Improved procedures for the selective chemical fragmentation of rhamnogalacturonans

The structural characterization of branched rhamnogalacturonans (RGs) requires the availability of methods that selectively cleave the Rhap-(1-->4)-alpha-GalAp linkage and thereby generate oligosaccharide fragments that are suitable for mass spectrometric and NMR spectroscopic analyses. Enzymic cleavage of this linkage is often ineffective, especially in highly branched RGs. Therefore, we have developed an improved chemical fragmentation method based on beta-elimination of esterified 4-linked GalpA residues. At least 85% of the carboxyl groups of the GalA residues in Arabidopsis thaliana seed mucilage RG is esterified using methyl iodide or 3-iodopropanol in Me(2)SO containing 8% water and 1% tetrabutylammonium fluoride. However, beta-elimination fragmentation at pH 7.3 and 120 degrees C is far more extensive with hydroxypropyl-esterified RG than with methyl-esterified RG. The non-reducing 4-deoxy-beta-l-threo-hex-4-enepyranosyluronic acid residue formed by the beta-elimination reaction is completely removed by treatment with aqueous N-bromosuccinimide, thereby simplifying the structural characterization of the chemically generated oligoglycosyl fragments. This newly developed procedure was used to selectively fragment the branched RG from peppergrass seed mucilage. The products were characterized using MALDI-TOF mass spectrometry, glycosyl residue composition analysis, and 1 and 2D NMR spectroscopy. Our data show that the most abundant low-molecular weight fragments contained a backbone rhamnose residue substituted at O-4 with a single sidechain, and suggest that peppergrass seed mucilage RG is composed mainly of the repeating unit 4-O-methyl-alpha-d-GlcpA-(1-->4)-beta-d-Galp-(1-->4)-[-->4)-alpha-d-GalpA-(1-->2)-]-alpha-l-Rhap-(1-->.