Cinnamic acid bridges between cell wall polymers in wheat and phalaris internodes

Abstract

A method has been devised for the quantitative determination of cinnamic acids participating in ester—ether bridges between cell wall polymers based on the different reactivities of free carboxylic acids and their esters towards borohydride reductants and the different susceptibilities of cinnamic acid ester and benzyl ether linkages to alkaline treatments. Lignin—polysaccharide containing fractions extracted with dioxane—H 2O from cell walls of wheat ( Triticum aestivum) and phalaris ( Phalaris aquatica) internodes are hydrogenated using a Pd/C catalyst at room temperature to convert cinnamic acids to their corresponding dihydrocinnamic acids. The sample is subsequently reduced with LiBH 4 in ether—toluene to convert ester-linked dihydrocinnamates to their corresponding alcohols, hydrolysed with 4 M NaOH at 170°, and the dihydrocinnamic acid derivatives released from their etherified forms determined by GC. Using model compounds it was shown that these reactions proceeded quantitatively. The results indicate that all of the etherified ferulic acid in the dioxane—H 2O-soluble fractions of walls of wheat and phalaris internodes is also ester-linked. It has been calculated that there are nine to 10 ferulic acid ester—ether bridges for every 100 C 6—C 6 lignin monomers. p-Coumaric acid is not involved in ester-ether bridges.