Phospholipids in Motion: High-Resolution P NMR Field Cycling Studies.

Diverse phospholipid motions are key to membrane function but can be quite difficult to untangle and quantify. High-resolution field cycling P NMR spin-lattice relaxometry, where the sample is excited at high field, shuttled in the magnet bore for low-field relaxation, then shuttled back to high field for readout of the residual magnetization, provides data on phospholipid dynamics and structure. This information is encoded in the field dependence of the P spin-lattice relaxation rate (). In the field range from 11.74 down to 0.003 T, three dipolar nuclear magnetic relaxation dispersions (NMRDs) and one due to P chemical shift anisotropy contribute to of phospholipids. Extraction of correlation times and maximum relaxation amplitudes for these NMRDs provides (1) lateral diffusion constants for different phospholipids in the same bilayer, (2) estimates of how additives alter the motion of the phospholipid about its long axis, and (3) an average P-H angle with respect to the bilayer normal, which reveals that polar headgroup motion is not restricted on a microsecond timescale. Relative motions within a phospholipid are also provided by comparing P NMRD profiles for specifically deuterated molecules as well as C and H field dependence profiles to that of P. Although this work has dealt exclusively with phospholipids in small unilamellar vesicles, these same NMRDs can be measured for phospholipids in micelles and nanodisks, making this technique useful for monitoring lipid behavior in a variety of structures and assessing how additives alter specific lipid motions.