Probing the effect of acylation on arabinofuranose ring conformation in di- and trisaccharide fragments of mycobacterial arabinogalactan.
A major component of the cell wall of mycobacteria is the mycolyl-arabinogalactan (mAG) complex. The arabinose and galactose residues in mAG are found solely in the furanose form, and it has been suggested that the flexibility of these five-membered rings allows for the tight packing of mycolic acids. In order to probe the "flexible scaffold hypothesis", we designed and synthesized glycolipids 3-6 and 8-11 as simple models of the terminal portion of mAG. A set of donors and acceptors were explored for preparing the key beta-(1-->2) linkage in 2-6, and the best selectivity and yield can be obtained by using the electron-rich thioglycoside donor 14 and the O-5 p-methoxybenzyl-protected acceptor 17. Both alpha-linkages in the trisaccharides 7-11 were formed in a one-pot reaction. The conformations of compounds 2-11 were studied using solution-state NMR spectroscopy, but little change was observed in the coupling constants for the ring protons between 2 and 3-6 or between 7 and 8-11. However, the rotamer populations about the C-4-C-5 bond for the beta-linked ring in disaccharide 2 did change upon acylation at O-5.
Liu
Chunjuan
C
Alberta Ingenuity Centre for Carbohydrate Science and Department of Chemistry, The University of Alberta, Edmonton, Alberta, T6G 2G2 Canada.
Richards
Michele R
MR
Lowary
Todd L
TL
D000215:Acylation; D001089:Arabinose / Q000031:analogs & derivatives* / Q000737:chemistry; D005685:Galactans / Q000737:chemistry*; D006031:Glycosylation; D009682:Magnetic Resonance Spectroscopy; D008968:Molecular Conformation; D015335:Molecular Probes; D009161:Mycobacterium / Q000737:chemistry*
D005685:Galactans; D015335:Molecular Probes; C431714:arabinofuranose; D001089:Arabinose; C005653:arabinogalactan