Structural and rheological studies of a polysaccharide mucilage from lacebark leaves (Hoheria populnea A. Cunn.)

A water-soluble mucilage extracted from the leaves of Hoheria populnea was chemically and physically studied. Monosaccharide composition and linkages were determined by high performance anion exchange chromatography (HPAEC), gas chromatography-mass spectrometry (GC-MS) and nuclear magnetic resonance (NMR) spectroscopy. Lacebark mucilage was composed of rhamnose, galactose, galacturonic acid and glucuronic acid (2:1:2:1). Proton and C-13 NMR spectroscopy, and linkage analysis, revealed a predominantly rhamnogalacturonan I-type (RG I-type) structure comprising of a backbone of -> 4]-alpha-D-GalpA-[1 -> 2]-alpha-L-Rhap-[1 ->. Data indicated the mucilage likely comprises of a polymer containing several structurally discrete domains or possibly more than one discrete polymer. One domain contains a RG I-type backbone with branching at 0-3 of GalpA residues to terminal beta-D-GlcpA residues, another similarly contains a RG I-type backbone but is branched at 0-4 of the Rhap residues to terminal GalpA residues or oligosaccharides containing alpha-linked 4-Galp and terminal GalpA residues. A possible third domain contains contiguous 2-Rhap residues, some branched at 0-3. Hydrated mucilage exhibited pseudoplastic flow behaviour and viscoelastic properties of an entangled biopolymer network. These rheological behaviours were only slightly affected by pH and may prove advantageous in potential end-product applications including oral pharmaceuticals or as a food ingredient. (C) 2018 Elsevier B.V. All rights reserved.