Structure of the O-deacetylated glucuronoxylomannan from Cryptococcus neoformans serotype C as determined by 2D 1H NMR spectroscopy

The primary structure of the O-deacetylated capsular glucuronoxylomannan (GXM) isolated from Cryptococcus neoformans serotype C was investigated by 2D NMR spectroscopy. Assignment of the 1H NMR chemical shifts for the polysaccharide was accomplished from the analysis of DQF-COSY, TOCSY, NOESY and/or ROESY spectra of three isolates (298, 34, and 401). These isolates contain the same polysaccharide glycosyl residues but in different proportions. The serotype C GXM consists of two repeating polysaccharide units that have the following structures: It is not known if these repeating units comprise a single or two separate polymer chains. The relative amounts of the more highly branched octasaccharide 1 in the isolates studied (i.e., approximately 75% in isolate 34, 50% in isolate 298, and 25% in isolate 401) can be used to explain the serological specificity of these isolates with C. neoformans factor sera, as was previously determined by ELISA in this laboratory. The octasaccharide 1 component is the one previously postulated as the structure of the serotype C GXM although definitive placement of the β-Xyl-(1 → 4) residues had previously not been determined. The heptasaccharide 2 component is uniformly found as the repeating unit in the polysaccharide from serotype B isolates. Additionally, GXM 401 was found to contain a small amount of the hexasaccharide repeating unit usually attributed to serotype A GXM.