TRANSFER AND TRAPPING OF EXCITATION ENERGY IN PHOTOSYSTEM II AS STUDIED BY CHLOROPHYLL a2 FLUORESCENCE QUENCHING BY DINITROBENZENE AND CAROTENOID TRIPLET THE MATRIX MODEL

1. The curves representing the reciprocal fluorescence yield of chlorophyll a of Photosystem II (PS II) in Chlorella vulgaris as a function of the concentra-tion of m~linitrobenzene i the states P Q and P Q-, are found to be straight parallel ines; P is the primary donor and Q the primary acceptor of PS II. In the weakly trapping state P Q- the half-quenching of dinitrobenzene is about 0.2 mM, in vitro it is of the order of 10 mM. The fluorescence yield as a func-tion of the concentration of a quencher is described for three models for the structure of pigment systems: the model of separate units, the model of limited energy transfer between the units, and the matrix model. If it is assumedthat the rate constant of quenching by dinitrobenzene is high and thus the number of dinitrobenzene molecules per reaction center low, it can be concluded that the pigment system of PS II in C. vulgaris is a matrix of chlorophyll molecules in which the reaction centers are embedded. Theoretical and experimental evidence is consistent with such an assumption. For Cyanidium caldarium the zero fluorescence yield ~0 and its quenching by dinitrobenzene were found to be much smaller than the corresponding quantities fo r C. vulgaris. Nevertheless, our measurements on C. caldarium could be interpreted by the assumption that the essential properties (rate con-stants, dinitrobenzene quenching) of PS II are the same for these two species belonging to such widely different groups