Lipid bilayer conformational equilibria and dynamics studied by 13C PMAS NMR : influence of hydration and of incorporation of detergents

High-resolution "C CPMAS NMR spectra of anhydrous and of hydrated dipalmitoylphosphatidylcholine (DPPC) in gel-phase bilayers have been obtained. Judged from I3C NMR chemical shifts one can conclude that no substantial changes in the conformational equilibria of the acyl chains take place upon hydration, in contrast to conclusions drawn earlier from vibrational spectra. Incorporation of n-tetradecyltrimethylammonium bromide in the bilayers does not cause conformational changes in the chains. Measurements of relaxation times in the rotating frame, TI,, both for 13C and IH lead to the result that the mobilities on the lo5 Hz time scale of the lecithin acyl chains and head groups are progressively decreased upon solubilization of more detergents. Opposite trends are found for the detergents. Those results are in agreement with previously published findings for, e.g., cholesterol solubilization in lipid bilayers, provided that one defines a cross-over region in the frequency domain at ca. lo5 Hz. This view is supported strongly by the results of cross-polarization time (TcH) measurements. The previously postulated squeezing action of phospholipids on solubilized detergents in vesicles is shown to exist also in the gel phase