Ultrafast carotenoid-to-chlorophyll singlet energy transfer in the light-harvesting complex LHCII.

Abstract

Carotenoids serve as accessory light-harvesting pigments in photosynthesis absorbing light in the blue-green spectral region and efficiently transferring energy to neighbouring chlorophylls. In the light-harvesting complex LHCII of higher plants there are a large number of possible pathways between the lowest excited singlet states of the different carotenoids and chlorophylls a and b. We have made fluorescence upconversion measurements of the excited S2 state decay lifetimes of both lutein and the mixture of xanthophylls extracted from LHCII and dissolved in simple solvents. In benzyl alcohol, chosen as a model of the protein environment, the carotenoid emission shows a major decay component of ~120 fs. In contrast, the carotenoid emission observed upon excitation of the LHCII trimers at 501 nm was only ~26 fs. The same measurement made after addition of the detergent TX100, which disrupts energy transfer from both chlorophyll b and the xanthophylls to chlorophyll a, resulted in a decay time constant of 116 fs. These results suggest an ultrafast energy transfer pathway from the S2 state of lutein to the chlorophylls with a ~78% efficiency. The emission wavelength dependence of the chlorophyll rise kinetics was also studied after excitation of LHCII at 501 nm, or 470 nm, and the observation of an ultrafast component in the rise kinetics is consistent with energy transfer from the carotenoid S2 state.