Order and Disorder in a Liquid Crystalline Bilayer: Pulsed Field Gradient NMR Studies of Lateral Phase Separation

Abstract

The pulsed field gradient NMR method utilized for determination of lipid lateral diffusion in macroscopically aligned lipid bilayers is described. This technique is proposed to have an appreciable potential for biophysical investigations in the field of lipid (lipidomics) and membrane biology. Topics like transport of molecules both across and in the plane of the membrane can be studied successfully, as well as the formation of lipid domains and their intrinsic dynamics can be scrutinized. In this review some recent results on lipid membrane systems are presented. In particular investigations, not easily monitored with other spectroscopic methods, of so‐called “raft membranes” are presented. In these membranes two‐phase coexistence between liquid disordered (l d) and liquid ordered (l o) phases occurs. Methods for obtaining good oriented lipid membranes, necessary for the NMR method to be efficiently used, are also described. Quite unexpectedly, cholesterol (CHOL) seems to partition into both phases to roughly the same extent, indicating that CHOL has no particular preference for any of the l d and l o phases, and that there are no specific interactions between CHOL and saturated lipids. Another interesting observation is that the lateral diffusion is the same for all components, independent of the molecular structure (thus including CHOL), if they reside in the same domain in the membrane. Furthermore, it is found that the domain formation process is sensitive to small changes in the phospholipid chain composition and the structure of the sterol. However, more studies are needed to explain the effect on the raft‐forming properties from changes in the positions and the number of double bonds in the sterol ring system.