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 labeled lactam analog of ganglioside G(M4) in an oriented bilayer system composed of L-alpha-dimyristoylphosphatidylcholine (DMPC) and 3-((cholamidopropyl)dimethylammonio)-2-hydroxy-1-propanesulfonate (CHAPSO). This discoidal bilayer system is used to mimic a biological membrane, the natural environment 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 amide C-13 and N-15 labeled sites were measured in G(M4) lactam, using both 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 bilayer 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 families of structures was readily eliminated based on chemical shift measurements, and another was eliminated based on energetic considerations, leaving a single structure to represent the average conformation of the G(M4) lactam headgroup at a membrane surface.