Molybdenum pterin complexes

The earliest period of work on pterin models for Moco followed the discovery of the pterin unit within Moco, and occurred prior to the confirmation of the dithiolene chelate. These early studies explored the coordination chemistry between molybdenum and pterins or other structurally related molecules such as pteridines (Figure 2.1, top). The resulting themes of this body of work include the favorable coordination by molybdenum in several oxidation states to the 04, N5 chelate site in pterin (see Figure 2.1 for numbering), a variety of reactivities exhibited by Mo -tetrahydropterin systems and the highly delocalized electronic structures in molybdenum-pterin complexes that defy formal oxidation state assignments to Mo and pterin. 

The failure to form a stable dioxo-Mo-(H4pterin) complex can be attributed to competition between the two extant Mo=0 groups on the molybdenum reagent with the incipient Mo=N5 bond to the pterin. The competition is relieved in protic environments where one 0x0 ligand can be protonated and removed as water. One may consider that the Mo(=0)(=N5) unit of the molybdenum pterin complexes substitutes for the common Mo(=0)2 core frequently observed in Mo(vi) complexes. 

Formal Oxidation States in Molybdenum-Pterin Complexes. 

Both BVS and XPS methods, although different in approach, converge on the same conclusion the molybdenum pterin complexes, as a group, are most consistent with an oxidation state assignment of +5 for molybdenum. To... 

Alkyne-substituted quinoxalines (10) and pterins have been converted to molybdenum-dithiolenes via Eq. (2) (54) and Eq. (3) (55) by exploiting the reaction of activated acetylenes with molybdenum polysulfide complexes 9, 27, 28). The Mo-S vibration of 11, the final... 

Pterin-Molybdenum Redox Chemistry. The early studies focused on Mo(vi) and tetrahydropterin reagents to closely mimic the Mo and pterin portions of Moco. These studies showed that a variety of dioxo-Mo(vi) complexes reacted with tetrahydropterins to produce intensely colored mono-0x0 Mo complexes, but the reports offered different interpretations of what reaction had occurred and what resulting oxidation states of molybdenum and pterin had been produced, even when X-ray structures of the product Mo-pterin complexes were available. A clear picture of the electronic structure was eventually developed from redox titrations, reactivity studies, theoretical calculations and X-ray photoelectron spectroscopy (XPS) studies. 

Several general conclusions may be made regarding synthesis of reduced pterin complexes of molybdenum. The stability of these complexes depends on formation of a Mo=N bond, which, electronically, replaces one of the two Mo=0 groups of the reagent as detailed in Scheme 2.12. Loss of an oxo ligand is facilitated by protonation by the two equivalents of HCl associated with... 

Scheme 2.22 Synthetic strategy for preparing molybdenum pterin dithiolene complex.

H. L. Kaufmann, L. Liable-Sands, A. L. Rheingold and S. J. N. Burgmayer, Molybdenum-Pterin Chemistry. 1. The Five-Electron Oxidation of an Oxo Molybdenum Dithiolate Complex of a Reduced Pterin, Inorg. Chem., 1999, 38, 2592-2599. 

Chemical systems of relevance to the molybdenum and tungsten enzymes include synthetic pterins, a-phosphorylated ketones (as precursor models), and a variety of molybdenum and tungsten oxido, sulfido, and 1,2-enedithiolate complexes. These compounds have been used to (1) confirm the identity of MPT derivatives (2) define steps in MPT biosynthesis (3) calibrate spectroscopic observations (4) give precise geometries and reactivities that can be used as input for theoretical studies and (5) provide options for mechanistic consideration. 

The Taylor route is also very useful in the synthesis of more complex structures as demonstrated in the total synthesis of deoxyurothione (381), ( )-urothion (382) <89JA285>, and analogous 6,7-dihydrothieno[3,2- ]pterins (384) which are model substances of the oxidative breakdown product (383), termed form B of the molybdenum cofactor (399) <88JOC5839>. 

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. 

In summary, a 6-substituted pterin was first identified as a structural component of the molybdenum cofactor from sulfite oxidase, xanthine oxidase and nitrate reductase in 1980 (24). Subsequent studies provided good evidence that these enzymes possessed the same unstable molyb-dopterin (1), and it seemed likely that 1 was a constituent of all of the enzymes of Table I. It now appears that there is a family of closely related 6-substituted pterins that may differ in the oxidation state of the pterin ring, the stereochemistry of the dihydropterin ring, the tautomeric form of the side chain, and the presence and nature of a dinucleotide in the side chain. In some ways the variations that are being discovered for the pterin units of molybdenum enzymes are beginning to parallel the known complexity of naturally occurring porphyrins, which may have several possible side chains, various isomers of such side chains, and a partially reduced porphyrin skeleton (46). 

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