A COMPARATIVE ELECTRON-SPIN ECHO ENVELOPE MODULATION STUDY OF THE PRIMARY ELECTRON-ACCEPTOR QUINONE IN ZN-SUBSTITUTED AND CYANIDE-TREATED PREPARATIONS OF PHOTOSYSTEM-II

The electron spin echo envelope modulation spectra of the reduced prim ary acceptor quinone, Q(A), in two preparations of plant photosystem I I, 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 diamagnet ic state using a potassium cyanide treatment. A comparative analysis o f two-dimensional three-pulse electron spin echo envelope modulation s pectra has shown similar structure of the binding site of QA in both p reparations. 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 p rotein. The other nitrogen has been assigned to the peptide group of a lanine 261 of the D2 protein. The numerical simulations of the electro n spin echo envelope modulation spectra have shown that both nitrogens are simultaneously bound to Q(A). (C) 1998 American Institute of Phys ics.