1H NMR chemical shifts of hydroxyprotons 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 (³J_CH,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, ³J_CH,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.