Macroscopic orientation effects in broadline NMR-spectra of model membranes at high magnetic field strength

摘要

The partial orientation of multilamellar vesicles (MLV) in high magnetic fields has been studied and a method to prevent such effects is herewith proposed. The orientation effect was measured with ²H-, ³¹P-NMR and electron microscopy on MLVs of dipalmitoyl phosphatidylcholine with 30 mol% cholesterol. We present the first freeze—etch electron microscopy data obtained from MLV samples that were frozen directly in the NMR magnet at a field strength of 9.4 Tesla. These experiments clearly show that the MLVs adopt an ellipsoidal (but not a cylindrical) shape in the magnetic field. Best fit ³¹P-NMR lineshape calculations assuming an ellipsoidal distribution of molecular director axes to the experimentally obtained spectra provide a quantitative measure of the average semiaxis ratio of the ellipsoidal MLVs and its change with temperature. The application of so-called spherical supported vesicles (SSV) is found to prevent any partial orientation effects so that undistorted NMR powder pattern of the bilayer can be measured independently of magnetic field strength and temperature.The usefulness of SSVs is further demonstrated by a direct comparison of spectral data such as ³¹P-and ²H-NMR lineshapes and relaxation times as well as ²H-NMR dePaked spectra obtained for both model systems. These experiments show that spectral data obtained from partially oriented MLVs are not unambiguous to interpret, in particular, if an external parameter such as temperature is varied.