Water-soluble p-carboxybenzylated beechwood 4- O-methylglucuronoxylan: structural features and properties

Abstract

The structural features of the water-soluble p-carboxybenzyl 4- O-methylglucuronoxylan derivative (CBGX), prepared by reaction of xylan with p-carboxybenzyl bromide in aqueous alkali, were characterised by various one-dimensional (1D)- and 2D-NMR techniques applied to the intact derivative as well as to both the oligosaccharide (XO) and resistant (XR) fractions obtained by digestion of CBGX with endo-β- d-xylanase. The real substitution of the xylan backbone with carboxybenzyl (CB) groups was unambiguously determined by the HMBC-NMR technique. The CB groups were located predominantly at the O-2 position of the xylosyl residues. The degrees of substitution of CBGX with 4- O-methylglucuronic acid units (DS U) and CB groups (DS CB) were 0.10 and 0.11, respectively. Thus, every fifth xylose unit of the main chain, on an average, bears either an acidic side chain or a CB substituent. The different proportions of both the substituents in XO and XR fractions suggested the distribution along the xylan backbone to be blockwise rather than uniform. The physicochemical properties of CBGX were characterised by means of viscometry, sedimentation velocity and HPGPC giving a relatively low intrinsic viscosity (53 ml g −1) and similar values of the mean molecular mass ( M w) 27,000 g mol −1 and ( M app) 34,000 g mol −1, respectively. CBGX exhibited remarkable emulsifying and protein foam-stabilising activities.