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

The 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, Δ OD, 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, I P, no significant interference with the signal due to the probe pulse was observed. The data obtained reveal: a) at a fixed time delay of 50 ns between pump and probe pulse the fluorescence yield of the latter drastically decreased with increasing I P, b) the recovery of the fluorescence yield in the μs time domain exhibits kinetics which are dependent on I P, c) Δ OD at 507 nm induced by the pump pulse and monitored by the probe pulse with a delay of 50 ns (reflecting carotenoid triplets) increases with I P without reaching a saturation level at highest I P values, d) an analogous feature is observed for the bleaching at 675 nm but it becomes significant only at very high I P values, e) the relaxation of Δ OD at 507 nm occurs via a monophasic kinetics at all I P values whereas Δ OD at 675 nm measured under the same conditions is characterized by a biphasic kinetics with τ values of about 1 μs and 7–9 μs. The latter corresponds with the monoexponential decay kinetics of Δ OD at 507 nm. 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.