Liquid crystal NMR techniques were used to study an isotopicallylabeled lactam analog of gangliosideG
M4 in an oriented bilayer system composed of
L-
-dimyristoylphosphatidylcholine (DMPC) and3-((cholamidopropyl)dimethylammonio)-2-hydroxy-1-propanesulfonate (CHAPSO). Thisdiscoidal bilayer system is used to mimic abiological membrane, the natural environment of G
M4 andother glycolipids. Residual dipolar couplings(
13C-
13Cand
15N-
13C) and chemical shift anisotropyeffects for the amide
13C and
15N labeled siteswere measured in G
M4lactam, using both one- and two-dimensional NMR methods. Thedipolar coupling data were interpreted using atorsional search for preferred geometry about the two elements of theglycosidic bond attaching the headgroup to thebilayer surface. This yielded three independent families ofstructures, all of which were consistent with the dipolarcoupling data. An order matrix analysis was used to compareexperimentally measured changes in chemical shiftsupon orientation to those predicted by chemical shift tensors derivedfrom
ab initio calculations. One of thethreefamilies of structures was readily eliminated based on chemical shiftmeasurements, and another was eliminatedbased on energetic considerations, leaving a single structure torepresent the average conformation of the G
M4lactamheadgroup at a membrane surface.