Synthesis and NMR spectroscopic analysis of acylated pentasaccharide fragments of mycobacterial arabinogalactan. 2011

Chunjuan Liu, and Michele R Richards, and Todd L Lowary
Alberta Ingenuity Centre for Carbohydrate Science and Department of Chemistry, The University of Alberta, Gunning-Lemieux Chemistry Centre, Edmonton, Alberta, Canada.

The mycolyl-arabinogalactan (mAG) complex, a large glycolipid composed of arabinofuranose and galactofuranose monosaccharides and mycolic acid lipids, provides mycobacteria with substantial protection from their environment. It has been proposed that the presence of flexible furanose rings in the mAG facilitates the packing of the hydrophobic mycolic acids, forming a dense protective barrier of low permeability. In a previous article, we probed this "flexible scaffold hypothesis" through the synthesis and NMR analysis of di- and trisaccharide fragments of the mAG acylated with linear fatty acids. However, we saw few conformational changes due to the presence of the acyl chains. We proposed that branched pentasaccharide glycolipids 5-8 might exhibit larger changes due to the presence of more acyl chains, and studies with these compounds are described here. The carbohydrate portion of 5-8 was synthesized in a 1 + 2 + 2 manner. First, the monosaccharide diol was treated with an excess of appropriately protected thioglycoside donor to give a trisaccharide, which, following selective deprotection to a diol, was converted to the pentasaccharide in a one-pot glycosylation. The resulting differentially protected pentasaccharide 20 gave glycolipids 5-8 upon removal of the protecting groups at the primary positions, acylation, and hydrogenolysis. The conformations of 5-8 were probed using NMR spectroscopy, and chemical shift selective filtering 1D-TOCSY spectra allowed for the determination of all the ring coupling constants. It was found that the addition of four fatty acids to the parent pentasaccharide had little effect on the conformation of the compounds in solution.

UI MeSH Term Description Entries
D009682 Magnetic Resonance Spectroscopy Spectroscopic method of measuring the magnetic moment of elementary particles such as atomic nuclei, protons or electrons. It is employed in clinical applications such as NMR Tomography (MAGNETIC RESONANCE IMAGING). In Vivo NMR Spectroscopy,MR Spectroscopy,Magnetic Resonance,NMR Spectroscopy,NMR Spectroscopy, In Vivo,Nuclear Magnetic Resonance,Spectroscopy, Magnetic Resonance,Spectroscopy, NMR,Spectroscopy, Nuclear Magnetic Resonance,Magnetic Resonance Spectroscopies,Magnetic Resonance, Nuclear,NMR Spectroscopies,Resonance Spectroscopy, Magnetic,Resonance, Magnetic,Resonance, Nuclear Magnetic,Spectroscopies, NMR,Spectroscopy, MR
D009844 Oligosaccharides Carbohydrates consisting of between two (DISACCHARIDES) and ten MONOSACCHARIDES connected by either an alpha- or beta-glycosidic link. They are found throughout nature in both the free and bound form. Oligosaccharide
D005685 Galactans Polysaccharides composed of repeating galactose units. They can consist of branched or unbranched chains in any linkages. Galactan,Polygalactose,Polygalactoses
D000215 Acylation The addition of an organic acid radical into a molecule.
D015394 Molecular Structure The location of the atoms, groups or ions relative to one another in a molecule, as well as the number, type and location of covalent bonds. Structure, Molecular,Molecular Structures,Structures, Molecular

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