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, Delta OD, induced by 2-ns pump pulses. By using a special array for the measurement of the probe fluorescence (Schodel R., F, Hillman, T. Schrotter, 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 mu s time domain exhibits kinetics which are dependent on I-P, c) Delta 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 Delta OD at 507 nm occurs via a monophasic kinetics at all I-P values whereas Delta OD at 675 nm measured under the same conditions is characterized by a biphasic kinetics with tau values of about 1 mu s and 7-9 mu s. The latter corresponds with the monoexponential decay kinetics of Delta 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.