Effect of sterol structure on the bending rigidity of lipid membranes: A 2H NMR transverse relaxation study

The effect of incorporation of 3–43 mol% sterol on the lipid order and bilayer rigidity has been investigated for model membranes of dimyristoylphosphatidylcholine or dipalmitoylphosphatidylcholine. 2H NMR spectra and spin-lattice relaxation rates were measured for macroscopically aligned bilayers. The characteristics of spectra obtained at temperatures between 0–60 °C are interpreted in terms of a two-phase coexistence of the liquid disordered and the liquid ordered phases and the data is found to be in agreement with the phase diagram published by Vist and Davis (Biochemistry 29 (1990), pp. 451–464). The bending modulus of the bilayers was calculated from plots of relaxation rate vs. the square of the order parameter at 44 °C. Clear differences were obtained in the efficiency of the sterols to increase the stiffness of the bilayers. These differences are correlated to the ability of the sterols to induce the liquid ordered phase in binary as well as in ternary systems; the only exception being ergosterol, which was found to be unable to induce lo phases and also had a relatively weak effect on the bilayer stiffness in contrast to earlier reports.