Abstract
P-31 single-pulse and cross-polarization (CP) nuclear magnetic resonance spectra were obtained of aqueous dispersions of pure phospholipids. Dimyristoyl phosphatidylcholine, dipalmitoylphosphatidylcholine, 1-palmitoyl-2-oleoyl phosphatidylcholine, egg phosphatidylcholine, bovine brain sphingomyelin, and transphosphatidylated (from egg phosphatidylcholine) phosphatidylethanolamine were studied. The spectra from all the phospholipids, taken in the usual single-pulse mode, showed the pseudo-axially symmetric powder pattern typical of phospholipids in a hydrated lamellar form. P-31 CP spectra of all the phosphatidylcholines and phosphatidylethanolamine revealed a decrease in intensity in the vicinity of the isotropic chemical shift as long as the lipid was above the gel-to-liquid crystalline phase transition temperature. This intensity pattern has been observed previously for C-13 CP spectra of molecules rotating rapidly about a single well-defined axis (e.g., solid benzene) (Pines, A., M.G. Gibby, and J.S. Waugh, 1973, J. Chem. Phys., 59:569-590). Pure lipid dispersions below their gel-to-liquid crystalline phase transition temperature, including dipalmitoylphosphatidylcholine and sphingomyelin, do not exhibit a local minimum in the CP spectrum at the position of the isotropic chemical shift. Thus, below the phase transition temperature, there is not the same rapid rotation of the headgroup about a well-defined axis. A dramatic change in the rate of headgroup rotation is shown to take place at the pretransition of dipalmitoylphosphatidylcholine. P-31 CP spectra were also obtained for bovine rod outer segment disk membranes, rabbit muscle sarcoplasmic reticulum membranes, a total lipid extract of the latter, and a recombined membrane containing human erythrocyte glycophorin. The CP spectra were similar to the single-pulse spectra, indicating a substantial difference in behavior from pure phospholipid dispersions. This is interpreted in terms of a slower headgroup rotation.