Lignin cross-links with cysteine- and tyrosine-containing peptides under biomimetic conditions

The work presented here investigates the cross-linking of various nucleophilic amino acids with lignin under aqueous conditions, thus providing insight as to which amino acids might cross-link with lignin in planta. Lignin dehydrogenation polymer (DHP) was prepared in aqueous solutions that contained tripeptides with the general structure XGG, where X represents an amino acid with a nucleophilic side chain. Fourier-transform infrared spectroscopy and energy dispersive X-ray spectroscopy showed that peptides containing cysteine and tyrosine were incorporated into the DHP to form DHP-CGG and DHP-YGG adducts, whereas peptides containing other nucleophilic amino acids were not incorporated. Scanning electron microscopy showed that the physical morphology of DHP was altered by the presence of peptides in the aqueous solution, regardless of peptide incorporation into the DHP. Nuclear magnetic resonance (NMR) spectroscopy showed that cysteine-containing peptide cross-linked with lignin at the lignin α-position, whereas in the case of the lignin-tyrosine adduct the exact cross-linking pathway could not be determined. This is the first study to use NMR to confirm cross-linking between lignin and peptides under biomimetic conditions. The results of this study may indicate the potential for lignin-protein linkage formation in planta, particularly between lignin and cysteine- and/or tyrosine-rich proteins.