Kinetic analysis of nonphotochemical quenching of chlorophyll fluorescence. 2. Isolated light-harvesting complexes

The chlorophyll fluorescence yield of purified photosystem II light-harvest ing complexes can be lowered by manipulation of experimental conditions. In several important respects, this quenching resembles the nonphotochemical quenching observed in isolated chloroplasts and leaves, therefore providing a model system for investigating the underlying mechanism. A methodology b ased on the principles of enzyme kinetic analysis has already been applied to isolated chloroplasts, and this same experimental approach was used here with purified LHCIIb, CP26, and CP29. It was found that the kinetics of th e decrease in fluorescence yield robustly fitted a second-order kinetic mod el with respect to time after induction of quenching. The second-order rate constant was dependent upon the complex that was analyzed, the detergent c oncentration, the solution pH, and the presence of exogenous xanthophyll cy cle carotenoids. In contrast, the formation of an absorbance chan-e at 683 nm that accompanies quenching displayed first-order kinetics. The reversal of quenching also displayed second-order kinetics. These data show that que nching results from a binary reaction, possibly arising between two chlorop hyll molecules. On the basis of these data, a model for the regulation of n onphotochemical quenching based upon the allosteric control of the conforma tion of light-harvesting complexes by protonation and xanthophyll binding i s presented.