New evidence for covalent coupling of peptides to humic acids based on 2D NMR spectroscopy: A means for preservation

Nitrogen immobilization in soils and sediments involving the preservation of peptides is an important yet not well-understood process. Several hypotheses have been proposed for the preservation of peptides in these systems; however, to date, there is no direct molecular-level evidence for any one of several hypotheses. In this study, a synthesized peptide with 15N-labeled amino acid residues was utilized to examine the mechanism by which humic substances from a peat might chemically interact with peptides to induce preservation. Two-dimensional 15N-1H HSQC NMR experiments were performed to evaluate the bonding and nonbonding interactions between 15N-labeled peptide and an Everglades peat humic acid. The observed changes in cross peaks provide the first direct spectroscopic evidence for what we believe to be covalent binding between these substances. Noncovalent interactions between the 15N-labeled peptides and humic acid molecules are also observed in the spectra. Quinone structures in humic acids are suggested to be the important reactive groups, based on reaction of the labeled peptide with a model quinone which shows similar nitrogen–proton correlated peak patterns as is observed in the HSQC NMR spectra of humic acids reacted with the peptide. The Michael reaction with quinone structures is proposed, and this type of reaction provides a mechanism that is consistent with previous observation for the chemistry of sedimentary nitrogen species.