Molecular organization and motions of crystalline monoacylglycerols and diacylglycerols: a C-13 MASNMR study.

Six saturated acylglycerols (1-myristoyl-sn-glycerol, 1-palmitoyl-sn-glycerol, 1,2-dimyristoyl-sn-glycerol, 1,2-dipalmitoyl-sn-glycerol, 1,2-dipalmitoyl-rac-glycerol, and 1,3-dimyristoylglycerol) were studied in their various polymorphic forms (sub-alpha, alpha, beta') by natural abundance C-13 nuclear magnetic resonance (NMR) with magic angle spinning (MASNMR). C-13 MASNMR does not require single crystals and can observe relatively disordered crystals, distinct advantages over crystallographic diffraction methods. Well resolved spectra were obtained for each acylglycerol, and the chemical shifts of corresponding carbons were different for each crystalline phase and the isotropic liquid phase; moreover, in the case of monoacylglycerols, the symmetrically nonequivalent molecules in the same crystalline structure gave distinct C-13 resonances for the same carbon. The C-13 chemical shifts corresponding to each polymorphic phase were interpreted in terms of differences in intramolecular bond distances, intermolecular interactions (such as H bonding), and molecular motions. Mobilities of the glycerol backbone and acyl chains were assessed by the C-13 linewidths and the C-H dipolar relaxation rates. The chemical shift anisotropy(ies) (delta sigma) of the carbonyl group(s) of each acylglycerol was determined from slow-spinning MAS spectra, and was discussed in terms of the conformational and/or motional changes for the carbonyl carbon(s).