The electron spin echo envelope modulation spectra of the reduced primary acceptor quinone, QA, in two preparations of plant photosystem II, have been studied. In one of these preparations the Fe2+ ion in the quinone-iron complex has been substituted by diamagnetic Zn2+. In the other preparation this iron ion has been converted into the diamagnetic state using a potassium cyanide treatment. A comparative analysis of two-dimensional three-pulse electron spin echo envelope modulation spectra has shown similar structure of the binding site of QA in both preparations. Two nitrogen nuclei have been found to contribute to the spectra in both preparations. One of these nitrogens is, most probably, an amino nitrogen in the imidazole ring of histidine 215 of the D2 protein. The other nitrogen has been assigned to the peptide group of alanine 261 of the D2 protein. The numerical simulations of the electron spin echo envelope modulation spectra have shown that both nitrogens are simultaneously bound to QA.
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