Coenzyme models. 34. Synthesis and oxidation ability of a metal-coordinative flavin.

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A new metal-coordinative flavin(2,4,7-trimethylquino[8,7-g]pteridine-9,11(7H,10H)-dione: (1)) which has a phenanthroline-like skeleton within the molecular structure was synthesized. Being different from conventinal flavins, the new flavin showed a significant affinity toward metal ions and the absorption spectra were sensitively affected by added metal ions. The spectral patterns were classified into three types, which were assigned respectively to the different coordination structures. It was demonstrated through oxidation of dihydropyridine derivatives, phenylhydrazine, and L-ascorbic acid that complexed metal ions are able to "activate" 1 as oxidizing agent. The complexes with Zr4+ and Fe3+ even oxidized alcohols at ambient temperature. Unexpectedly, it was found that tetrahedral coordination metals such as Cd2+, Zn2+, and Co2+ frequently give the stable semiquinone radical of 1. The remarkable stabilization effect of these metals was attributed to a bent, butterflywing structure of the semiquinone radical. These findings suggest that the 1–metal complexes are useful not only as a model system of metalloflavoproteins but also as an in vitro oxidation catalyst.
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