Reactivity of polynuclear zinc-thiolate sites

The proton affinities of a series of monoand polynuclear zinc-thiolate compounds, mimicking zing finger protein and metallothioneins sites, have been investigated by Density Functional Theory calculations. This allows to evaluate the intrinsic nucleophilicity of synthetic and natural zinc-bound thiolates and to compare their relative reactivities. The site specificity of the experimental thiolate alkylation for the Zn4Cys11 clusters in metallothioneins is well reproduced, as well as the relative reactivities of ZnHis2Cys2, ZnHisCys3 and ZnCys4 zinc finger sites. Our results also show that terminal thiolates, bound to only one Lewis acid, are more reactive than bridging thiolates. Synthetic inorganic clusters and the Zn4Cys9His2 cluster found in a cyanobacterial metallothionein are less reactive than Zn4Cys11 and Zn3Cys9 clusters in metallothioneins. These results allow discussing the influence of the protein backbone and residues on the reactivity of these natural clusters.