Conversion of phenylalanine into styrene by 2,4-decadienal in model systems.

2,4-Decadienal was heated under an inert atmosphere and in the presence of phenylalanine to investigate whether this secondary lipid oxidation product is a final product of lipid oxidation or it reacts with the amino acid. The results obtained showed that, in the presence of the alkadienal, the amino acid was degraded to styrene. This reaction was favored in dry systems at pH approximately 6 and in the absence of oxygen. If oxygen was present, the alkadienal was oxidized and the Strecker degradation of the amino acid was produced. The activation energy for the formation of styrene from phenylalanine was 150.4 kJ/mol. The reaction mechanism is suggested to be produced either by an electronic rearrangement of the imine produced between the aldehyde and the amino acid with the formation of styrene, 2-pentylpyridine, carbon dioxide, and hydrogen, or by Michael addition of the amino compound to the alkadienal followed by beta-elimination to produce the same compounds. Both reaction schemes were supported on the results obtained by studying both the degradation of phenylethylamine and phenylalanine methyl ester produced by 2,4-decadienal, and the formation of ethylbenzene in decadienal/phenylalanine reaction mixtures heated in the presence of platinum oxide. All these results suggest that, analogously to carbohydrates, certain lipid oxidation products may degrade appropriate amino acids to their corresponding vinylogous derivatives.