Orientation and Frequency Dependent NMR Relaxation Studies of Bilayer Membranes: Characterisation of the Lipid Motions

Abstract

Lipid dynamics have been studied by pulsed proton and deuteron nuclear magnetic resonance (’H and 2H NMR) of unoriented and macroscopically aligned bilayer membranes over a wide temperature range, covering three different membrane phases. Longitudinal and transverse relaxation times, T, and TZE, have been recorded as a function of temperature, magnetic field strength and macroscopic orientation of the bilayer normal with respect to the magnetic field. The presence of various minima both in T1z and T2E temperature plots as well as the angle dependence of these relaxation times allows a description of the lipid dynamics by three types of motion, i.e., intramolecular, intermolecular and collective motions. The intramolecular motions consist of local internal reorientations such as trans-gauche isomerisation of the aliphatic chains. These motions are the fastest in the hierarchy of time with correlation times varying from 10−12 s to 10−5 s in the temperature range investigated. The intermolecular motions are assigned to phospholipid long axis rotation and fluctuation. They have correlation times ranging from 1011 s at high temperatures to 10−’s at low temperatures. The slowest motion affecting the ‘H and 2H NMR observables is assigned to viscoelastic moles, i.e., the so-called order director fluctuations and is only detected at high temperatures above the gel-to-fluid phase transition. Comprehensive analysis of the lipid motions is achieved by a dynamic NMR model based on the stochastic Liouville equation. In addition to correlation times, this analysis yields activation energies and order parameters for the various motions, and a value for the bilayer elastic constant.