Bonding of hydroxycinnamic acids to lignin: ferulic and p-coumaric acids are predominantly linked at the benzyl position of lignin, not the β-position, in grass cell walls

Abstract

A suspension in dichloromethane–water (18:1, v/v) of various fractions containing hydroxycinnamic acid ester–ether bridges between lignin and polysaccharides prepared from cell walls of matured oat ( Avena sativa L.) intemodes, and a solution of their acetates in the same solvent, were treated with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). This reagent selectively cleaves benzyl ether and ester linkages of negatively charged aromatic nuclei. The sample treated with DDQ was directly hydrolysed either under mild (1 M NaOH, overnight at 37°C) or severe (4 M NaOH, for 2 h at 170°C) conditions. The hydroxycinnamic acids released in the hydrolysate were methylated with diazomethane and analysed quantitatively using gas chromatography. Significant portions of ether linkages between hydroxycinnamic acids and lignin were cleaved with DDQ, which suggests that most of the hydroxycinnamic acids were ether-linked at the benzyl position, and not the β-position, of the lignin side chain as previously claimed.