Rate of Carotenoid Triplet Formation in Solubilized Light-Harvesting Complex II (LHCII) from Spinach

AbstractIn the present study the rate of triplet transfer from chlorophyll to carotenoids in solubilized LHCII was investigated by flash spectroscopy using laser pulses of ∼2ns for both pump and probe. Special attention has been paid to calibration of the experimental setup and to avoid saturation effects. Carotenoid triplets were identified by the pronounced positive peak at ∼507nm in the triplet-singlet difference spectra. ΔOD (507nm) exhibits a monoexponential relaxation kinetics with characteristic lifetimes of 2–9μs (depending on the oxygen content) that was found to be independent of the pump pulse intensity. The rise of ΔOD (507nm) was resolved via a pump probe technique where an optical delay of up to 20ns was used. A thorough analysis of these experimental data leads to the conclusion that the kinetics of carotenoid triplet formation in solubilized LHCII is almost entirely limited by the lifetime of the excited singlet state of chlorophyll but neither by the pulse width nor by the rate constant of triplet-triplet transfer. Within the experimental error the rate constant of triplet-triplet transfer from chlorophyll to carotenoids was estimated to be kTT>(0.5ns)−1. This value exceeds all data reported so far by at least one order of magnitude. The implications of this finding are briefly discussed