The hex-5-enose degradation: cleavage of glycosiduronic acid linkages in modified methylated Sterculia gums

Abstract

Can. J. Chem. 65,2069 (1987. Reduction of uronic acid residues in permethylated polysaccharides from Sterculin urens and S. caudara gums, followed by transformation of the resulting hexose into 6-deoxy-6-iodohexose residues, affords modified polysaccharides that undergo depolymerization on treatment with zinc dust. The main products after reduction with sodium borohydride are hexenitolterminated oligosaccharides. Their structures have been assigned as members of a series of 0-[a-L-rhamnopyranosyll-(1 +3)-1,2-dideoxyhex-I-enitol derivatives, with and without attendant P-D-galactopyranosyl substituents, on the basis of nuclear magnetic resonance and mass spectral data and compositional analysis of trideuteriomethylated derivatives. The structures of the polysaccharides have been reassessed in the light of these and other experiments, and may now be formulated as containing a backbone of alternating 4-linked a-D-galacturonic acid and 2-linked a-L-rhamnopyranose residues bearing a minimum of three types of side chains attached at specific sites. Characterization of some of the degradation products provides evidence that limited degradation had occurred during methylation of the glycuronans.GERALD OLIVER ASPINALL, LEV KHONDO et BRUCE ALAN WILLIAMS. Can. J. Chem. 65,2069Chem. 65, (1987. La reduction de rtsidus d'acide uronique de polysaccharides permtthylts provenant de gommes de Sterculia urerls et de S. caudata, suivie d'une transformation de l'hexose qui en rtsulte en rtsidus dtoxy-6 iodo-6 hexose, fournit des polysaccharides modifits qui subissent une dtpolymerisation lorsqu'on les traite par de la poudre de zinc. Les produits principaux provenant d'une rtduction par le borohydrure de sodium sont des oligosaccharides se terminant par un hexknitol. En se basant sur des donnees de rtsonance magnttique nucltaire et de spectre d e masse ainsi que sur des analyses de composition des dtrivts trideuttromtthylts, on a attribut leurs structures aux membres d'une strie de dtrivts 0-[a-L-rhamnopyrannosyll-(1-3)-didtoxy-1,2 hexene-1 itols qui pouvaient aussi porter des groupements P-D-galactopyrannosyles. On a rttvalut les structures des polysaccharides a la lumiere de ces experiences ainsi que d'autres et on peut maintenant affirmer qu'elles contiennent une unite de base formte d'acide a-D-galacturonique lit par la position 4 qui alterne avec un rtsidu du a-L-rhamnopyrannose lit par la position 2; cette unite repetitive porte un minimum de trois types de chaines lattrales qui sont attachtes ides sites sptcifiques. La caractirisation de quelques produits de dtgradation fournit des donntes permettant d'affirmer qu'il s'est produit une degradation limitee au cours de la mtthylation des glycuronanes.[Traduit par la revue]Exudate gums from the Sterculia genus contain structurally similar, partially acetylated polysaccharides with residues of D-glucuronic acid, D-galacturonic acid, D-galactose, and L-rhamnose (1-5). The most detailed studies have been carried out on S . urens gum and led to the formulation of two structural regi...

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