Alkyl and Other Major Structures in 13C-Labeled Glucose-Glycine Melanoidins Identified by Solid-State Nuclear Magnetic Resonance

The high molecular weight fraction of melanoidins formed in the Maillard reaction between isotopically labeled glucose and glycine has been characterized comprehensively using advanced C-13 and N-15 solid-state NMR with spectral editing. We have focused on the fate of glucose in a 1:1 molar ratio with glycine, heated as a coprecipitated powder at 125 degrees C for 2 h. Quantitative C-13 NMR spectra show that aromatic and alkene carbons make up only 40% of the total in the melanoidin. Spectra of melanoidins made from specifically labeled ((13)C1, (13)C2, (13)C3, and (13)C6) glucose are strikingly different, proving that specific structures of various types are formed. More than half of the glucose-C1 carbons form new C-C bonds, not just C-O and C-N bonds. Most C2 carbons are bonded to N or O and not protonated, while C3 shows the reverse trends. C4 and C5 remain significantly in alkyl OCH sites or become part of heterocyclic aromatic rings. C6 undergoes the least transformation, remaining half in OCH2 groups. Functional groups characteristic of fragmentation are relatively insignificant, except for N/O-C2=O groups indicating some C-1 + C-5 and C-2 + C-4 fragmentation. On the basis of C-13-C-13 and N-15-C-13 correlation spectra, 11 "monomer units" have been identified, including several types of alkyl chain or ring segments, furans, pyrroles, imidazoles, and oxazoles; these are mixed on the nanometer scale. This complexity explains why simple models cannot represent the structure of melanoidins. While none of the "monomer units" represents more than 15% of all C, the 11 units identified together account for more than half of all glucose carbon in the melanoidin.