene, (b) the higher energy levels of neurosporene in GM309 induce a lower B800→B850 energy transfer rate and efficiency as compared to that in RS601 resulting from the relatively higher band gap between the donor of B800 and the bridge of the carotenoids, (c) the same lifetime of the B850 excited singlet state is observed in these two LH2 complexes.