Covalent bond formation between amino acids and lignin: Cross-coupling between proteins and lignin

The present study characterized the products formed from the reaction of amino acids and in turn, proteins, with lignin resulting in cross-coupling. When added to reaction mixtures containing coniferyl alcohol, horseradish peroxidase and H₂O₂, three amino acids (Cys, Tyr, and Thr) are able to form adducts. The low molecular weight products were analyzed by HPLC and from each reaction mixture, one product was isolated and analyzed by LC/MS. LC/MS results are consistent with bond formation between the polar side-chain of these amino acids with Cα. These results are consistent with the cross-coupling of Cys, Tyr and Thr through a quinone methide intermediate. In addition to the free amino acids, it was found that the cross-coupling of proteins with protolignin through Cys or Tyr residues. The findings provide a mechanism by which proteins and lignin can cross-couple in the plant cell wall.