Cross-linking of maize walls by ferulate dimerization and incorporation into lignin

Cross-linking of xylans and lignin by ferulates was investigated with prima ry maize walls acylated with 2% ferulate and with ferulate ethyl esters. Pe roxidase-mediated coupling of wall ferulate and ethyl ferulate yielded most ly 8-coupled products, including three new dehydrodimers. Significant quant ities of 5-5-coupled diferulate formed only within walls, suggesting that m atrix effects influence dimer formation. Over 60% of wall ferulate dimerize d upon H2O2 addition, suggesting that xylan feruloylation is highly regulat ed during wall biosynthesis to permit extensive dimer formation at the onse t of lignification. During lignification, ferulate and 5-5-coupled diferula te copolymerized more rapidly and formed fewer ether-linked structures with coniferyl alcohol than 8-5-, 8-O-4-, and 8-8-coupled diferulates. The pote ntial incorporation of most ferulates and diferulates into lignin exceeded 90%. As a result, xylans become extensively cross-linked by ferulate dimeri zation and incorporation to lignin, but only a small and variable proportio n of these cross-links is measurable by solvolysis of lignified walls.