1H NMR chemical shifts of hydroxy protons in conformational analysis of disaccharides in aqueous solution

Abstract

In a continuing study on the use of hydroxy protons in conformational analysis of saccharides in aqueous solution by NMR spectroscopy, a number of disaccharides with 1,3-, 1,4- and 1,6-linkages have been investigated. The 1H-NMR chemical shifts, vicinal coupling constants (3JCH,OH), temperature coefficients and rates of exchange with the solvent have been measured for the hydroxy protons. Most of the hydroxy proton signals in the disaccharides have chemical shifts similar (|Δδ| ≤ 0.3 ppm) to those in the corresponding methyl monosaccharides. Three hydroxy protons, however, O(2)H in β-L-Fucp-(1→3)-α-D-Glcp-OMe, O(3)H in β-L-Fucp-(1→4)-α-D-Galp-OMe and O(6)H in β-L-Fucp-(1→4)-α-D-Glcp-OMe show a large upfield shift (Δδ ≤ −0.3 ppm) attributed to the proximity of the hydroxy proton to the ring oxygen of the neighbouring sugar. In β-L-Fucp-(1→4)-α-D-Galp-OMe, the chemical shift, 3JCH,OH and temperature coefficient of the O(3)H signal indicate that a weak hydrogen bond interaction exists between O(3)H and O(5′). In β-L-Fucp-(1→4)-α-D-Glcp-OMe, the chemical shift and the NOEs involving the O(6)H signal suggest a preference for the trans-gauche conformation around the C5–C6 bond.