ИСТИНА

During the past decade the electron transfer processes in photosystem I (PS I) from Synechocystis sp. PCC 6803 were mainly investigated under single-flash excitation, however the mechanisms of the electron transfer in PS I under continuous illumination have not been investigated so intensively. Here we compare the kinetics of the redox-state changes of P700, the primary electron donor in PS I under single-flash and under continuous illumination. The P700 redox-state changes were observed under single-flash laser excitation by measuring transient absorption changes in submillisecond time range at 820 nm or under continuous white-light illumination using X-band transient EPR. Intact or FA/FB-depleted (FX-core) PS I complexes were purified from the cyanobacteria Synechocystis sp. PCC 6803. The kinetics were registered in the presence of the increasing concentration of the redox-mediator sodium ascorbate (Asc). The results clearly show that Asc efficiently accepts electrons from intact PS I not only under continuous illumination, which has been shown before (Trubitsin et al., 2014), but also under flash excitation. It was shown that under flash excitation Asc is unable to accept electrons from FX-core complexes, most probably because it cannot compete for the electrons with the back electron transfer reaction. At the same time, the concentration dependence of the P700 oxidation kinetic in FX-core complexes was observed under continuous illumination, which shows that under these conditions the backward electron transfer reaction is slowed down in comparison to the single flash excitation. The data presented here discover the difference between the electron transfer reactions in PS I complexes under single-flash and continuous illumination.