Quenching of Chlorophyll Fluorescence by Triplets in Solubilized Light-Harvesting Complex II (LHCII)

AbstractThe quenching of chlorophyll fluorescence by triplets in solubilized trimeric light harvesting complexes was analyzed by comparative pump-probe experiments that monitor with weak 2-ns probe pulses the fluorescence yield and changes of optical density, ΔOD, induced by 2-ns pump pulses. By using a special array for the measurement of the probe fluorescence (Schödel R., F. Hillman, T. Schrötter, K.-D. Irrgang, J. Voight, and G. Renger, 1996. Biophys. J. 71:3370–3380) the emission caused by the pump pulses could be drastically reduced so that even at highest pump pulse intensities, IP, no significant interference with the signal due to the probe pulse was observed. The data obtained reveal: a) at a fixed time delay of 50ns between pump and probe pulse the fluorescence yield of the latter drastically decreased with increasing IP, b) the recovery of the fluorescence yield in the μs time domain exhibits kinetics which are dependent on IP, c) ΔOD at 507nm induced by the pump pulse and monitored by the probe pulse with a delay of 50ns (reflecting carotenoid triplets) increases with IP without reaching a saturation level at highest IP values, d) an analogous feature is observed for the bleaching at 675nm but it becomes significant only at very high IP values, e) the relaxation of ΔOD at 507nm occurs via a monophasic kinetics at all IP values whereas ΔOD at 675nm measured under the same conditions is characterized by a biphasic kinetics with τ values of about 1μs and 7–9μs. The latter corresponds with the monoexponential decay kinetics of ΔOD at 507nm. Based on a Stern-Volmer plot, the time-dependent fluorescence quenching is compared with the relaxation kinetics of triplets. It is shown that the fluorescence data can be consistently described by a quenching due to triplets