FORMATION OF MELANOIDINS IN THE PRESENCE OF H2S

The formation of melanoidins from d-lysine in the presence of H2S and N2 was examined to clarify the effect of H2S on the condensation of sugars and amino acids. Significant amounts of dark brown-colored precipitates formed in a mixture of glucose and lysine heated at 80°C for 20 days under N2. However, the same solution heated at 80°C for 30 days under H2S showed no major change apart from a slight change in color to light brown. This observation indicates that the condensation of glucose and lysine proceeds very slowly in the presence of H2S compared to their condensation in the presence of N2. Instead, significant amounts of organosulphur compounds formed from the glucose-lysine solution and H2S as confirmed by py-GC (pyrolysis gas chromatography) using both FID (flame ionization detector) and FPD (flame photometric detector). The rapid reaction of H2S with reactive carbonyl groups of reducing sugars is proposed to hinder the progess of the condensation between reducing sugars and amino acids. The incorporation of sulphur from H2S into sedimentary organic matter in the very early stage of diagenesis could be one of important controlling factors governing the formation of melanoidin-like geopolymers in Recent sediments. The organic geochemical characteristics of kerogens in carbonate rocks (relatively low abundance and high organic sulphur contents) could be interpreted by the rapid reaction between H2S and reducing sugars in the very early stage of diagenesis. © 1990.