VERY RAPID, COOPERATIVE 2-ELECTRON TWO-PROTON REDOX REACTIONS OF [3FE-4S] CLUSTERS - DETECTION AND ANALYSIS BY PROTEIN-FILM VOLTAMMETRY/

The ''hyper-reduced'' [3Fe-4S](2-) cluster (consisting formally of thr ee Fe(II) atoms and four sulfides (S2-)) possesses a remarkable capabi lity for very rapid and reversible two-electron/two-proton oxidation ( E-o' > -650 mV below pH 7) that is strongly suggestive of disulfide-ba sed (as opposed to Fe-based) redox chemistry. This otherwise elusive r eactivity is most readily revealed by performing fast-scan protein-fil m voltammetry on ferredoxins that contain a [3Fe-4S] cluster using an electrolyte composed of D2O. Fast, cooperative two-electron/two-proton transfer is observed after first generating the fully reduced [3Fe-4S ](2-) state and then cycling rapidly to more oxidizing potentials. The unusual voltammetric characteristics can be modeled by using a couple d electron-transfer scheme involving multiple states of both the [3Fe- 4S](0) and [3Fe-4S](0) forms. Rapid two-electron/two-proton oxidation produces an unstable species, most likely a disulfide, which may eithe r be rapidly re-reduced or undergo an internal redox reaction to produ ce the normal [3Fe-4S](0) form, which formally comprises two Fe(III) a nd one Fe(II). Relaxation to the normal ''0'' form is a factor of 4 fa ster when the experiment is conducted in H2O, thus making the fast cou ple more difficult to observe-the retardation observed in D2O may be a ttributed to the need to rearrange the hydrogen-bonding interactions i n the cluster binding domain.