Arabinofuranosyl Oligosaccharides from Mycobacteria:  Synthesis and Effect of Glycosylation on Ring Conformation and Hydroxymethyl Group Rotamer Populations

Abstract

A series of R-D-arabinofuranosyl oligosaccharides (2-8) that are 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 acceptors. 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 those available in the parent monosaccharide, methyl R-D-arabinofuranose (16), allowed the determination of the effect of glycosylation upon ring conformation. At equilibrium, 16 exists as an approximately equimolar mixture of O T 4 (North, N) and 2 T 3 (South, S) conformers. These studies showed that glycosylation of 16 at OH 5 resulted in no significant change in conformer identity or population relative to 16. However, glycosylation of OH 3 resulted in a change in the identity of the N species (to O E) 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.

Approach for the Simulation and Modeling of Flexible Rings: Application to the α-d-Arabinofuranoside Ring, a Key Constituent of Polysaccharides from Mycobacterium tuberculosis

“…13 A histogram of the behavior of this torsion is shown in Figure 5. All three rotamers are populated, but the tg rotamer (180°) is visited infrequently.…”

“…Although intriguing, there are scant data to support the flexible scaffold hypothesis. As part of a program dedicated to understanding the conformation of mycobacterial AG (and LAM), we have carried out a series of NMR studies on the arabinofuranose-containing oligosaccharides 13,14 and coupled these experimental studies with high-level ab initio and density functional theory calculations on methyl α-D-arabinofuranoside (1, Figure 1) 15,16 and related analogs. [17][18][19][20] Given their inherent flexibility, the conformational analysis of furanosides is more complicated than comparable studies with pyranosides as more than one ring conformer must be considered.…”

Design, synthesis and activity of thio-linked arabinofuranosyl disaccharides against mycobacterial tuberculosis (MTB) and Mycobacterium avium complex (MAC)

“…The syrup was acetylated with dry pyridine (3 mL) and Ac2O (3 mL) followed by usual work up and column chromatography (cyclohexane : EtOAc, 9 : 1 to 8 : 2) gave 13 (30 mg, 25%) and 14 (20 mg, 16%). (19). To a solution of 17 (230 mg, 0.48 mM) in anhydrous MeOH (5 mL), freshly prepared solution of MeONa (1M, 0.5 mL) was added drop wise at -78 0 C as reported in 13 and 14 to give thiosodium 18.…”

Determination of rotamer populations and related parameters from NMR coupling constants: a critical review

“…Since that time this program has become widely used in the conformational analysis of five-membered ring containing compounds [59,60,61,62,63,64]. This program was further improved to include 3 J CH coupling constants in the analysis [65].…”