American Chemical Society
jf9020587_si_001.pdf (222.57 kB)

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

Download (222.57 kB)
journal contribution
posted on 2009-11-25, 00:00 authored by Xiaowen Fang, Klaus Schmidt-Rohr
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 13C and 15N solid-state nuclear magnetic resonance (NMR) techniques to 13C- and 15N-labeled melanoidins formed in dry and solution reactions. Quantitative 13C NMR shows that ∼23% of carbon is from glycine; the ∼2% loss compared to the 25% glycine C in the reactants is due to the COO moiety being liberated as CO2 (Strecker degradation). 13C J-modulation experiments on melanoidins made from doubly 13C-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; ∼5% of amides have also been detected. The C2−N bond stays intact for ∼70% of the incorporated glycine. The 15N 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.