Protonation and substitution reactions of Fe-S "basket' clusters including extracted FeMo-cofactor of nitrogenase

In an extension to our earlier work on the mechanisms of reactions of Fe-S-based clusters, we now report studies on the substitution kinetics of structurally more 'open' Fe-S clusters. In particular, we concentrate on protonation of the 'basket' cluster [Fe6S6Cl2(PEt3)(4)]. Protonation at a mu(n)-S can either accelerate or inhibit the rate of substitution of the cluster depending on: the number of protons added and whether substitution is associative or dissociative. Analysis of the kinetics allows us to calculate the pK(a) values of the protonated clusters and comparison with a variety of other Fe-S-based clusters shows that, irrespective of the structure and charge, the first (pK(a)(1) = 17.9-18.9) is rather insensitive to the cluster ligands whilst the second (pK(a)(2) = 13.6-16.9) is markedly more sensitive. Analogous studies on extracted FeMo-cofactor [MoFe7Sy(R-homocitrate)(NMF)(2)] (NMF = N-methylformamide) from the nitrogenase of Klebsiella pneumoniae shows the same general protonation/substitution characteristics as synthetic Fe-S-based clusters. (C) 1999 Elsevier Science S.A. All rights reserved.