NMR spectroscopy of hydroxy protons of 3,4-disubstituted methyl α-D-galactopyranosides in aqueous solution

Abstract

The ¹H NMR chemical shifts, coupling constants, temperature coefficients, exchange rates, inter-residue NOEs, and the deuterium isotope effects on ¹³C chemical shifts have been measured, in aqueous solution, for the hydroxy protons of two 3,4-disubstituted galactopyranosides, β-L-Fucp-(1→3)-[α-L-Fucp-(1→4)]-α-D-Galp-OMe and α-D-Glcp-(1→3)-[β-D-Glcp-(1→4)]-α-D-Galp-OMe, and their constituent monomeric methyl glycosides. All the hydroxy proton resonances could be assigned and for both trisaccharides the data indicated hydrogen bonding interactions between the hydroxy groups at the C-2 position of the two non-reducing sugars. Using the hydroxy protons, the number of inter-residue NOEs for the two trisaccharides was increased from 4 to 6 and from 4 to 7, respectively, relative to those obtained in D₂O solutions. These results show that the NMR data obtained from hydroxy protons can provide important information in terms of hydrogen bonding interactions and inter-residue NOEs, which could be further used in structural and conformational analysis to improve the characterisation of oligosaccharides in aqueous solution.