MEMBRANE AND SOLUTION CONFORMATIONS OF MONOGALACTOSYLDIACYLGLYCEROL USING NMR MOLECULAR MODELING METHODS

The conformation of uniformly C-13-labeled monogalactosyldiacylglycerol (MGDG) is studied both in a membrane environment and in solution using NMR spectroscopy. Analysis of the membrane-bound conformation of MGDG is based on measurement of dipolar interactions between C-13-C-13 and H-1-C-13 spin pairs and on measurement of C-13 chemical shift anisotropies which appear in magnetically-oriented phospholipid-based membrane fragments. Potential energy maps for glycosidic torsions phi, psi, and theta 1 calculated with a membrane interaction energy are used to aia in the interpretation of experimental data. The membrane-bound description for MGDG is most consistent with a set of low-energy conformations that extend the galactose headgroup away from the membrane surface. Analysis of the conformation of MGDG dissolved in CD3OD is based on measured (3)J(CH) and (3)J(HH) scalar couplings. The description of the solution conformation is modeled as a mixture of low-energy conformers predicted in the absence of a membrane interaction term and involves more extensive motional averaging than the model for MGDG embedded in the lipid matrix. Clearly the presence of a membrane interface influences preferred conformations of the galactose headgroup of MGDG when anchored to a membrane surface.