Molybdenum reactions with oxidized pterins

Subsequent Mo-pterin model investigations addressed a seeond provocative aspeet of Rajagopalan s proposed Moeo strueture. The 1982 proposal for Moco paired an oxidized Mo center with a redueed tetrahydropterin - a juxtaposition of the highest molybdenum oxidation state with the most reduced form of pterin - that seemed incompatible and implied possible redox reactions between the metal and the organie eofactor. This seetion deseribes studies direeted at exploring whether molybdenum and pterin redox reactions might occur. These studies were conducted from 1989 to 1999 and inelude reactivity studies of oxidized molybdenum(vi) with redueed pterins, and studies of redueed molybdenum(iv) with oxidized pterins and pteridines. 

Molybdenum(iv) Reactions with Oxidized Pterins. Long before a pteridine was discovered in Moco, the reaction of Mo(iv) and flavin was reported in 1974 (Scheme 2.14), yielding oxo-Mo" OCl3(flavinH) (15) complex, which was characterized by spectroscopic and analytical data. This research was called into question shortly after its publication. ... 

Molybdenum hydroxylases (i.e., AO and XO) are flavoproteins that contain in addition to a FAD, a pterine cofactor coordinated to a molybdenum atom, and an iron sulfur center for their catalytic activity. They catalyze the two-electron oxidation of substrates with transfer to molecular oxygen to produce H2O2, and insert an atom of oxygen from water into a wide range of N-hctcrocycies and aldehydes via two-electron redox reaction as shown in equation 1.4 ... 

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. 

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