LIPO-OLIGOSACCHARIDIC ANTIGEN FROM MYCOBACTERIUM-GASTRI - COMPLETE STRUCTURE OF A NOVEL C4-BRANCHED 3,6-DIDEOXY-ALPHA-XYLO-HEXOPYRANOSE

The following incomplete structure, alpha-X-(1-->3)-[beta-L-Xylp-(1-->4)](6)-3-O-Me-alpha-Rhap-(1-->3)-beta-D-Galp-(1-->3)-beta-D-Glcp-(1--> 4)-2-O-acyl-alpha-D-Glcp-(1<->1)-4,6-di-O-acyl-alpha-D-Glcp, was previously established for the antigenic lipo-oligosacccharide typifying Mycobacterium gastri, namely, LOS-III. The partial structure of the distal monosaccharide (X) was assigned as 3,6-dideoxy-4-C-(1,3-di-O-methylpropyl)-alpha-hexopyranose, which corresponds to a new-found monosaccharide in nature [Gilleron M., Vercauteren J., and Puzo G. (1993) J. Biol. Chem. 268, 3168-3179]. This article reports the complete structure of X, which was determined from the FAB-MS and 2D NMR analysis of the peracetylated LOS-III. The comparative analyses of the native and per-O-acetylated LOS-III FAB-MS spectra revealed, for the monosaccharide X, a molecular mass of 370 Da and five hydroxyl groups that could be acetylated. Additionally, the 1D H-1 NMR spectrum of the per-O-acetylated LOS-III showed a dramatically increased dispersion of the protons, which resonated between 3 and 4 ppm in the spectrum of the underivatized LOS-III. Thus, thanks to 2D NMR sequences (COSY, HOHAHA, HMQC, HMQC-HOHAHA, and HMBC), the complete assignment of the H-1 and C-13 signals was achieved. Starting from the quaternary C4 resonance, the spin system of the C-alkyl chain was assigned, allowing us to propose the following structure, 3,6-dideoxy-4-C-(1,3-dimethoxy-4,5,6,7-tetrahydroxyheptyl)-alpha-xylo-hexopyranose. The xylo configuration was established from the ROESY spectrum.