Structure and dynamics of the sialic acid moiety of GM3-ganglioside at the surface of a magnetically oriented membrane

13C-NMR techniques in oriented bilayer systems composed of DMPC and CHAPSO have been used to measure dipolar interactions between 13C-13C and 13C-1H pairs located in the sialic acid moiety of ganglioside GM3. These interactions are reduced to a structural and motional model for the headgroup of this glycolipid using an order matrix approach. The analysis shows an average structure possessing a high degree of order and is most consistent with structures that are well extended from the membrane surface. Saturation of the ganglioside environment with high concentrations of Ca2+ (0.28 M) produces a small perturbation of the headgroup. GM3 dissolved in a DMPC/CHAPSO system can also be demonstrated to bind wheat germ agglutinin (WGA). The minimal perturbation of structure-dependent parameters suggests that the dominant structure of the sialic acid moiety in the isolated membrane system is favored for binding by the protein.