Rapid oxidation processes relevant to the degradation of [4Fe-4S] clusters in Clostridium pasteurianum ferredoxin were studied via direct (unmediated) heterogeneous electron transfer at a pyrolytic graphite electrode. Differential-pulse voltammograms of native [4Fe-4S] ferredoxin showed two well-defined oxidation peaks corresponding to apparent E-values of +793 and +1120 mV at 5°C. Direct involvement of the cluster was established through parallel experiments with the 2[4Fe-4Se] derivative for which peak positions were shifted. Square-wave voltammetry showed that the product of the first electron transfer, which may correspond to the ‘super-oxidised’ [4Fe-4S]3+ oxidation level, undergoes rapid degradation (t ½ < 1.6 ms at 5°C). The second oxidation process, as characterised by a significant (⪢100 mV) negative shift upon selenium substitution, very likely represents oxidation of S(Se) still associated with the protein and possibly contained within the remaining FE-S(Se) substructure.
