Hydroxy protons in conformational study of a Lewis b tetrasaccharide derivative in aqueous solution by NMR spectroscopy

Abstract

The 1H NMR chemical shifts, vicinal coupling constants, temperature coefficients, and exchange rates of the hydroxy protons of a Lewis b tetrasaccharide derivative, α- l-Fuc p-(1→2)-β- d-Gal p-(1→3)[α- l-Fuc p-(1→4)]-β- d-Glc pNAc-1-O(CH 2) 2NHCOCHCH 2, have been measured in aqueous solution. The data did not show any evidence for persistent hydrogen bonds participating in the stabilization of the structure. While most of the hydroxy proton signals have chemical shifts similar to those of the corresponding methyl glycosides, four of them, O(3)H, O(4)H, and O(6)H of Gal p, and O(2)H of the Fuc p linked to Glc pNAc, exhibit large upfield shifts. This shielding effect has been attributed to the orientation of the hydroxy protons toward the amphiphilic region constituted by the hydroxy groups of the Gal p residue and mainly the ring and methyl hydrogens of the Fuc p unit attached to the Glc pNAc. The close face to face stacking interaction between the Fuc p linked to the Glc pNAc and the Gal p residues, as well as the steric interaction between the Fuc p linked to the Gal p and the Glc pNAc are confirmed by the additional inter-residue NOEs of the exchangeable protons in sugar units which are not directly connected.