H-1-NMR STUDY OF GM2 GANGLIOSIDE - EVIDENCE THAT AN INTERRESIDUE AMIDE CARBOXYL HYDROGEN-BOND CONTRIBUTES TO STABILIZATION OF A PREFERRED CONFORMATION

Several properties of the exchangeable amide protons of the ganglioside G(M2) were studied in detail by H-1-NMR spectroscopy in fully deuterated dimethylsulfoxide [H-2(6)]DMSO/2% H2O, and compared with data obtained for the simpler constituent glycosphingolipids G(A2) and G(M3). In addition to chemical shifts, 3J2,HN coupling constants, and temperature shift coefficients, the kinetics of NH/H-2 chemical exchange were examined by following the disappearance of the amide resonances in [H-2(6)]DMSO/2% (H2O)-H-2. The results included observation of an increase in half-life of the N-acetylgalactosamine acetamido HN by more than an order of magnitude in G(M2) compared to G(A2), attributable to the presence of the additional N-acetylneuraminic acid residue. Additional one-dimensional dipolar cross relaxation experiments were also performed on nonexchangeable protons of G(M2). The results of all of these experiments support a three-dimensional model for the terminal trisaccharide in which a hydrogen bond is formed between the N-acetylgalactosamine acetamido NH and the N-acetylneuraminic acid carboxyl group. The interaction is proposed to be of the pi-acceptor type, a possibility which has not yet been explored in the literature on carbohydrates. The proposed model is discussed in comparison with that of Sabesan et al. (1984, Can J Chem 62:1034-45), and the models of G(M1) proposed more recently by Acquotti et al. (1990, J Am Chem Soc 112:7772-8) and Scarsdale et al. (1990, Biochemistry 29:9843-55).