Study of the PS I acceptor side by double and triple flash experiments

Abstract

Electron transfer from P-700 to the PS I electron acceptors has been studied by flash absorption spectroscopy after double or triple excitation at room temperature, in the presence of glycerol, in pea chloroplasts and in PS I particles supplemented with purified plastocyanin. After two flashes, the reduction of P-700 + occurs with t 1 2 = 250 μ s when the excitations are separated by less than 10 ms. This reduction is attributed to the recombination reaction between P-700 + and the reduced iron-sulfur center X. Under conditions where bound plastocyanin can rapidly reduce P-700 + after the second excitation, a third flash induces an oxidation of P-700 which decays with a t 1 2 of 30 μs. This new kinetic phase is tentatively attributed to a back-reaction between P-700 + and an electron acceptor more primary than the iron-sulfur center X. Addition of glycerol to the samples is shown to inhibit the reduction of the iron-sulfur center A at room temperature. Data are compared with results obtained under background illumination and reducing conditions.