Transfer and trapping of excitation energy in Photosystem II as studied by chlorophyll a2 fluorescence quenching by dinitrobenzene and carotenoid triplet. The matrix model

Abstract

1. The curves representing the reciprocal fluorescence yield of chlorophyll a of Photosystem II (PS II) in Chlorella vulgaris as a function of the concentration of m-dinitrobenzene in the states P Q and P Q -, are found to be straight parallel lines; 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 function 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 assumed that 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 Ф 0 and its quenching by dinitrobenzene were found to be much smaller than the corresponding quantities for C. vulgaris. Nevertheless, our measurements on C. caldarium could be interpreted by the assumption that the essential properties (rate constants, dinitrobenzene quenching) of PS II are the same for these two species belonging to such widely different groups. 2. The measured dinitrobenzene concentrations required for half-quenching in vivo and other observations are explained by (non-rate-limiting) energy transfer between the chlorophyll a molecules of PS II and by the assumptions that dinitrobenzene is approximately distributed at random in the membrane and does not diffuse during excitation. 3. The fluorescence kinetics of C. vulgaris during a 350 ns laser flash of variable intensity could be simulated on a computer using the matrix model. From the observed fluorescence quenching by the carotenoid triplet (C T) and the measurement of the number of C T per reaction center via difference absorption spectroscopy, the rate constant for quenching of C T is calculated to be k T = 3.3 · 10 11 s −1 which is almost equal to the rate constant of trapping by an open reaction center (Duysens, L.N.M. (1979) CIBA Foundation Symposium 61 (New Series), pp. 323–340). 4. The fluorescence quenching by C T in non-treated spinach chloroplasts after a 500 ns laser flash (Breton, J., Geacintov, N.E. and Swenberg, C.E. (1979) Biochim. Biophys. Acta 548, 616–635) could be explained within the framework of the matrix model when the value for k T is used as given in point 3. 5. The observations mentioned under point 1 indicate that the fluorescence yield Ф 0 for centers in trapping state P Q is probably for a fraction exceeding 0.8 emitted by PS II.