Ester and ether linkages between hydroxycinnamic acids and lignins from wheat, rice, rye, and barley straws, maize stems, and fast-growing poplar wood

Abstract

Treatment of solvent extracted wheat, rice, rye, and barley straws, maize stems, and fast-growing poplar wood with 60% aqueous ethanol in 0.2 M HCl at 75 °C for 3 h released 51.8, 51.2, 47.2, 43.7, 54.0, and 16.7% of the original lignin, and 44.3, 50.3, 30.9, 36.1, 40.0, and 25.5% of the original hemicelluloses, respectively. It was found that the bulk of p-coumaric acid (PCA) (67.0–83.5%) was esterified at the lignin side chains, while ferulic acid (FA) is linked to lignin side chains through both ether bonds (51.6–68.3%) and ester bonds (31.7–48.4%), indicating that FA may form intra- and/or inter-molecular ester–ether bridges between lignin fragments, which is first proposed in this study. In addition to p-hydroxybenzoic acid esterified to lignins in the cell walls of wheat straw and fast-growing poplar wood, a small portion of ether-linked p-hydroxybenzoic acid in the lignin preparations, obtained from rice, rye, and barley straws and maize stems, was also detected. It was also detected that noticeable amounts of syringic and vanillic acids were predominantly esterified to the lignin molecules in the cell walls of the materials studied.