In the Maillard reaction, independent degradations of amino acids play an important role in the generation of amino-acid-specific products, such as Strecker aldehydes or their Schiff bases. Such oxidative decarboxylation reactions are expected to be enhanced in the presence of metals. Preliminary studies performed through heating of alanine and various metal salts (Cu, Fe, Zn, and Ca) under pyrolytic conditions indicated that copper(II) and iron(III) because of their high oxidation potentials were the only metals able to induce oxidative decarboxylation of amino acids and formation of Strecker aldehyde or its derivatives as detected by gas chromatography/mass spectrometry. Furthermore, studies performed with synthetic alanine and glycine copper complexes indicated that they constituted the critical intermediates undergoing free-radical oxidative degradation, followed by the loss of carbon dioxide and the generation of Strecker aldehydes, which were detected either as stable Schiff base adducts or incorporated in moieties, such as pyrazine or pyridine derivatives.
The transformation of α-amino acids into their hydroxymethyl derivatives during the Maillard reaction is an intriguing possibility for catalysis by metal salts in the presence of Strecker aldehydes; the process is commonly known as the Akabori reaction. The mechanism of this reaction was studied in the presence of glucose, using glycine copper complex and paraformaldehyde as Akabori model system in aqueous mixtures heated at 110 °C for 2 h and subsequently analyzed by qTOF/ESI/MS. Isotope-labeling studies of the various products identified have provided for the first time mass spectrometric evidence for the detailed mechanism of Akabori transformation, particularly the formation of Schiff base adducts prior to the final conversion into serine and hydroxymethyl-serine. Furthermore, the results have indicated that sugars do not interfere with such transformations and, on the contrary, the presence of glycine–copper complexes in the Maillard model systems can enhance the production of Maillard reaction intermediates.
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