NMR analysis of the configuration and conformation of N-acetylneuraminosyl-(2 -> 3)-lactose

1D and 2D C-13 NMR spectra of N-acetylneuraminosyl-(2 --> 3)-lactose were r ecorded and the sign and magnitude of the geminal couplings between C-2 and the H-3 protons of the N-acetylneuraminic acid residue were determined. Th e C-2,H-3ax coupling constant was -8.0 Hz, indicative of an alpha-configura tion at C-2. The vicinal coupling between C-2 and H-3' of the beta-D-galact ose unit was measured to be 5.2 Hz. This result gave a set of four solution s for the dihedral angle psi at the 2 --> 3 glycosidic linkage: +/-16 degre es; +/-149 degrees. NOE experiments established close contacts between the sialic acid and the galactose protons. Force field calculations determined up to eight conformational minima with comparable energy distributed over t he whole range of phi angles but only within a band of psi angles, -60 degr ees < psi < 60 degrees. This indicated a highly flexible 2 --> 3 glycosidic linkage. However, by considering only the two most favored conformations ( phi, psi, = -161 degrees, -21 degrees and -67 degrees, -4 degrees), good ag reement with the coupling constant and the NOE data was found. Copyright (C ) 2000 John Wiley & Sons, Ltd.