TEMPERATURE-DEPENDENT TRIPLET AND FLUORESCENCE QUANTUM YIELDS OF THE PHOTOSYSTEM-II REACTION-CENTER DESCRIBED IN A THERMODYNAMIC MODEL

A key step in the photosynthetic reactions in photosystem II of green plants is the transfer of an electron from the singlet-excited chlorop hyll molecule called P680 to a nearby pheophytin molecule. The free en ergy difference of this primary charge separation reaction is determin ed in isolated photosystem II reaction center complexes as a function of temperature by measuring the absolute quantum yield of P680 triplet formation and the time-integrated fluorescence emission yield. The to tal triplet yield is found to be 0.83 +/- 0.05 at 4 K, and it decrease s upon raising the temperature to 0.30 at 200 K. It is suggested that the observed triplet states predominantly arise from P680 but to a min or extent also from antenna chlorophyll present in the photosystem II reaction center. No carotenoid triplet states could be detected, demon strating that the contamination of the preparation with CP47 complexes is less than 1/100 reaction centers. The fluorescence yield is 0.07 /- 0.02 at 10 K, and it decreases upon raising the temperature to reac h a value of 0.05-0.06 at 60-70 K, increases upon raising the temperat ure to 0.07 at similar to 165 K and decreases again upon further raisi ng the temperature. The complex dependence of fluorescence quantum yie ld on temperature is explained by assuming the presence of one or more pigments in the photosystem II reaction center that are energetically degenerate with the primary electron donor P680 and below 60-70 K tra p part of the excitation energy, and by temperature-dependent excited state decay above 165 K. A four-compartment model is presented that de scribes the observed triplet and fluorescence quantum yields at all te mperatures and includes pigments that are degenerate with P680, temper ature-dependent excited state decay and activated upward energy transf er rates. The eigenvalues of the model are in accordance with the life times observed in fluorescence and absorption difference measurements by several workers. The model suggests that the free energy difference between singlet-excited P680 and the radical pair state P680(+)1(-) i s temperature independent, and that a distribution of free energy diff erences represented by at least three values of about 20, 40, and 80 m eV, is needed to get an appropriate fit of the data.