Slow-Motional NMR Line Shapes for Very Anisotropic Rotational Diffusion. Phosphorus-31 NMR of Phospholipids

The model of Mason, Polnaszek, and Freed for ESR-slow-tumbling spectra is extended to the case of NMR line shapes for arbitrary tilt of the internal axis of relatively rapid rotation with respect to the principal axes of the chemical shielding tensor for a decoupled I = 1 / 2 nucleus. The theory is applied to an analysis of 31P NMR spectra from partially hydrated dipalmitoylphosphatidylcholine (DPPC) molecules over a range of temperature. Generally, very good agreement with experiment is obtained enabling the determination of the rate of the “average” or “effective” internal rotation and the orientation of this “average” axis in the molecule. The basis on which the simple model reflects the spectral effects of the combined internal and overall motions is discussed in detail in an appendix, and the extension to more complex models is outlined,