Selective chemical depolymerization of rhamnogalacturonans

A method was developed to selectively methyl esterify and then cleave Ga1A residues in pectic polysaccharides. The method was optimized using a rhamnogalacturonan (RG) from Arabidopsis mucilage as a model compound. The carboxyl group of the GalA residues in the RG was selectively methyl esterified using tetrabutylammonium fluoride and iodomethane in Me2SO containing 8% water. A 1D HMQC NMR method to determine the degree of methyl esterification was developed using C-13-iodo-methane as the methylating agent. The methyl-esterified pectins were fragmented by beta-elimination in 0.2 M sodium borate, pH 7.3, at 125 degrees C. The resulting oligoglycosyl fragments, which contain a nonreducing 4-deoxy-beta-L-threo-hex-4-enepyranosyluronic acid residue, were characterized using MALDI-TOF mass spectrometry, monosaccharide composition analysis, and 1D and 2D H-1 and C-13 NMR spectroscopy. Application of this method to branched RG from potato generated low-molecular-weight fragments containing two residues from the RG backbone and a single side chain. In contrast, the fragments obtained when RG is treated with RG lyase contain a minimum of four backbone residues. The chemical method thus facilitates the release and structural characterization of the side-chain structures of RG obtained from various plant sources. The method also provides a convenient method for generating fully or partially methyl-esterified homogalacturonans. (c) 2006 Elsevier Ltd. All rights reserved.