Molybdenum Cofactor Model Reveals Remarkable Redox Activity at Both Molybdenum and the Pyranopterin Dithiolene Ligand

The molybdenum (Moco) and tungsten (Tuco) cofactors are uniquely found in pyranopterin dithiolene (PDT) molybdenum and tungsten enzymes, yet the roles of this electronically complex PDT ligand in the catalytic cycles of these enzymes has yet to be revealed. After more than a decade of effort, we have synthesized and characterized a model compound containing a reduced PDT ligand coordinated to a diamagnetic d² low-spin Mo(4+) ion, mimicking the MoO(PDT) structure common to most Mo enzyme active sites. A combination of 1D and 2D NMR spectroscopies, augmented by molecular geometry optimization computations, confirms that both R,R- and S,S-diastereomers coexist in the synthetic final product. Redox processes at both the Mo ion and the pyranopterin are detected by cyclic voltammetry. The two-electron oxidant DCIP oxidizes the pterin component of the ligand in methanol, whereas no reaction occurs in aprotic acetonitrile. Addition of 1 equiv of the one-electron oxidant Fc⁺ stoichiometrically oxidizes the Mo(4+) ion to the paramagnetic d¹ Mo(5+) species, a result supported by electron paramagnetic resonance (EPR) spectroscopy. However, the addition of more than 1 equiv of Fc⁺ results in oxidation of the reduced pyranopterin to yield a Mo(4+) complex of the oxidized pyranopterin dithiolene ligand, a result supported by both the cyclic voltammetry and electronic absorption titrations. The concrete examples from these model studies suggest how the unique electronic structure of the PDT ligand in Moco and Tuco may enable variable redox reactivity in enzymatic catalysis, highlighting its role as a complex noninnocent biological ligand.