The position of cytochrome b559 relative to QA in Photosystem II studied by electron–electron double resonance (ELDOR)

AbstractThe electron–electron double resonance (ELDOR) method was applied to measure 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 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 b+559Y⋅DQ−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 b+559-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 QA to Cyt b559 and the membrane normal was determined to be 80±5°. The position of Cyt b559 relative to QA 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 than 50 Å