CROSS-LINKING OF CELL-WALL PHENOLIC ARABINOXYLANS IN GRAMINEOUS PLANTS

O-[5-O-((E)-p-coumaroyl)-α-l-arabinofuranosyl]-(1→3)-O-β-d-Xylopyranosyl-(1→4)-d-xylopyranose (PAXX) and O-[5-O-((E)-feruloyl)-α-L-arabinofuranosyl]-(1→3)-O-β-D-Xylopyranosyl-(1→4)-D-xylopyranose (FAXX) were isolated from cell walls of Cynodon dactylon by treatment with cell wall carbohydrases. The esters were exposed to light for several days. The p-coumaric acid (P) component of PAXX dimerized to 4,4′-dihydroxy-α-truxillic acid (Mr twice P, i.e. a `PP' type stereodimer) and to other PP type dimers esterified to oligosaccharide. FAXX behaved in a similar manner to PAXX; its ferulic acid (F) component dimerized to esterified 4,4′-dihydroxy-3,3′-dimethoxy-α- truxillic acid (Mr twice F, i.e. an `FF' type stereodimer) plus other esterified FF type dimers. A mixture of PAXX plus FAXX gave esterified 4,4′-dihydroxy-3-methoxy-α-truxillic acid (Mr P plus F, i.e. a `PF' type stereodimer) as the major product. It seems likely that similar dimerization reactions occur in the cell walls of the growing graminaceous plant. Such a mechanism could be involved in cross-linking adjacent p-coumaroyl and feruloyl groups in arabinoxylans leading to increased cell wall rigidity and decreased biodegradability of the wall polysaccharides. © 1990.