Arabinofuranosyl oligosaccharides from mycobacteria: Synthesis and effect of glycosylation on ring conformation and hydroxymethyl group rotamer populations

A series of alpha-D-arabinofurnnosyl oligosaccharides (2-8) that an fragments of the arabinan portions of two polysaccharides present in the cell wall of Mycobacterium tuberculosis have been synthesized. Preparation of the oligosaccharides involved the sequential addition of arabinofuranosyl residues from thioglycoside donors to methyl glycoside accepters. High-resolution NMR studies on the final products provided all (3)J(H,H) values, which were in turn used in PSEUROT 6.2 calculations to determine both the identity and equilibrium populations of preferred conformers for each furanose ring in these glycans. Comparison of the ring conformers present in 2-8 with these available in the parent monosaccharide, methyl alpha-D-arabinofuranose (16), allowed the determination of the effect of glycosylation upon ring conformation. At equilibrium, 16 exists as an approximately equimolar mixture of T-0(4) (North, N) and T-2(3) (South, S) conformers. These studies showed that glycosylation of 16 at OH5 resulted in no significant change in conformer identity or population relative to 16. However, glycosylation of OH3 resulted in a change in the identity of the N species (to E-O) and a significant favoring of this conformer at equilibrium. These trends were seen in all of the oligosaccharides. The populations of the three possible staggered rotamers (gg, gt, tg) about the C4-C5 bond were essentially the same for all residues in 2-8, and thus this equilibrium does not appear to be sensitive to glycosylation.