Pteridines chelation

The metal is chelated by an ene-1,2-dithiolate (dithiolene) of the dihydropyran ring which is fused to a partially reduced pteridine 178. The syntheses of these molybdopterin-related proligands were described as model compounds for verification of the stability in vitro <2001CC 123>. The proligand 179 is a relatively stable compound however, after several weeks, the pyran ring opened and oxidation to the diol derivatives 180 was observed (Scheme 35). 

Pyrimido[4,5-r/]pyrimidines are stronger acids and weaker chelating agents than either purines or pteridines. Thus, the pKa values of pyrimido[4,5-form strong internal hydrogen bonds.106... 

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. 

A striking feature of Mo(iv)-oxidized pterin systems is that metal chelation is coupled to pteridine protonation. This protonation is evidence of a change in the electronic structure within the Mo-pterin unit, where electron density is increased in the pteridine system through delocalization via... 

It required a decade of study to develop a clear understanding of the principles and outcome of pterin reactions to molybdenum. The results summarized here tell the story of non-innocent ligand behavior of pterin and pteridine ligands. During this period the first crystal structure of molybdenum enzymes emerged from which the dithiolene chelation of molyb-dopterin was proved. While protein structures unequivocally confirmed the dithiolene coordination to molybdenum, the alternative coordination mode described in this section underscores the electronic flexibility of the pterin system partnered with molybdenum. 

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