Analyses of the chemical structures of melanoidins by 13C NMR, 13C and 15N CP-MAS NMR spectrometry.

Abstract

Nondialyzable melanoidins, which were prepared from a glucose-glycine system at 95°C for 7, 26 and 44 hr, were subjected to NMR analyses before and after ozonolysis. On ¹³C CP-MAS NMR, signals in the regions of 10-50, 60-75, 105-115 (I), 130-140 (II), 170-180 and 190-205 (III) ppm were observed for the melanoidins. The signals of I, II and III completely disappeared on ozone-treatment. These findings indicate that the melanoidins were composed mainly of saturated and aliphatic carbons. The ¹³C NMR spectra of glucose (1-¹³C)-glycine melanoidins showed that C-1 carbons originating from glucose in the melanoidins were distributed at the positions of aliphatic alkyl methyl (IV), saturated, unsaturated or aromatic and carbonyl (V) carbons. The signals of IV and V disappeared on ozone-treatment. Studies using glycine-2-¹³C revealed that most of the glycine is incorporated into the melanoidins. On ¹⁵N CP-MAS NMR, weak broad signals in the region of 0 - 70 ppm and strong signals in the region of 70-120 ppm with a shoulder at 120-170 ppm (VI) were observed for the melanoidins. The signals of VI disappeared on ozonetreatment. The main linkage of nitrogen (70-120ppm) was remarkably shifted from that of the original glycine and was determined to be a conjugated enamine linkage.