Structure of an alpha-Keto beta-Amido Acid, 3-(Phenylacetamido)pyruvic Acid, and Its Methyl Ester in the Solid State and in Organic and Aqueous Solvents.
3-(Phenylacetamido)pyruvic acid and its methyl ester both exist in the enol form in the solid state and when dissolved in organic solvents. In aqueous solution, the pK(a)'s of the enol acid are 3.28 (CO(2)H) and 10.14 (enolic OH) and that of the ester is 8.14 (enolic OH). The thermodynamically stable form of both acid and ester in aqueous solution, however, is the keto species. Ketonization rates of 3-(phenylacetamido)pyruvic acid were determined spectrophotometrically in acid and in buffered solution at neutral pH. The dominant reaction at low pH is the pH-independent protonation of the enol carboxylate monoanion with a rate constant of 0.062 s(-)(1) M(-)(1). At neutral pH, the ketonization in water alone is very slow but is strongly catalyzed by buffer acids. Rapid, partial (ca. 30%) hydration of the keto form also occurs in aqueous solution. The pK(a) of the pyruvate as a carbon acid is around 12.5. These results are compared with literature data for pyruvic acid itself. Application of peptidyl pyruvates and their derivatives as protease inhibitors requires careful assessment of the complications illustrated by the behavior of 3-(phenylacetamido)pyruvic acid and its methyl ester in aqueous solution.