In order to investigate the stabilities of different tautomeric forms of pteridine derivatives with a phenacetyl "ide chain in the 6-or 7-position, we have performed ab initio quantum chemical geometry optimizations of both the keto form and the vinylogous amide form of 6-and 7-phenacetyl pterin. The results are in accordance with experimental expectations: the keto fom1 is slightly more stable for the 6-substituted derivatiye (1 .8 kcal per mol). while the vinylogous amide form is

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... more stable for the 7-substituted compound (8.7 kcal per mol). The vinylogous amidc forms are planar molecules, and this geometry enables the interaction between the hydrogen atom at N-5 (or N-8) and the carbonyl function of the side chain. Inspe1:tion of the calculated electron densities (population analysis) underlines the suggestion that the vinylogous amide form of 7-phenacetyl pterin is stabilized by a shift of electron density from the nitrogen honded to C-2 to the carbonyl oxygen atom of the side chain (resonance stabilization via an additional highly conjugated vinylogous amide structure).