Molecular motions in lipid bilayers. II. Magnetic resonance of multilamellar and vesicle systems

Abstract

The chain motion model developed in part I is shown to be consistent with a range of NMR data on Lα phase lipid multilayers and vesicles. A simple treatment of correlation effects within the intramolecular (crankshaft) rotations in the weak collision limit leads to a realistic T1 profile along the acyl chain, despite the assumption of a constant order parameter profile. The observed dependence of T1 on chain length is not reproduced however. The measured activation energy (∼13 kJ/mol) for T1 relaxation is consistent with the model, as are the ’’anomalously’’ low T1 and C–D order parameters observed on the methene groups in monounsaturated chains. A very small angular dependence for the 2H methylene T1 at high frequency (≳50 MHz) is suggested. This accords with observation and is a direct consequence of the crankshaft geometry. At temperatures sufficiently above Tm, the nematic fluctuation model of part I semiquantitatively predicts the components of multilayer T1 and T1ρ relaxation rates linear in ω−1/2. However, the description fails for ω∼108 and T∼Tm, in a manner possibly indicating nonrigid chain axis fluctuation on this time scale. A two-dimensional formulation of the nematic model provides a simple description of small vesicle 1H and 2H NMR linewidths, consistent with the experimental data.