The use of chemical shifts of hydroxy protons of oligosaccharides as conformational probes for NMR studies in aqueous solution. Evidence for persistent hydrogen bond interaction in branched trisaccharides

Abstract

The ¹H NMR chemical shifts, vicinal coupling constants, temperature coefficients, exchange rates with solvent and NOEs have been measured for the hydroxy protons of a series of 15 branched trisaccharides in aqueous solution. These compounds are methyl α-D-galactopyranosides substituted at the 3- and 4-positions with either L-fucosyl or D-glucosyl groups. While most of the hydroxy protons in the trisaccharides have chemical shifts similar to those in the corresponding methyl monosaccharides (Δδ ≤ ±0.20 ppm), some hydroxy protons are found to exhibit large upfield shifts. The NMR data together with HSEA and MM2 calculations show a correlation between these large upfield shifts and the proximity of the hydroxy group to oxygen atoms. The largest upfield shifts are observed for hydroxy protons which are in close proximity to ring oxygens O(5). This dependency of chemical shifts of hydroxy protons on the electronic environment might be used as a conformational probe to improve the determination of conformation of oligosaccharides. The NMR data also show that in three branched trisaccharides, α-D-Glcp-(1→3)-[α-D-Glcp-(1→4)]-α-D-Galp-OMe, β-L-Fucp-(1→3)-[α-D-Glcp-(1→4)]-α-D-Galp-OMe and α-D-Glcp-(1→3)-[β-L-Fucp-(1→4)]-α-D-Galp-OMe, there is a persistent hydrogen bond interaction between the O(2′)H of the 3-linked sugar residue and the O(5″) of the 4-linked sugar residue. In the three compounds, a large upfield shift relative to the constituent methyl monosaccharide is observed for the hydroxy proton O(2′)H involved in hydrogen bonding with the O(5″) oxygen. Additional information about the conformation could also be obtained from the inter-residue NOEs involving the exchangeable hydroxy protons. These additional NOEs are in good agreement with a conformation in which the O(2′)H proton and the O(5″) oxygen atoms are involved in a hydrogen bond interaction.