Electrochemical and spectroscopic studies of ferredoxins

Fast diffusion-dominated electron transfer between bacterial ferredoxins and pyrolytic graphite 'edge' electrodes promoted by aminoglycosides permits detailed voltammetric studies and preparation of oxidation states inacessible by chemical titration. Low temperature EPR and magnetic circular dichroism spectroscopy identifies the cluster types, and magnetic states at defined oxidation levels. In the 7Fe ferredoxin, Fd 11, Azotobacter &roococcum, the (3Fe-4S) cluster exhibits pH dependent El/ values (3OOC): El 2 (alkaline) = -460 f 10 mV vs NHE, -dE1/2/d(pH) = 55 mV, pK = 5.8. The (4Fe-4S) duster is characterised by an unusually low reduction potential, -645f 10 mV vs NHE at pH 8.3. FdIII, PesulphovibriQj&icanyS, contains one (3Fe-4S), and one (4Fe-4S) cluster when isolated aerobically. Cyclic voltammetry shows that w reduced the (3Fe-4SIo centre reacts rapidly with ferrous ion to form a (4Fe-4S) cluster. Since the protein sequence contains only seven cysteines this cluster must possess non-cysteinyl ligation, which we propose to be the carboxylate side-chain of aspartic acid, residue 14. The magnetic properties of this cluster in the reduced state are S = 3/2. 9: