Fate of the Amino Acid in Glucose-Glycine Melanoidins Investigated by Solid-State Nuclear Magnetic Resonance (NMR)

The fate of the amino acid in the model Maillard reaction between glucose and glycine in a 1:1 molar ratio has been investigated by applying advanced C-13 and N-15 solid-state nuclear magnetic resonance (NMR) techniques to C-13- and N-15-labeled melanoiclins formed in dry and solution reactions. Quantitative C-13 NMR shows that similar to 23% of carbon is from glycine; the similar to 2% loss compared to the 25% glycine C in the reactants is due to the COO moiety being liberated as CO2 (Strecker degradation). C-13 J-modulation experiments on melanoidins made from doubly C-13-labeled glycine show that the C-C backbone bond of about two-thirds of the incorporated amino acid stays intact, and about half of all glycine is incorporated as N-CH2-COO without fragmentation. Degradation processes without CO2 loss affect about one-eighth of glycine in dry reaction and about one-fourth in solution. These results indicate that Strecker degradation affects about one-fourth (dry reaction) to one-third (in solution) of all glycine but is not the main pathway of glycine incorporation. Spectra of Strecker degradation products show that C2 of glycine reacts to form N-CH3, C-CHn-C, or aromatic units, but not pyrazines or pyridines. The gycine-C1 carbon incorporated into the melanoidins remains >= 90% part of COO moieties; similar to 5% of amides have also been detected. The C2-N bond stays intact for similar to 70% of the incorporated glycine. The N-15 spectra show many peaks, over a 200 ppm range, documenting a multitude of different chemical environments of nitrogen, but no enamines or imines. The majority (>78%) of nitrogen, in particular most pyrrolic N, is not protonated. Because N-H predominates in amino acids and proteins, nonprotonated nitrogen may be a characteristic marker of Maillard reaction products.