Electron Transfer between exogenous electron donors and reaction center of photosystem 2

Transfer of electrons between artificial electron donors diphenylcarbazide (DPC) and hydroxylamine (NH₂OH) and reaction center of manganese-depleted photosystem 2 (PS2) complexes was studied using the direct electrometrical method. For the first time it was shown that reduction of redox-active amino acid tyrosine Yz by DPC is coupled with generation of transmembrane electric potential difference (δψ). The amplitude of this phase comprised ∼17% of that of the δψ phase due to electron transfer between YZ and the primary quinone acceptor QA. This phase is associated with vectorial intraprotein electron transfer between the DPC binding site on the protein-water interface and the tyrosine YZ. The slowing of AW decay in the presence of NH2OH indicates effective electron transfer between the artificial electron donor and reaction center of PS2. It is suggested that NH2OH is able to diffuse through channels with diameter of 2.0-3.0 Å visible in PS2 structure and leading from the protein-water interface to the Mn4Ca cluster binding site with the concomitant electron donation to YZ. Because the dielectrically-weighted distance between the NH2OH binding site and YZ is not determined, the transfer of electrons from NH₂OH to YZ could be either electrically silent or contribute negligibly to the observed electrogenicity in comparison with hydrophobic donors.