A Fe2+ ion on the acceptor side of plant photosystem II has been substituted by Zn2+ and an anion radical of the primary acceptor quinone, Q−A, has been studied by electron spin echo method. The electron spin echo modulation shows the interaction of the unpaired electron of Q−A with nitrogen nuclei of the histidine and, probably, alanine residues situated nearby. The comparison of the modulation spectra of Q−A with those of the anion radical of plastoquinone-9 stabilized in protonated and deuterated isopropanol matrices allows one to distinguish between the spectrum lines due to the quinone protons and due to the protons of other molecules that form hydrogen bonds with the oxygen atoms of the quinones.

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