Formation of sulfur and nitrogen cross-linked macromolecules under aqueous conditions

Polysulfides and ammonia are abundant in young depositional environments and play an important role in the formation of macromolecular structures such as protokerogen and humics. In this work, we study the co-incorporation of polysulfides and ammonia into simple carbonyl model compounds, octanal and trans 2-octenal, in order to study their effect on the formation of a cross-linked macromolecule and suggest a feasible mechanism. The reactions, performed in aqueous solutions at ambient temperature and pH ∼6 to 9, simulate formation of S and N cross-linked polymers in the natural environment. The complex S and N containing polymer was studied by ¹⁵N enrichment coupled to 2D NMR (¹H, ¹³C, ¹⁵N) techniques and chemical degradation of S-S bonds followed by deuterium labeling and GC-MS analyses. In addition, molecular modeling techniques were used to provide theoretical interpretations and important insights at the molecular level. The results indicate that polysulfide out competes ammonia in the formation of Michael adducts while ammonia is equally competitive with polysulfides when the reaction is addition to the carbonyl position. The co-incorporation of ammonia and polysulfides into carbonyls rapidly forms N and S cross-linked polymers. The effects of ammonia and amines on the polymerization processes are by two means: (i) reaction with carbonyls through an imine functionality to form oligomers and polymers and (ii) catalysis of sulfur nucleophiles onto carbonyls by transfer of a proton which enhances the rate of polymerization. A similar catalytic effect is observed when glycine is used instead of ammonia. This mechanism is especially important under basic to neutral conditions like those that prevail in marine environments. The results show that ammonia and glycine or possibly other amino acids and/or peptides are intimately involved with sulfur nucleophiles throughout the polymerization processes that occur at low temperatures and thus are suggested as key reactants in diagenetic formation of protokerogen and humics.