Melanoidins and Soil Organic Matter: Evidence of Strong Similarities Revealed by 13C CP‐MAS NMR

Abstract

Melanoidins of molecular weight higher than 12000 were synthesized by reaction of xylose with glycine (I), xylose with ammonium sulfate (III), and glucose with glycine (II) and compared with a well-humified fraction of an organic soil. Microanalytical data for all four substances were very similar. The CP-MAS spectra of the three melanoidins strongly resemble that of the organic soil fraction. Melanoidins (I) and (II), like the organic soil fraction, are highly aliphatic and have the same distribution of carbon types: aliphatic, carbons bonded to the electronegative atoms O and N, aromatic, carboxyl, and carbonyl. Acid hydrolysis of melanoidin (I) yielded no xylose and < 1% glycine, indicating that these starting components are not main constituents of the polymer. These findings not only add weight to Maillard's hypothesis, that melanoidin formation “may be at the origin of humic substances in nature,” but allow us to suggest that some of the nitrogen in soil which is still unidentified may exist in melanoidin-type polymers.