Pterin redox chemistry

A brief summary of pterin redox chemistry is presented first to provide the necessary context for understanding the interpretations of the redox studies, followed by discussions of a series of studies pursuing the outcomes of oxo-Mo and Mo" complexes reacted with tetrahydropterins and pterins. 

Pterin Redox Chemistry. Participation in redox processes is one of the fundamental roles of pteridines in biology, and pterins most often appear in biological systems as a redox component of a catalytic process. Their redox reactivity is a consequence of their nitrogen heterocyclic structure that can support several reduetion levels and multiple tautomeric forms. The N-heterocyclic structure of the bicyelic pterin system exhibits a wealth of redox reactions in ways similar to the related N-heterocyele isoalloxazine in FAD. Unlike FAD where redox reaetions are limited to 2e , 2H proeesses, pterins are able to transfer up to 4e , 4H units in sequential reaetions. 

The fully oxidized state, the semi-redueed or dihydro state and the fully reduced or tetrahydro state (Seheme 2.2a-e) are the three main redox states of pterins interconverted by 2e , 2H reaetions. The complexity of pterin redox chemistry results from many tautomeric forms of the semi-redueed state (Scheme 2.2d-h), whieh, unless highly substituted, will eventually rearrange to the most thermodynamieally stable form, ca. the 7,8-dihydropterin (Scheme 2.2b)... 

A thorough investigation using voltammetry revealed the key features of pterin redox chemistry that are generally applicable for simple pterins such as 6- and 6,7-methylated pterins. These key features are illustrated for unsubstituted pterin in Seheme 2.3. The scheme is arranged with the... 

As an introduction to the chemistry of the pterin piece of molybdopterin, to be elaborated in Section III, pterin redox reactivity is now addressed. Pterins... 

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. 

The above examples underline that the dithiolene unit can also be an important redox component of the molybdenum cofactor. Much of the discussion in this chapter has focused on the redox chemistry of pterin and its... 

Figure 7. Analogy between redox chemistry ofS-oxoG and pterins andflavins.

A range of chemical analogs of the catalytic centers of Mo and W dithiolene-containing enzymes (pterins) have been prepared. In particular, the rich chemistry of multisulfur transition metal systems allows ligand redox, internal electron transfer, and intermediate redox states. Such redox flexibility may facihtate coupled proton/electron transfer and/or 0x0-transfer mechanisms, which are employed by Mo and W enzymes. 

---