Comparative structural studies of lignin-carbohydrate complexes from Digitaria decumbens (pangola grass) before and after chlorite delignification

Abstract

Lignin-carbohydrate complexes (LCCs) were extracted with methyl sulphoxide from cellulase-digested stem cell-walls of mature pangola grass. Comparative studies, using gel filtration, g.l.c., and spectroscopy, were conducted concurrently on digested cell-walls which had also been treated with sodium chlorite. All the LCCs contained hemicellulose and protein with low contents of hydroxyproline. Gel filtration showed markedly increased polydispersity of the LCCs from chlorite-treated residues. Chlorite treatment caused loss of arabinose and galactose, with a concomitant increase in the proportion of xylose degradable by sodium metaperiodate. The LCCs from untreated cell-walls comprised polysaccharide, esterified p-coumaric acid, and a water-insoluble, lignified “hemicellulose A” component (6% carbohydrate) having a low xylose-to-arabinose ratio and a high proportion of hexoses. These components were liberated with alkali, as were, unexpectedly, some free xylose and arabinose. Chlorite-treated walls gave LCCs with ester bonds still present, but p-coumaric acid was degraded to lignin-like products, and a ”hemicellulose A“ xylan (94% carbohydrate) was isolated. The structural relationship of the components in the LCCs from untreated material was complex, and appeared to involve monomer and oligomer pentose units, diesterified at least, in bridging structures with p-coumaric acid. Xylose reducing-units also appeared to be liberated by alkali, suggesting esterification at C-1 in some xylan in the LCC. Thus, chlorite delignification causes significant structural changes in the polysaccharides and non-carbohydrate components of the cell walls, and it is suggested that chlorite treatment be omitted when in situ structural data on plant cell-walls are being sought.