BOROHYDRIDE-SOLUBLE LIGNIN CARBOHYDRATE COMPLEX ESTERS OF PARA-COUMARIC ACID FROM THE CELL-WALLS OF A TROPICAL GRASS

Lignin-carbohydrate complex (LCC), solubilised by treatment of the cell walls of pangola grass (Digitaria decumbens) with borohydride, was fractionated from solution in methyl sulphoxide with ethanol into insoluble (LCC-P) and soluble (LCC-S) components. LCC-P had a higher concentration of carbohydrate (mainly pentoses) but less esterified p-coumaric acid than LCC-S. Galactose was found only in LCC-P. Treatment of LCC-P and LCC-S with alkali liberated most of the pentose and galactose in water-soluble material which, on gel filtration, gave two polysaccharide fractions from LCC-P and one oligosaccharide from LCC-S, together with p-coumaric acid (from LCC-P and LCC-S) and small amounts of p-hydroxybenzoic and vanillic acids (only from LCC-P). Of the alkali-degraded LCC-P and LCC-S, ∼75% remained insoluble in neutral solution and contained only ∼4% of carbohydrate. Methylation analyses established the water-soluble carbohydrates to be mainly branched arabinoxylans. The neutral-insoluble material contained (1 → 4)-linked glucose residues in addition to a short-chain arabinoxylan. Most of the pentosan was linked in the LCC by alkali-labile bonds that did not involve uronic acid esters. The presence of glycosidic bonds between lignin and a small proportion of arabinoxylan, and also between lignin and a glucan, was indicated. © 1990.