The Effect of Catechol Interaction with the Maillard Reaction on Abiotic Humification as Catalyzed by Birnessite.
A.G. Hardie1, L.M. Kozak2, and P.M. Huang2. (1) Univ of Saskatchewan, Dept of Soil Science, 51 Campus Drive, Saskatoon, SK S7N5A8, Canada, (2) Dept of Soil Science, Univ of Saskatchewan, 51 Campus Drive, Saskatoon, SK S7N5A8, Canada
The Maillard reaction, involving condensation reactions between sugars and amino acids, is regarded as an important pathway in natural humification processes (1). Likewise the polyphenol pathway of humification has been intensely studied. Various soil mineral colloids can serve as catalysts in humification, and birnessite has been found to be one of the most powerful catalysts of these reactions (2). In nature it is likely that the Maillard reaction and polyphenol pathways interact with each other since sugars, amino acids and polyphenols all coexist in soil solutions and aquatic environments. An integrated humification pathway was first studied by using birnessite as the catalyst and using equimolar concentrations of glucose, glycine and catechol (3). The aim of our study was to examine the effect of the molar ratio of catechol to Maillard reagents on the humification process as catalyzed by birnessite. The integrated system (glucose, glycine and catechol) was compared with the Maillard reaction and catechol only systems. A number of treatments with an increasing concentration of catechol present in the reaction systems were conducted, and then the degree of humification and the nature of the products formed were investigated. The experiment was performed under environmentally relevant conditions, namely at pH 7.0 and at 45° C. Forty five degrees centigrade is the approximate temperature of the topsoil on a day with an ambient air temperature of 25° C. Sterile conditions were maintained throughout the experiment. The final pH, Eh, Mn concentration and visible absorbance of the reaction systems was determined after a period of 15 days. Visible, EPR and FTIR spectroscopy were used to investigate the nature of the reaction products. Our results showed that 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. The visible spectroscopic data show that the threshold value required to observe significant increases in the degree of humification was a molar ratio of catechol to Maillard reagents greater than 0.05 (i.e., 0.0025 mol catechol to 0.05 mol glucose + 0.05 mol glycine). The data indicate that this synergistic effect is a result of the multiple processes which can occur in the integrated reaction system, namely, the Maillard reaction, the polymerization of catechol and polycondensation of catechol and glycine. It can also be seen from the FTIR spectroscopic data that increasing the molar ratio of catechol to Maillard reagents higher than 0.05 in the integrated system results in the formation of humic substances with an increasingly aromatic character. The findings obtained in the present study are of fundamental significance in understanding the abiotic pathway of humification in natural environments. References: (1) Ikan, R., Rubinsztain, Y., Nissenbaum, A. and Kaplan, I.R. 1996. Geochemical aspects of the Maillard reaction. In: Ikan, R. (ed.): The Maillard Reaction: consequences for the chemical and life sciences. John Wiley and Sons, Inc. Chichester, UK, pp. 1-25. (2) Huang, P.M. 2004. Soil mineral-organic matter-microorganism interactions: fundamentals and impacts. Advances in Agronomy 82, 391-472. (3) Jokic, A., Wang, M.C., Liu, C., Frenkel, A.I. and Huang, P.M. 2004. Integration of the polyphenol and Maillard reactions into a unified abiotic pathway for humification in nature: the role of ä-MnO2. Organic Geochemistry 35, 747-762.