ESEEM study of the location of spin-polarized chlorophyll-quinone radical pair in membrane-oriented spinach photosystems I and II complexes

Abstract

Light-induced spin-polarized radical pairs, P700+A1 − in spinach photosystem (PS) I particles and P680+QA − in Zn-substituted PS II core complexes, in oriented membranes were studied by pulsed electron paramagnetic resonance (EPR). Based on the determined distance of 25.2 ± 0.2 A between P700 and A1, the angular dependence of the spin-polarized electron spin echo envelope modulation (ESEEM) spectra on the magnetic field suggests that the angle between $$R_{P700 - {\rm A}_1 } $$ , the radius-vector connecting P700 and A1, and the membrane normaln was 24 ± 4° in PS I particles. Obtained angle and distance of P700-A1 axis suggested QK side in the molecular geometry of cofactors presented in a recent X-ray crystallography of cyanobacterial PS I reaction center to be an active branch of electron transfer. The distance between P680 and QA was determined to be 27.4 ± 0.3 A for a nonoriented PS II. The angle between $$R_{P680 - Q_{\rm A} } $$ , the radius-vector connecting P680 and QA, andn was determined to be 21 ± 5°. The angle of P680-QA axis was close to that of 20° of P870-QA axis reported in X-ray analysis of the purple bacterial reaction center crystal.