| Abstrakt: |
Fast diffusion-dominated electron transfer between Azotobacter chroococcum7Fe ferredoxin, FdI, and pyrolytic graphite ‘edge’ electrodes, promoted by aminoglycosides, permits detailed voltammetric studies and preparation of oxidation states inaccessible by chemical titration. The [3Fe-4S] cluster exhibits pH dependent E12values (30°C); E12(alkaline) = 460 ± 10 mV vs NHE, −d E12/d(pH) = 55 mV, p K= 7.8. The [4Fe-4S] cluster is characterised by an unusually low reduction potential, −645 ± 10 mV vs NHE, at pH 8.3, with a slight pH dependence, −d E12/d(pH) ∼25 mV over the pH range 8.5-7.0. No redox couple is observed at potentials between −300 and +600 mV vs NHE. This shows that the [4Fe-4S] cluster is not an HIPIP-type centre. The electron paramagnetic resonance spectrum, centred at g= 1.93, of the product resulting from bulk electrolysis at −835 mV is assigned to a [4Fe-4S] +cluster interacting magnetically with a reduced [3Fe-4S] cluster. |
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