The acidic specific capsular polysaccharide of Rhodococcusequi serotype 3. Structural elucidation and stereochemical analysis of the lactate ether and pyruvate acetal substituents

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Abstract
The specific capsular polysaccharide produced by Rhodococcus equi serotype 3 was found to be a high-molecular-weight acidic polymer composed of D-glucose, D-galactose, D-glucuronic acid, 4-O-[(S)-l-carboxyethyl]-D-mannose, and pyruvic acid in equal molar proportions. Structural analysis, employing a combination of chemical and nuclear magnetic resonance techniques, established that the polysaccharide is composed of linear repeating tetrasaccharide units, [GRAPHICS] in which (R)-1-carboxyethylidene groups bridge the 0-2 and 0-3 positions of the beta-D-glucuronic acid residues. The H-1 and C-13 NMR resonances of the native and depyruvulated.serotype 3 polysaccharides were fully assigned by homo- and heteronuclear chemical shift correlation methods. The absolute configurations of the lactate-substituted mannopyranosyl residues and the pyruvate acetals were determined from H-1-H-1 NOE measurements on the intact polysaccharide. Unequivocal determination of the absolute chirality of the 4-O-[(S)-1-carboxyethyl]-beta-D-mannopyranose residues was achieved by H-1-H-1 NOE measurements on an acetylated lactone derivative of the glycose.