BINDING OF CARBOXYLATE ANIONS AT THE NONHEME FE(II) OF PS-II .1. EFFECTS ON THE Q(A)(-)FE(2-SPECTRA AND THE REDOX PROPERTIES OF THE IRON() AND Q(A)FE(3+) EPR)

We have examined the effects of a number of carboxylate anions on the iron-quinone complex of Photosystem II (PS II). Typical effects are th e following. In the state Q(A)(-)Fe(2+) oxalate enhances significantly the g=1.84 EPR resonance while, for example, glycolate and glyoxylate suppress it. The anions have variable effects on the iron midpoint po tential. Formate and oxalate raise significantly the E(m) of the iron. Glycolate lowers the E(m) significantly and the E(m) shows a weak pH dependence. In the presence of glycolate the native plastosemiquinone (Q(B)(-)/Q(B)H(2) couple) can oxidise the iron. Glyoxylate also lowers the E(m), but the E(m) shows a greater pH dependence than with glycol ate but still weaker than the -60 mV/pH unit of the untreated iron. Th e Fe3+ EPR spectra are characterised by small but distinct shifts, whi le in addition an unusual resonance at close to g=4.3 is observed. The se as well as the temperature dependence of the spectra are analysed b y a spin-Hamiltonian model. Comparison with competition studies in the companion paper indicates that the anions bind as iron ligands displa cing bicarbonate.