Phospholipid reorientation at the lipid/water interface measured by high resolution 31P field cycling NMR spectroscopy.

The magnetic field dependence of the 31P spin-lattice relaxation rate, R1, of phospholipids can be used to differentiate motions for these molecules in a variety of unilamellar vesicles. In particular, internal motion with a 5- to 10-ns correlation time has been attributed to diffusion-in-a-cone of the phosphodiester region, analogous to motion of a cylinder in a liquid hydrocarbon. We use the temperature dependence of 31P R1 at low field (0.03-0.08 T), which reflects this correlation time, to explore the energy barriers associated with this motion. Most phospholipids exhibit a similar energy barrier of 13.2 +/- 1.9 kJ/mol at temperatures above that associated with their gel-to-liquid-crystalline transition (Tm); at temperatures below Tm, this barrier increases dramatically to 68.5 +/- 7.3 kJ/mol. This temperature dependence is broadly interpreted as arising from diffusive motion of the lipid axis in a spatially rough potential energy landscape. The inclusion of cholesterol in these vesicles has only moderate effects for phospholipids at temperatures above their Tm, but significantly reduces the energy barrier (to 17 +/- 4 kJ/mol) at temperatures below the Tm of the pure lipid. Very-low-field R1 data indicate that cholesterol inclusion alters the averaged disposition of the phosphorus-to-glycerol-proton vector (both its average length and its average angle with respect to the membrane normal) that determines the 31P relaxation.