Ground and excited state spectral comparisons of models for sulfite oxidase

Abstract

Determining the contributions of thiolate and ene-1,2-dithiolate donors to the underlying electronic structure of the Mo site in sulfite oxidase (SO) has been a goal pursued by inorganic chemists for almost twenty years. The use of the hydrotris(3,5-dimethyl-1-pyrazolyl)borate ligand provided the framework for the first low-symmetry oxo-molybdenum(V) models in which the oxo donor was cis to the dithiolene moiety of benzene-1,2-dithiolate. Recent work from the Carrano and Kirk laboratories has produced a second-generation model of the SO active site based upon the (2-dimethylethanethiol)bis(3,5-dimethylpyrazolyl)methane heteroscorpionate ligand. Use of this heteroscorpionate framework provides for an oxo-molybdenum(V) complex with both a thiolate and an ene-1,2-dithiolate in the equatorial plane. In addition to providing a good structural model, the heteroscorpionate thiolate offers a constrained O–Mo–S thiolate–C torsion angle of ∼115°, closer to that of the ∼90° O–Mo–S Cys–C torsion angle observed in the chicken liver SO active site than can be obtained with thiophenolate ligands. This constrained torsion angle allows for the examination of how second-coordination sphere effects influence the electronic structure of the Mo site. Here we compare and contrast the molecular and electronic structures of these first- and second-generation models of the SO active site. Additionally, the electron paramagnetic resonance (EPR) parameters for both of these models are compared with those measured for the Mo(V) form of both low pH ( lpH) and high pH ( hpH) forms of SO in order to determine what insight these structural models can offer about the electronic structure of the active site of SO.