The Study of the Primary Photoprocesses in Photosystem I of Chloroplasts. Recombination Luminescence, Chlorophyll Triplet State and Triplet-triplet Annihilation

The dependence of the delayed luminescence of Photosystem I on the state of the reaction centers has been studied. Light flash induces a charge separation in the centers: P-700-P-430 hv in equilibrium P-700+-P-430-. Dark recombination of charges is accompanied by the recombination luminescence with tau1/2 congruent to 20ms. If the centers are in the P-700-P-430- state or if P-430 is inactivated by heat, then flashing of Photosystem I generates the triplet state chlorophyll with tau1/2 congruent to 0.5 ms. The triplet state has been measured by the delayed fluorescence of chlorophyll at 20 degrees C and 77 degrees K and by the chlorophyll phosphorescence at 77 degrees K. The delayed fluorescence at 20 degrees C arises from the thermal activation of the triplet state up to the excited singlet level of chlorophyll and at 77 degrees K it is due to triplet-triplet annihilation. The quantum yield of the triplet formation, estimated by a comparison of the light saturation curves of delayed fluorescence at 20 degrees C and of P-700 photooxidation under the same experimental (optical) conditions, is approximately 0.9 of the P-700+ yield. Only one triplet of chlorophyll can be generated per P-700. Under heat inactivation of P-430 the triplet formation is not observed when P-700 is oxidized. It is assumed that the triplet-triplet annihilation at 77 degrees K is related with the strong interaction between the chlorophyll molecules in the pigment complex of Photosystem I. The possibility of a triplet participation in the primary processes of photosynthesis is discussed.