ABSTRACT
Signals A and B were assigned, respectively, as the [3Fe-4S]+/0 and [4Fe-4S]2+/+ couples, based on EPR spectra of samples prepared by controlled potential bulk electrolysis [47]. What was striking at the time the first experiments were carried out in the similar protein from Azotobacter chroococcum (1988) was the revelation that the [4Fe-4S]2+/1+ cluster has an unusually negative reduction potential, −645 mV at pH 8.3 [47]. The cluster is coordinated within the classic binding motif, i.e., Cys-Xaa-Xaa-CysXaa-Xaa-Cys-……Cys-Pro that ligates other [4Fe-4S]2+/+ clusters; however, it had previously been proposed, incorrectly, that this cluster shuttles between 3+ and 2+ oxidation levels (see Figure 2). The paramagnetic ‘1+’ cluster is too reducing to be accessed by chemical titration with dithionite; yet it is quite unreactive and does not reduce water over the course of several hours. Samples of [4Fe-4S]1+ for EPR spectroscopy could be prepared easily by bulk electrolysis at −850 mV, using a cell incorporating a high surface area PGE electrode. It was thereby established that the reduced form has the typical spin state S=1/2 that is well established for most other ‘1+’ clusters: as expected, there is also a magnetic interaction with the [3Fe-4S]0 cluster (S=2) [47].
