Thermodynamic investigation into the mechanism of the chlorophyll fluorescence quenching in isolated photosystem II light-harvesting complexes.

Chlorophyll fluorescence quenching can be stimulated in vitro in purified photosystem II antenna complexes. It has been shown to resemble nonphotochemical quenching observed in isolated chloroplasts and leaves in several important respects, providing a model system for study of the mechanism of photoprotective energy dissipation. The effect of temperature on the rate of quenching in trimeric and monomeric antenna complexes revealed the presence of two temperature-dependent processes with different activation energies, one between approximately 15 and 35 degrees C and another between approximately 40 and 60 degrees C. The temperature of the transition between the two phases was higher for trimers than for monomers. Throughout this temperature range, the quenching was almost completely reversible, the protein CD was unchanged, and pigment binding was maintained. The activation energy for the low temperature phase was consistent with local rearrangements of pigments within some of the protein domains, whereas the higher temperature phase seemed to arise from large scale conformational transitions. For both phases, there was a strong linear correlation between the quenching rate and the appearance of an absorption band at 685 nm. In addition, quenching was correlated with a loss of CD at approximately 495 nm from Lutein 1 and at 680 nm from chlorophylls a1 and a2, the terminal emitters. The results obtained indicate that quenching of chlorophyll fluorescence in antenna complexes is brought about by perturbation of the lutein 1/chlorophyll a1/chlorophyll a2 locus, forming a poorly fluorescing chlorophyll associate, either a dimer or an excimer.