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This contribution describes the path of my nearly forty-year quest to understand the special ligand coordinated to molybdenum and tungsten ions in their respective enzymes. Through this quest, I aimed to discover why nature did not simply use a methyl group on the dithiolene that chelates Mo and W but instead chose a complicated pyranopterin. My journey sought answers through the synthesis of model Mo compounds that allowed systematic investigations of the interactions between molybdenum and pterin and molybdenum and pterin-dithiolene and revealed special features of the pyranopterin dithiolene chelate bound to molybdenum. [ABSTRACT FROM AUTHOR]

Research was conducted to examine a series of molybdenum-pterin complexes generated from the reactions of pterin and molybdenum reagents in different oxidation states. X-ray photoelectron spectroscopic studies of the molybdenum-pterin complexes revealed binding energies that are shifted by 1.5-1.8 eV to lower energies as compared to those exhibited by oxo-Mo(VI) reagents. Results demonstrate a considerable delocalization of electron density over the molybdenum-pterin framework.

Research was conducted to examine the coordination of alloxazine and pterins to molybdenum(IV). The synthesis and crystal structures of MoOCL3(pteridineH) were investigated starting from Mo(IV)Cl4(acetonitrile)2 and the pteridine ligand in methanol or chloroform. Results strongly suggest that pteridine protonation is facilitated by molybdenum(IV) coordination due to partial reduction of pteridine ring through electronic delocalization from Mo to the pteridine ligand.

The identity of (MoOCl(detc)(H3dmp))Cl was described along with its synthesis and isolation from an alternative preparation method. In the course of the study, it was discovered that mono-oxo molybdenum dithiocarbamate complexes of reduced pterin had the ability to reduce DMSO, thus, demostrating the oxygen atom transfer reaction characteristic of molybdenum enzymes and many Mo(IV) complexes. The results from the acid/base studies showed that (MoOCl(detc)(H3dmp))Cl exists as a hydrogen bound dimer in DMSO and DMF.

A combination of X-ray crystallography, electronic absorption spectroscopy, resonance Raman spectroscopy, and bonding calculations are applied to understand the considerable thiolate-thione character possessed by the monoanionic dithiolene ligand resulting from an admixture of an intraligand charge transfer excited state into the ground state wave function. A highly versatile donor-acceptor character exhibited by the unusual dithiolene which remarkably affected the Mo(IV/V) redox couple provide insight into a potentially noninnocent role of the pterin fragment in pyranopterin Mo enzymes.

An overview of the pyranopterin dithiolene (MPT) component of the molybdenum cofactor (Moco) and how MPT may contribute to enzymatic catalysis is presented. The chapter begins with a brief review of MPT and Moco biosynthesis and continues to explore the nature of what is arguably the most electronically complex ligand in biology. To explore this complexity, we have dissected MPT into its relevant molecular components. These include the redox-active ene-1,2-dithiolate (dithiolene) and pterin moieties, which are bridged by a pyran that may be found in ring-opened or ring-closed configurations. The various redox possibilities of MPT bound to Mo are presented, along with the electronic structure of the redox components. MPTs are found to display a remarkable conformational variance in pyranopterin Mo enzymes. This is discussed in terms of a relationship to enzyme function and the potential for the observed non-planer distortions to reflect different MPT oxidation and tautomeric states. The chapter ends with a series of case studies featuring model compounds that highlight how biomimetic small molecule studies have contributed to furthering our understanding of the roles this remarkable ligand plays in the catalytic cycles of the enzymes. [ABSTRACT FROM AUTHOR]