The position of cytochrome b(559) relative to Q(A) in Photosystem II studied by electron-electron double resonance (ELDOR)

The electron-electron double resonance (ELDOR) method was applied to measur e the dipole interaction between cytochrome (Cyt) b(559)(+) and the primary acceptor quinone (Q(A)(-)), observed at g = 2.0045 with the peak to peak w idth of about 9 G, in Photosystem II (PS II) in which the non-heme Fe2+ was substituted by Zn2+. The paramagnetic centers of Cyt b(559)(+)Y(D)(.)Q(A)( -) were trapped by illumination at 273 K for 8 min, followed by dark adapta tion for 3 min and freezing into 77 K. The distance between the pair Cyt b( 559)(+)-Q(A)(-) was estimated from the dipole interaction constant fitted t o the observed ELDOR time profile to be 40 +/- 1 Angstrom. In the membrane oriented PS II particles the angle between the vector from Q(A) to Cyt b(55 9) and the membrane normal was determined to be 80 +/- 5 degrees. The posit ion 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 b(559)(+) Y-D(.) spin pair, suggesting the distance between them is more t han 50 Angstrom. (C) 2000 Elsevier Science B.V. All rights reserved.