Effect of glycerol on the translational and rotational motions in lipid bilayers studied by pulsed-field gradient 1 H NMR, EPR and time-resolved fluorescence spectroscopy

Glycerol can replace water in both lipid vesicles and lyotropic liquid-crystalline phases. 1,2-Dioleoyl-sn-glycero(3)phosphocholine (DOPC) forms a lamellar (Lα) liquid-crystaline phase in arbitrary mixtures of glycerol and water (Biochim. Biophys. Acta, 1993, 1149, 285.). Monoolein (MO) forms Lα and also cubic liquid-crystalline phases in glycerol–water mixtures. The present study is focussed on characterizing the influence of glycerol on the molecular dynamics in the lipid bilayer. By EPR and time-resolved fluorescence spectroscopy we measure the rotational mobility of spin-labelled fatty acids [2-(3-carboxypropyl)-4, 4-dimethyl-2-tridecyloxazolidin-3-yloxyl (5-DS) and 2-(14-carboxytetradecyl)-2-ethyl-4, 4-dimethyloxazolidin-3-yloxyl (16-DS)] and a hydrophobic fluorophore, 2, 5, 8, 11-tetra-tert-butylperylene (TBPe), respectively. The translational diffusion of MO in the cubic phase is obtained by pulsed-field gradient 1H FT NMR experiments. The rotational rate of 16-DS and TBPe decreases continuously with increasing glycerol concentration, being a factor of 2–3 lower at 100 % glycerol. A continuous decrease in the lipid translational diffusion coefficient, D, is also found with increasing glycerol content, so that D= 12.6 × 10–12 m2 s–1 at 0 % and D= 1.9 × 10–12 m2 s–1 at 100 % glycerol. The effects of glycerol on both the translational diffusion of the lipid in the bilayers and the rotational dynamics of the probe molecules residing in the interior of the hydrophobic regions are ascribed to changes of the viscosity in the interbilayer regions.