The identification and quantitation of the hydroxycinnamic acid substituents of a polysaccharide extracted from maize bran

Abstract

The composition and carbohydrate structure of hemicelluloses namely branan ferulate isolated from maize bran has been determined. This novel polysaccharide material is the base polysaccharide of Sterigel™, a Seton Scholl preparation which is being used successfully as a wound management aid. The structure of the polysaccharide was determined by methylation followed by GC-MS analysis, and 1H- and 13C-NMR spectroscopic analysis. It was shown to be composed of a (1→4)-β-D-xylan backbone substituted with a variety of side-chain components, such as single α-D- glucuronic acid residues (via an α-D-GlcA-(1→2)-linkage to the xylan backbone), single α-L-arabinose residues (via α-L-Araf-(1→2)- and/or α-L-Araf-(1→3)-linkages to the xylan backbone) and longer chains of 2–3 monosaccharide residues containing arabinose, xylose and galactose (in the form α-D- Galp-(1→4)-β-D-Xylp-(1→2)-α-L-Araf, linked to the xylan backbone via α-L-Araf-(1→2)- and/or α-L- Araf-(1→3)-linkages). In addition to the carbohydrate, non-carbohydrate moieties such as hydroxycinnamic acids and some proteins, detected using scanning UV spectroscopy (190–450 nm), are present in the branan ferulate. The hydroxycinnamic acids appear to be bound to the polysaccharide chains via ester linkages between their carboxylic acid groups and the C-5 hydroxyl group of a proportion of the α-L-Araf residues, the location of which are detailed above. Predominantly trans-ferulic acid (0.38% w/w), and small amounts of cis-ferulic acid (0.06% w/w) and trans-p-coumaric acid (0.02% w/w) were extracted with ethyl acetate after hydrolysis using 2 M NaOH and identified using GC and GC-MS. Hydrolysis with an aryl ester hydrolase (EC 3.1.1.2) was carried out to confirm the presence of the ferulate ester linkages. HPLC results confirmed that the ferulic acid is released by the enzyme (0.25% w/w). GPC analysis using UV (280 nm) and RI detection confirmed that the ferulic acid was bound to the polysaccharide chain since the distributions directly overlapped, indicating the presence of little or no free ferulic acids. Indeed, free ferulic acid contents of <0.02% w/w were determined in extracts of solid branan ferulate and an aqueous solution of branan ferulate, by HPLC. © 1999 Society of Chemical Industry