Structures of the O-linked oligosaccharides of a complex glycoconjugate from Pseudallescheria boydii

Nonreducing 0-linked oligosaccharides were obtained from the peptidorhamnomannan of mycelia of Pseudallescheria boydii by alkaline β-elimination under reducing conditions. They were separated by gel filtration chromatography to give three oligosaccharide fractions. The major oligosaccharide from fraction 1 was characterized by a combination of techniques including electrospray ionization quadrupole time-of-flight tandem mass spectrometry (ESI MS/MS), matrix-assisted laser desorption ionization mass spectrometry (MALDI MS), nuclear magnetic resonance (NMR), and methylation gas-liquid chromatography-mass spectrometry (GC-MS) analysis. It was branched, with a principal chain of α-Rhap-(1 → 3)-α-Rhap-(1→ 3)-α-Manp-(1→ 2)-Man-ol substituted at 0-6 of mannitol with an α-Glcp-(1→ 4)-β-Galp group. Species containing one and two additional α-Glcp-(1→ 4) sub. stituents in the rhamnose branch were also present. The major component of fraction 2 was a substructure of oligosaccharide-1, lacking a hexose from the Glc-Gal branch. Fraction 3 contained a mixture of smaller, unbranched, oligosaccharides. In hapten inhibition tests, fractions 1 and 2 blocked the reaction between peptidorhamnomannan (PRM) and rabbit anti' P. boydii mycelium hyperimmune serum by ∼75%, whereas fraction 3 inhibited by ∼55%.