Formation of Strecker Aldehydes from Polyphenol-Derived Quinones and α-Amino Acids in a Nonenzymic Model System

Fruits and vegetables contain naturally occurring polyphenolic compounds that can undergo enzyme-catalyzed oxidation during food preparation. Many of these compounds contain catechol (1,2-dihydroxybenzene) moieties that may be transformed into o-quinone derivatives by polyphenoloxidases and molecular oxygen. Secondary reactions of the o-quinones include the Strecker degradation of ambient amino acids to form flavor-important volatile aldehydes. The purpose of this work was to investigate the mechanism of the polyphenol/o-quinone/Strecker degradation sequence in a nonenzymic model system. By using ferricyanide ion as the oxidant in pH 7.17 phosphate buffer at 22 °C, caffeic acid, chlorogenic acid, (+) catechin, and (−) epicatechin were caused to react with methionine and phenylalanine to produce Strecker aldehydes methional and phenylacetaldehyde in 0.032−0.42% molar yields (0.7−10 ppm in reaction mixtures). Also, by employing l-proline methyl ester in a reaction with 4-methylcatechol, a key reaction intermediate, 4-(2‘-carbomethoxy-1‘-pyrrolidinyl)-5-methyl-1,2-benzoquinone (7), was isolated and tentatively identified. Keywords: Strecker degradation; quinones; polyphenols; ferricyanide; aldehydes