3-DIMENSIONAL STRUCTURE OF GD1B AND GD1B-MONOLACTONE GANGLIOSIDES IN DIMETHYLSULFOXIDE - A NUCLEAR OVERHAUSER EFFECT INVESTIGATION SUPPORTED BY MOLECULAR-DYNAMICS CALCULATIONS

A comparative study on the conformational features of the oligosaccharide moiety of GD1b and GD1b lactone gangliosides, in dimethylsulphoxide, has been carried out by nuclear Overhauser effect investigation; the experimental interresidue contacts have been used for restrained molecular mechanics and dynamics calculations. For GD1b, the tetrasaccharide beta-GalNAc-(1 --> 4)-[alpha-Neu5Ac-(2 --> 8)-alpha-Neu5Ac-(2 --> 3)]-beta-Gal has a circular arrangement leaving a highly hydrophobic region with seven hydrogens pointing towards the center. At one side of this region the three electron rich groups GalNAc-NH, external Neu5Ac-OH4 and internal Neu5Ac-COO- are grouped together; at the other side five polar groups (four hydroxy groups and the external Neu5Ac carboxylate) define a large annular hydrophilic region. The external Neu5Ac is close to the external Gal residue, and the external Neu5Ac-COO- is within van der Waals contact with the inner Neu5Ac-OH9 group. The beta-Gal-(1 --> 3)-beta-GalNAc glycosidic linkage shows a high degree of freedom. For GD1b-L, the trisaccharide beta-GalNAc-(1 --> 4)-[alpha-Neu5Ac-(2 --> 3)]-beta-Gal is disposed to forming rigid partially circular arrangement showing strong interresidue contacts between the inner Neu5Ac-H8 and both GalNAc-H1 and GalNAc-H5. The conformation of the lactone ring is the boat 9(A),2(B)B. The lactonization of the disialosyl residue induces a strong variation of the preexisting torsional glycosidic angles-phi and psi, leaving the external Neu5Ac far from the external Gal. In both GD1b and GD1b lactone gangliosides, the conformation of the sialic acid side chain is the same as that of the free sialic acid in which the H7 is trans to H8 and gauche to H6, thus indicating that the presence of glycosidic and/or ester linkages does not affect the conformational properties of sialic acid. Both GD1b and GD1b lactone containing sialic acid carboxylate anion(s) or undissociated carboxyl group(s) show the same three dimensional structure, indicating that the presence of charges does not affect the intrinsic conformational features of gangliosides.