Synthesis, NMR spectroscopy and conformational studies of the four anomeric methyl glycosides of the trisaccharide D-Glcp-(1→3)-[D-Glcp-(1→4)]-α-D-Glcp

Abstract

The four anomeric methyl glycosides of the vicinally disubstituted trisaccharide D-Glcp-(1→3)-[D-Glcp-(1→4)]-α-D-Glcp have been synthesized using silver trifluoromethanesulfonate mediated glycosylations. The ¹H and ¹³C NMR resonances have been assigned and used for extraction of glycosylation shifts, i.e. the differences between chemical shifts for signals from the trisaccharides and those of the respective monomers, as well as those derived by addition of the glycosylation shifts for each disaccharide element. Glycosylation shifts are up to 0.5 ppm for proton and 10 ppm for carbon. Deviations from additivity are –0.2–0.1 ppm for proton and –4.5–2.3 ppm for carbon, usually confined to the atoms at the linkage positions. The conformational space spanned for the trisaccharides, and the constituent disaccharides, has been investigated by Metropolis Monte Carlo simulations using the HSEA force field. The α-linked glucosyl groups show larger conformational changes with multiple energy minima, whereas the β-linked glucosyl groups have a single energy minimum, close to that identified for the constituent disaccharide.