Synthesis and EPR Characterization of New Models for the One-Electron Reduced Molybdenum Site of Sulfite Oxidase

Deconvoluting the different contributions of thiolate and ene-1,2-dithiolate donors to the underlying electronic structure of the Mo site in sulfite oxidase (SO) has proven to be a difficult task. One way in which these differences might be illuminated is by selectively substituting Se for S in model complexes which possess multiple sulfur donor ligand environments. Here we report the synthesis and structures of new oxo-Mo(V) complexes as effective models for the one-electron reduced active site of SO. We have used the tridentate heteroscorpionate ligand (2-dimethylethanethiol)bis(3,5 -dimethylpyrazolyl)methane (L3SH) in order to model the constrained cysteinyl sulfur (SCys) ligand environment observed in the crystal structure of the enzyme, and benzene-1,2-dithiol (bdt) as a mimic of the ene-1,2-dithiolate chelate. [(L3S)MoO(bdt)] and [(L3S)MoO(SPh)2] have been structurally characterized by X-ray crystallography, and as such, [(L3S)MoO(bdt)] is only the second known model compound that closely approximates the active site structure of reduced forms of SO. Additionally, benzenethiol (SPh) and benzeneselenol (SePh) have been used to perturb the equatorial ligand environment of [(L3S)MoO(bdt)].) This has provided much needed insight into the electronic structure of the one-electron reduced SO site and has allowed for increased understanding of the individual roles played by these different thiolate donors in the oxidative half-reaction of the enzyme. Interestingly, the EPR spectra of [(L3S)MoO(bdt)], [(L3S)MoO(SPh)2], and [(L3S)MoO(SePh)2] closely resemble that of both high pH (hpH) and low pH (lpH) SO, except for the fact that the magnitude of g1 is found to be consistently higher in the model spectra compared to that of the enzyme. It is suggested that this derives from an increase in Mo-S covalency in the models relative to hpH and lpH SO.