Pyridoxal- and metal ion-catalyzed oxidative deamination of alpha amino acids

Abstract

This paper describes the pyridoxal- and metal ion- catalyzed oxidation of alanine and phenylalanine to pyruvic acid and phenylpyruvic acid, respectively. The reactions were found to occur at moderate, easily measurable rates at 25 °C in aqueous and methanol solutions. Conversions of amino acid to keto acid of up to 89% were observed in the presence of relatively low concentrations of catalysts. The relative catalytic activities of the metal ions employed, as measured by initial rate constants and product yields are Mn(II) > Co(II) > Cu(II) > Ni(II) ∼O. Of special interest is the fact that little or no ammonia was evolved from the reaction mixture and no hydrogen peroxide was formed. However a considerable amount of the ammonia was accounted for by the formation of pyridoxal oxime, and the trapping of some hydroxylamine from the volatile products. The results are discussed in terms of two possible intermediates involving different reaction mechanisms, one with dioxygen coordinated to the metal ion, originally suggested by Hamilton, and a mechanism by which the oxygen attacks the carbanionic form of the delocalized α-deprotonated intermediate to give a hydroperoxide, which then undergoes OO fission to form the keto acid and coordinated pyridoxal oxime. The relative merits of these reaction mechanisms are discussed.