D.a. FdIII adsorbed as a film on a PGE electrode with neomycin as co-adsorbate. Left-hand side shows successive cycles (oxidative direction only) at 470 mV s : [Fe]=300 µM;

The [3Fe-4S] cluster thus behaves as a tridentate ligand that is rather similar to a crown thioether, but with the capability of varying its coordinating ability depending on oxidation level [13, 79]. Clearly, [3Fe-4S]0 is a much better ligand than [3Fe-4S]1+, on account of the increased electron density that is available to the bridging sulfido ligands; it is also relevant, as described in Section IIIB, that [3Fe-4S]0 is also a Brønsted base in certain proteins, such as A.v. FdI. Only in the so-called high-potential iron-sulfur proteins (HiPIPs) can a divalent ion (Fe(II)) be considered to bind to [3Fe-4S]1+, in this case to give [4Fe-4S]3+. The situation is different for monovalent thiophilic metals like Tl(I) and Cu(I), for which binding is detected not only to [3Fe-4S]0 but also to [3Fe-4S]+, although affinities are much lower in the latter case (KdOx for Tl(I) is 0.34 M) [73]. Paramagnetic clusters formulated as [Cu3Fe-4S]2+ have been spectroscopically characterized in

Cluster Eo′/mV vs SHE Kd/µM [3Fe-4S]+/0 −150

both D.a. FdIII and P.f. Fd [76, 77]. As with D.a. FdIII, voltammetry shows that uptake of Tl(I) is also fast and reversible for P.f. Fd (rates approach diffusion control in both cases); this means that Tl(I) coordinates with minimal reorganization of the cluster and its surrounding residues and suggests that it may be an effective probe for detecting solventexposed 3Fe clusters [59].