Reduced pterins, molybdenum complexes

The second method of establishing oxidation states employed redox titration using the redox dye dichlorophenolindophenol (DCIP) based on the knowledge that tetrahydropterins reduce DCIP instantaneously while qui-nonoid dihydropterins react slowly and 7,8-dihydropterins do not reduce DCIP at all. As previously mentioned in this chapter, DCIP oxidation of Moeo within several molybdoenzymes was determined to be a two-electron process, suggesting a dihydropterin reduction state that was speculated to be the quinonoid tautomer. The results of stoichiometric additions of DCIP to the molybdenum complexes of reduced pterins showed that no oxidation of... 

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... 

In the case of molybdenum complexes of oxidized pterin and pteridine ligands, a similar argument leads to the formulation of these complexes as Mo(v) bound to a protonated, one-electron reduced pteridine radical, or Mo -(Hpterin ) (Scheme 2.18). This interpretation is consonant with earlier work on Ru"-flavin complexes. Radical character on flavin arising from intramolecular electron transfer was consistent with a structural distortion of flavin observed crystallographically, interpreted as partial Ru" -flavinsemiquinone character. Similarly, the Mo complexes of oxidized pterins and pteridines display short M=N5 bonds and bent flavin or pterin planes, consistent with a similar delocalized electronic structure. 

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. 

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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