The use of proton-enhanced, natural abundance 13C NMR to study the molecular dynamics of model and biological membranes.

The effect of intrinsic membrane protein on the mobility of lipid bilayers has been the subject of many nuclear magnetic resonance (NMR) and electron spin resonance (ESR) investigations [l-6]. Early ESR experiments employing nitroxide probes were interpreted to show that some of the lipid within the membrane is immobilized by the presence of the protein [3]. This result has been subsequently confirmed in a number of reconstituted and model lipid-protein systems [3-61. Recent deuterium NMR studies of a series of lipid-protein dispersions have shown, however, that the quadrupolar splittings, derived from lipids with deuterated methylene groups, are freq uently reduced compared to those obtained from the same labels in pure lipid bilayers [ 2,7]. These observations have been interpreted as indicating an increase in the amplitude of the low frequency (-10’ Hz) motion undergone by the lipid acyl chains in the presence of the protein [2]. It is argued that the frequency of this additional lipid motion is insufficiently rapid to cause a reduction in the hyperfme field splittings seen in the ESR spectrum. Here we use proton-enhanced NMR [8] of the natural abundance 13C nuclei to study the lipid mobility in dispersions containing cholesterol, the polypeptide gramicidin A, and in membrane preparations derived from spinach chloroplasts and bovine brain myelin.