Nuclear Relaxation Studies of Lecithin Bilayers

Abstract

BILAYER phases of lipids have broad proton magnetic resonance (PMR) spectra, which have been attributed to incomplete averaging of the dipole–dipole interactions between various protons as a result of slow molecular motion1,2. Two pulsed nuclear magnetic resonance (NMR) studies, however, have proposed internal magnetic field gradients as the source of spectral broadening. Kaufman et al. concluded that these gradients led to inhomogeneity broadening3, whereas Hansen and Lawson proposed molecular diffusion through them4. These conclusions were based on the apparent dependence of the spectral width on the applied magnetic field5 as well as abrupt changes in transverse relaxation time (T2) without concomitant changes in the longitudinal relaxation time (T1) at the bilayer phase boundaries4,5. The dependence of T2 on pulse spacing in a Carr–Purcell experiment6 has also been taken as evidence for molecular diffusion through internal magnetic field gradients4.