The influence of membrane proteins on lipid dynamics.

The application of electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) to the study of phospholipid dynamics in membranes is discussed. Using these complementary spectroscopic techniques it is possible to investigate the dynamics of lipids in membranes over a time scale range of from 10(-10) to 1 s. A rather detailed, quantitative description of phospholipid dynamics in pure lipid/water bilayer dispersions has emerged. For example, the correlation time for phosphate group reorientation has been shown to be of the order of 10(-9) s. Chain dynamics can be modelled in terms of three basic types of motion: reorientation about the long axis, fluctuation of the long axis with respect to the bilayer normal, and gauche-trans isomerization about C-C bonds. In the fluid phase, all of these chain motions are in the fast limit on the NMR time scale, but only the gauche-trans isomerization is fast on the EPR time scale. In the gel phase, all of these motions are in the intermediate time scale regime for NMR. While a similarly detailed description of the influence of protein on lipid dynamics has not yet been obtained, these techniques have demonstrated their capability to perform that task. The limited data available suggest that the major effect of protein on lipid dynamics is to increase the relative importance of motions at lower frequency. This is most clearly evident as a slight increase in the correlation time for phosphate group reorientation. The strongest evidence for slower motion of the hydrocarbon chains is from NMR relaxation time and line width measurements. The interpretation of changes in lipid dynamics in terms of protein/lipid interactions will require further studies of protein/lipid phase equilibria as well as molecular dynamics.