The electron-electron double resonance (ELDOR) method was applied to measure the dipole interaction between cytochrome (Cyt) b559/+ and the primary acceptor quinone (Q(A)/-, observed at g=2.0045 with the peak to peak width of about 9 G, in Photosystem II (PS II) in which the non-heme Fe2+ was substituted by Zn2+. The paramagnetic centers of Cyt b559/+Y(D).Q(A)/- were trapped by illumination at 273 K for 8 min, followed by dark adaptation for 3 min and freezing into 77 K. The distance between the pair Cyt b559/+-Q(A)/- was estimated from the dipole interaction constant fitted to the observed ELDOR time profile to be 40 ± 1 Å. In the membrane oriented PS II particles the angle between the vector from Q(A) to Cyt b559 and the membrane normal was determined to be 80±5°. The position of Cyt b559 relative to Q(A) suggests that the heme plane is located on the stromal side of the thylakoid membrane. ELDOR was not observed for Cyt b559/+ Y(D)/. spin pair, suggesting the distance between them is more than 50 Å. (C) 2000 Elsevier Science B.V.
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