Conformational analysis of the Sd(a) determinant-containing tetrasaccharide and two mimics in aqueous solution by using H-1 NMR ROESY spectroscopy incombination with MD simulations

The conformational behaviour of the spacer-linked synthetic Sd(a) tetrasacc haride 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 investi gated by H-1 NMR spectroscopy in combination with molecular dynamics (MD) s imulations in water. Experimental 2D H-1 ROESY cross-peak intensities (ROEs ) of the tetrasaccharides were compared with calculated ROEs derived from M D 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-Ga lp fragments. The alpha-Neu5-Ac-(2 --> 3)-beta-D-Galp linkage occurred in t wo different energy minima in the three-dimensional structure of the compou nds 1-3 in aqueous solution. Experimental data and dynamics simulations sup ported the finding that the higher energy rotamer (CHEAT forcefield) was ab undant 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 termi nal monosaccharide (GalNAc or GlcNAc) unit. The conformational similarity b etween 1 and 3 leads to the suggestion that also their activities will be a like.