Orientation and dynamics of phospholipid head groups in bilayers and membranes determined from 31P nuclear magnetic resonance chemical shielding tensors.

Abstract

31P nuclear magnetic resonance (NMR) powder spectra have been used to determine the principal values of the chemical shielding tensors of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine, and phosphatidic acid. The shielding tensors in all cases were clearly nonaxial. The principal values for the monoester phosphatidic acid shielding tensor are -40, -4, and 48 ppm relative to 85% H3PO4. By contrast the diesters have values of -87, -25, and 119 ppm for phosphatidylcholine, -81, -20, and 105 ppm for phosphatidylethanolamine, and -80, -20, and 112 ppm for phosphatidylserine. This difference reflects the sensitivity of the 31P shielding tensor to chemical environment. Anisotropic motion of the molecules in lamellar dispersions of phospholipids caused an incomplete averaging of the shielding tensors resulting in partially narrowed spectra. Spectra of various phospholipid dispersions were recorded as a function of temperature and transitions observed at the gel-liquid crystalline phase transition temperatures. Using a reasonable set of initial conditions, it was shown that a simple model of molecular motion could successfully predict the observed spectra and their temperature dependences. The model includes rotations about the P-O(glycerol) bond and the molecular z axis and a wobble of the molecule about the bilayer normal. As the temperature increases, the wobble amplitude increases and the spectra narrow. A preliminary 31 P NMR spectrum of chick embryo fibroblasts is included. The similarities between this spectrum and those of the lamellar dispersions indicate that some of the predominant features are due to the phospholipid resonances.