Issue 1, 2011

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

Abstract

The mycolylarabinogalactan (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.

Graphical abstract: Synthesis and NMR spectroscopic analysis of acylated pentasaccharide fragments of mycobacterial arabinogalactan

Supplementary files

Article information

Article type
Paper
Submitted
14 Jul 2010
Accepted
08 Sep 2010
First published
21 Oct 2010

Org. Biomol. Chem., 2011,9, 165-176

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

C. Liu, M. R. Richards and T. L. Lowary, Org. Biomol. Chem., 2011, 9, 165 DOI: 10.1039/C0OB00423E

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