Electronic structure studies of molybdenum -thiolate complexes related to pyranopterin molybdenum enzymes

All forms of life from microorganisms to humans, aerobes and anaerobes, utilize pyranopterin molybdenum enzymes to catalyze reactions of organic and inorganic substrates. The diverse reactions catalyzed by these enzymes are an integral part of the global cycles of sulfur, nitrogen, and carbon. Different combinations of cysteine thiolate, ene-1,2-dithiolate, and terminal sulfido donor ligands are coordinated to molybdenum in the active sites of the three distinct families of pyranopterin molybdenum enzymes. Thus, understanding the bonding of these sulfur donor ligands to high-valent molybdenum centers is a prerequisite for obtaining fundamental insight into their respective roles in enzymatic catalysis. The electronic structures of mono-oxo molybdenum-thiolate and -dithiolate complexes ([MoO(phenylthiolate)4]- , [(L-N2S2)MoO(SR)], [MoO(ene-1,2-dithiolate)2]-) and des-oxo molybdenum-bisdithiolate ([Mo(O/S/Se-adamantyl)(dimethylethene-1,2-dithiolate)2] -) complexes have been investigated using a combination of electronic absorption, magnetic circular dichroism, and resonance Raman spectroscopies. The results have been used to evaluate DFT calculations that explore the electronic structure of the enzyme active site with respect to amino acid binding, geometric perturbations of the dithiolate ligand, and reaction mechanism.;Three primary conclusions have been made that are relevant to the electronic structures of the active sites of pyranopterin molybdenum enzymes: (1) extremely versatile mono-oxo molybdenum-thiolate orbital interactions are a direct function of the O-Mo-S-C dihedral angle, and a strong pi-type bonding interaction occurs when this angle is near 90°, (2) mono-oxo molybdenum-bisdithiolate bonding results in a unique pseudo-sigma bonding scheme whereby electron density is localized between the two sulfur atoms of the dithiolate, providing an efficient way to couple the metal redox orbital into electron transfer pathways, (3) the bisdithiolate ligation dominates the electronic structure of des-oxo molybdenum-bisdithiolate complexes and may be a stabilizing factor that reduces the reorganization energy needed by the enzyme during catalysis.