DISTANCE OF P680 FROM THE MANGANESE COMPLEX IN PHOTOSYSTEM-II STUDIED BY TIME RESOLVED EPR

The flash-induced transient EPR signal of p680+ was observed in PS II membrane at temperatures between 77 and 220 K. Below 180 K, the half decay time, t1/2, of P680+ transient signal was about 2 ms and re-reduction of P680+ has been ascribed to the back reaction with Q(A)-. Above 200 K, the decay rate increased with an activation energy of 15.6 kJ /mol. These results were coincident with previous optical data of P680+. Spin-lattice relaxation rates, 1/T1, of P680+ and Tyr.D+ EPR signals, were determined by microwave power saturation experiments and by T2 measurements using a pulsed EPR in the S1 and S2 states of oxygen-evolving complex (OEC) at 90 K. The ratio of distances of both radical species from the manganese cluster in OEC was derived from the difference of T1 values in the S1 and S2 states. The result has shown that the distance from OEC to P680 was 0.91 times as far as that to Tyr.D. On the basis of the temperature-dependence of the half saturation power P1/2 of Tyr.D+ in the S1QAFe2+ and S2QAFe2+ states we estimated that the distance from Tyr.D to the Mn-cluster in OEC to be 24-27 angstrom. The distance from R680 to the manganese cluster was estimated to be 21-26 angstrom. The distance of Tyr.D from Q(A)Fe2+ was supposed to be 26-33 angstrom, from 1/T1 values in Q(A)-Fe2+ and Q(A)Fe2+ at 90 K.