Biological dithiolene ligands

The biological studies summarized above have fully defined the oxidized active sites of the enzymes and provided idealized targets for MoHM studies. These targets are represented in structures (l)-(4), in rough order of prevalence in enzymes. In the enzymes, the dithiolene ligand represents MPT (or a dinucleotide derivative thereof) while in models it represents a synthetic simulant of this unique biological ligand. 

The synthesis of dithiolene complexes has been exhaustively reviewed by Rauchfuss. The most common methods used to prepare Mo enzyme model compounds are the reactions of alkynes with polysulfido-Mo complexes and of protected or free dithiolene ligands with Mo complexes. Dithiolene-Mo complexes fall into two broad classes, those containing arene-l,2-dithio-lates (pseudo-dithiolenes) and those containing true dithiolenes both are accorded equal status here. These synthetic methods and structural types all feature in the excellent, dithiolene-based models now available for enzymes of the SO and DMSOR families and biologically relevant mixed-ligand dithiolene and pterindithiolene complexes the reader is referred to... 

Ab initio and semiempirical methods have been applied to the interpretation of many aspects of dithiolene chemistry electronic spectra, ESR, Mossbauer, XPS, charge distributions, redox properties, reaction mechanisms, metal binding in biological systems and ligand-exchange behavior. We shall focus our attention on the theoretical deductions of some representative research groups. For computational details, the reader is referred to the original papers and references therein. 

The final topic addressed in this chapter is the biosynthesis of the dithiolene cofactor ligand and its coordination to molybdenum and tungsten in the enzymes. Nature has clearly devised a synthetic process to overcome the twin difficulties of building a reactive dithiolene unit bearing a complicated and equally reactive pterin substituent. Molecular biology has been the tool to elucidate the steps in this complex process. Although the dithiolene formation step remains mainly a subject of conjecture, definitive information about the reagent molecule that will eventually be converted to a dithiolene is known. 

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