Stable isotope fractionation of biomonomers during protokerogen formation

Abstract

The condensation of amino acids and sugars (Maillard reaction) is one possible diagenetic pathway for the formation of humic materials in sediments. In this study, aqueous solutions of alanine and glucose were heated (100°C) for up to 40 days. The δ¹³C- and δ¹³N-values of reactants and products were monitored. With increased heating time, the stable carbon and nitrogen isotope compositions of the unreacted alanine in solution were enriched by up to 8.8‰ and 2.7‰, respectively, relative to their initial compositions. In contrast, the insoluble melanoidin product and alanine recovered by acid hydrolysis of the melanoidin were both depleted in¹³C and¹⁵N relative to the starting materials. The magnitude of this isotopic fractionation varied as a function of the relatieve concetration of alanine to glucose in the starting solution. The CO₂ that evolved during the reaction is depleted in¹³C relative to the initial δ¹³C composition of alanine and its car☐yl group, suggesting that¹³C-depleted amino acids initially condense with glucose to form insoluble melanoidins. Subsequent to melanoidin formation, a second isotope fractionation takes place whereby¹³C-depleted car☐yl groups are preferentially cleaved from the melanoidin. Assuming that humic substances may form in natural environments via condensation reactions like the Maillard reaction, it is hypothesized that the stable isotope fractionation that occurs during the transformation of organic matter to humic materials and kerogen might be at least partially explained by kinetic effects during condensation reactions rather than decar☐ylation of the primary amino acids.