The catalytic role of birnessite in the Maillard reaction and the abiotic formation of humic substances
Humic substances are ubiquitous in soils and sediments and because of their agricultural and environmental importance have been the subject of much research. The Maillard (browning) reaction involving the polycondensation of carbohydrates and amino acids has been postulated as being an important abiotic pathway for humic substance formation. However, the catalytic role of minerals common in soils and sediments in the Maillard reaction and the resultant formation of humic substances in nature is little understood and the effect of light is unknown. In the present dissertation it is shown that birriessite, a tetravalent manganese oxide common in soil and sediment environments, exerted a pronounced catalytic effect on the Maillard reaction between glucose and glycine both at room temperature (25°C) and particularly at 45°C, a surficial soil temperature frequently encountered in tropical and subtropical regions, and even in temperate regions during the summer months. Light, even of moderate intensity, exerted asignificant promoting effect on browning in the glucose-glycine-birnessite system. The catalytic effect of birnessite on the Maillard reaction was still evident even under conditions of complete darkness and therefore may occur at any depth in soil and related environments. Molecular topological analysis was applied to investigate the initial reaction between glucose and glycine to form the Amadori compound fructosylglycine which is an intermediate product in the Maillard reaction. The calculations indicate that the reaction between glucose and glycine, in the absence of birnessite, to form the Amadori compound is slow. Polyphenols in terrestrial and aquatic environments are regarded as important precursors in the formation of humic substances. In the presence of catechol, the accelerating effect of birnessite on the polymerization and polycondensation in the glucose-glycine-catechol ternary system was even more dramatic compared with the catalytic effect on the Maillard reaction alone. This points to a linking of the polyphenol and Maillard reaction pathways into an integrated pathway--a significant advancement in the understanding of natural humification processes.