COMPETITION BETWEEN ANNIHILATION AND TRAPPING LEADS TO STRONGLY REDUCED YIELDS OF PHOTOCHEMISTRY UNDER PS-FLASH EXCITATION

Excitation of photosynthetic systems with short intense flashes is kno wn to lead to exciton-exciton annihilation processes. Here we quantify the effect of competition between annihilation and trapping for Photo system II, Photosystem I (thylakoids from peas and membranes from the cyanobacterium Synechocystis sp.), as well as for the purple bacterium Rhodospirillum rubrum. In none of the cases it was possible to reach complete product saturation (i.e. closure of reaction centers) even wi th an excitation energy exceeding 10 hits per photosynthetic unit. The parameter a: introduced by Deprez et al. ((1990) Biochim. Biophys. Ac ta 1015: 295-303) describing the competition between exciton-exciton a nnihilation and trapping was calculated to range between approximate t o 4.5 (PS II) and approximate to 6 (Rs. rubrum). The rate constants fo r bimolecular exciton-exciton annihilation ranged between (42 ps)(-1) and (2.5 ps)(-1) for PS II and PS I-membranes of Synechocystis, respec tively. The data are interpreted in terms of hopping times (i.e. mean residence time of the excited state on a chromophore) according to ran dom walk in isoenergetic antenna.