MEMBRANE AND SOLUTION CONFORMATIONS OF MONOGALACTOSYLDIACYLGLYCEROL USING NMR MOLECULAR MODELING METHODS

The conformation of uniformly C-13-labeled monogalactosyldiacylglycero l (MGDG) is studied both in a membrane environment and in solution usi ng NMR spectroscopy. Analysis of the membrane-bound conformation of MG DG is based on measurement of dipolar interactions between C-13-C-13 a nd H-1-C-13 spin pairs and on measurement of C-13 chemical shift aniso tropies which appear in magnetically-oriented phospholipid-based membr ane 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 de scription for MGDG is most consistent with a set of low-energy conform ations that extend the galactose headgroup away from the membrane surf ace. 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 confo rmers predicted in the absence of a membrane interaction term and invo lves more extensive motional averaging than the model for MGDG embedde d in the lipid matrix. Clearly the presence of a membrane interface in fluences preferred conformations of the galactose headgroup of MGDG wh en anchored to a membrane surface.