Polyphenol Interactions with the Maillard Reaction under the Influence of Birnessite: Impact on Humus and Carbonate Formation.
P. M. Huang, A.G. Hardie, and L.M. Kozak. Dept of Soil Science, Univ of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N 5A8, Canada
The Maillard reaction, involving condensation reactions between sugars and amino acids, is regarded as an important pathway in humification processes. Likewise the polyphenol pathway of humification has been intensely studied. The aim of our study was to examine the effect of the molar ratio of catechol to Maillard reagents in the integrated polyphenol-Maillard reaction system on humification and carbonate formation processes as catalyzed by birnessite. Experiments with an increasing concentration of catechol present in the integrated and catechol-only reaction systems were conducted at environmentally relevant conditions, i.e. pH 7.0 and 45° C. Subsequently the degree of humification and the nature of the products formed were examined. Increasing molar ratios of catechol to Maillard reagents, as catalyzed by birnessite, resulted in significant increases in the degree of humification in the integrated pathway. This increase in humification was substantially higher than the sum of the humification in the Maillard reaction system and in the catechol alone systems. FTIR spectroscopic data indicate that increasing the molar ratio of catechol to Maillard reagents resulted in formation of humic substances with an increasingly aromatic character. We also observed for the first time the biomolecule-induced formation of MnCO3 (rhodocrosite) as a result of the oxidation and cleavage of glucose, glycine, or catechol by birnessite in our reaction systems. The Maillard reaction system showed the greatest amount of rhodocrosite formation, and as the amount of catechol was increased in the integrated systems, there was a subsequent decrease in the amount and crystallinity of rhodocrosite formed. This is ascribed to the perturbing effect of the catechol-derived humic polymers on the crystallization of rhodocrosite as well as the final pH of the reaction systems. The findings obtained in the present study are of fundamental significance in understanding the abiotic pathway of both humification and carbonate formation in natural environments.