Abstract:
Order parameter profiles extracted from the NMR spectra of model membranes are a valuable
source of information about their structure and molecular motions. To al1alyze powder spectra
the de-Pake-ing (numerical deconvolution) ~echnique can be used, but it assumes a random
(spherical) dist.ribution of orientations in the sample. Multilamellar vesicles are known to deform
and orient in the strong magnetic fields of NMR magnets, producing non-spherical orientation
distributions. A recently developed technique for simultaneously extracting the anisotropies
of the system as well as the orientation distributions is applied to the analysis of partially
magnetically oriented 31p NMR spectra of phospholipids. A mixture of synthetic lipids, POPE
and POPG, is analyzed to measure distortion of multilamellar vesicles in a magnetic field. In
the analysis three models describing the shape of the distorted vesicles are examined. Ellipsoids
of rotation with a semiaxis ratio of about 1.14 are found to provide a good approximation of
the shape of the distorted vesicles. This is in reasonable agreement with published experimental
work. All three models yield clearly non-spherical orientational distributions, as well as a precise
measure of the anisotropy of the chemical shift. Noise in the experimental data prevented the
analysis from concluding which of the three models is the best approximation. A discretization
scheme for finding stability in the algorithm is outlined
