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

Lignin can cross-couple with the side-chain of amino acids, possibly through a quinone intermediate thereby creating covalent bonds between lignin and proteins. •The reaction of aromatic amino acids with intermediates of lignin biosynthesis was examined.•Products formed from reaction of amino acids...

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Bibliographic Details
Published in:Phytochemistry (Oxford) 2013-12, Vol.96, p.449-456
Main Authors: Cong, Fang, Diehl, Brett G., Hill, Joseph Lee, Brown, Nicole R., Tien, Ming
Format: Article
Language:English
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Summary:Lignin can cross-couple with the side-chain of amino acids, possibly through a quinone intermediate thereby creating covalent bonds between lignin and proteins. •The reaction of aromatic amino acids with intermediates of lignin biosynthesis was examined.•Products formed from reaction of amino acids with intermediates of lignin biosynthesis were identified by mass spectroscopy.•Amino acid adducts were formed with Tyr, Thr and Cys through a reaction of the side-chain with the quinone methide.•The cross-coupling was also shown to occur at the protein level.•These results show how peptide linkages can be introduced into the lignin polymer. 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.
ISSN:0031-9422
1873-3700
DOI:10.1016/j.phytochem.2013.09.012