Conformational analysis of the Sda determinant-containing tetrasaccharide and two mimics in aqueous solution by using 1H NMR ROESY spectroscopy in combination with MD simulations

The conformational behaviour of the spacer-linked synthetic Sd(a) tetrasaccharide beta-D-GalpNAc-(1 --> 4)-[alpha-Neu5Ac-(2 --> 3)]-beta-D-Galp-(1 --> 4)-beta-D-GlcpNAc-(1 --> O)(CH2)(5)NH2 (1) and the two mimics beta-D-Galp-(1 --> 4)-[alpha-Neu5Ac-(2 --> 3)]-beta-D-Galp-(1 --> 4)-beta-D-GlcpNAc-(1 --> O)(CH2)(5)NH2 (2) and beta-D-GlcpNAc-(1 --> 4)-[alpha-Neu5Ac-(2 --> 3)]-beta-D-Galp-(1 --> 4)-beta-D-GlcpNAc-(1 --> O)(CH2)(5)NH2 (3) were investigated by H-1 NMR spectroscopy in combination with molecular dynamics (MD) simulations in water. Experimental 2D H-1 ROESY cross-peak intensities (ROEs) of the tetrasaccharides were compared with calculated ROEs derived from MD trajectories using the CROSREL program. Analysis of these data indicated that the oligosaccharidic skeletons of the compounds 1-3 are rather rigid, especially the beta-D-Hex(NAc)-(1 --> 4)-[alpha-Neu5Ac-(2 --> 3)]-beta-D-Galp fragments. The alpha-Neu5-Ac-(2 --> 3)-beta-D-Galp linkage occurred in two different energy minima in the three-dimensional structure of the compounds 1-3 in aqueous solution. Experimental data and dynamics simulations supported the finding that the higher energy rotamer (CHEAT forcefield) was abundant in compounds 1 and 3 due to the existence of a hydrogen bond between the carboxyl group of the sialic acid and the acetamido group of the terminal monosaccharide (GalNAc or GlcNAc) unit. The conformational similarity between 1 and 3 leads to the suggestion that also their activities will be alike.