NMR-STUDIES OF A C-13,N-15-LABELED G(M4)-LACTAM GLYCOLIPID AT AN ORIENTED MODEL-MEMBRANE INTERFACE

Liquid crystal NMR techniques were used to study an isotopically label ed lactam analog of ganglioside G(M4) in an oriented bilayer system co mposed of L-alpha-dimyristoylphosphatidylcholine (DMPC) and opyl)dimet hylammonio)-2-hydroxy-1-propanesulfonate (CHAPSO). This discoidal bila yer system is used to mimic a biological membrane, the natural environ ment of G(M4) and other glycolipids. Residual dipolar couplings (C-13- C-13 and N-15-C-13) and chemical shift anisotropy effects for the amid e C-13 and N-15 labeled sites were measured in G(M4) lactam, using bot h one- and two-dimensional NMR methods. The dipolar coupling data were interpreted using a torsional search for preferred geometry about the two elements of the glycosidic bond attaching the headgroup to the bi layer surface. This yielded three independent families of structures, all of which were consistent with the dipolar coupling data, An order matrix analysis was used to compare experimentally measured changes in chemical shifts upon orientation to those predicted by chemical shift tensors derived from nb initio calculations, One of the three familie s of structures was readily eliminated based on chemical shift measure ments, and another was eliminated based on energetic considerations, l eaving a single structure to represent the average conformation of the G(M4) lactam headgroup at a membrane surface.