Arabinose Conjugates Diagnostic of Ferulate-Ferulate and Ferulate-Monolignol Cross-Coupling Are Released by Mild Acidolysis of Grass Cell Walls

Abstract : Ferulate (FA) units esterified to grass arabinoxylans are involved in cross-linking cell wall polymers. In this work, this contention is strengthened by the identification of FA homo-and heterodimers esterified to methyl arabinofuranoside (MeAra) units after their release from the xylan by mild acidolysis in dioxane/methanol/HCl. Acidolysis of poorly lignified maize bran cell walls provided diferulate (DFA) isomers, including those from 8−5, 8−O−4, and 5−5 interunit bonding, esterified to one or two MeAra units. Acidolysis of lignified grass samples released crossed dimers esterified to one MeAra unit and derived from the β−O−4 coupling of coniferyl alcohol to FA esters. The evaluation of these heterodimeric esters by LC-UV of their aglycones revealed that the parent structures occur in significant amounts in lignified cell walls (0.5−1 mg/g expressed as FA equivalents). The present results position mild acidolysis as an efficient strategy to obtain improved details regarding the FA-mediated cross-linking of grass cell walls.