MOLECULAR-MOTION AND TRANSITIONS IN SOLID TRIPALMITIN MEASURED BY DEUTERIUM NUCLEAR-MAGNETIC-RESONANCE

Deuterium nuclear magnetic resonance (NMR) quadrupole echo spectra of deuterated acyl chains and the glyceryl moiety of tripalmitin were found to depend on the crystal form. At 20-degrees-C, which is below melting points, line shapes indicate that molecular motions in the beta form (triclinic subcell) are more restricted than in beta' (orthorhombic) or alpha (hexagonal). Motional rates in excess of about 20 kHz are responsible for the line shapes. Spin-lattice relaxation, sensitive to motional frequencies in the tens of megahertz range, is much faster for methyl (CD2) groups, in alpha or beta' than in beta, indicating that fast motions are also governed by crystal form. General theoretical considerations suggest that motion of methylene groups is dynamically heterogeneous and that motion of methyl (CD3) groups may be averaged by motions other than rotation about the terminal C-C bond. The isothermal solid-state transition from alpha to beta, induced by increasing the temperature to 35-degrees-C, was accompanied by NMR lineshape changes consistent with immobilization. The reversible transition of alpha to "sub-alpha" upon cooling, accompanied by orthorhombic-like Bragg spacings and other changes in the X-ray pattern and by corresponding changes in the infrared spectrum, also produced a marked restriction in NMR-detected mobility of the kind seen in beta' relative to alpha. The advantages of H-2 NMR for studies of motions and transitions in solid glycerides are discussed.