The effect of glycerol on the structure of lecithin membranes; a study by freeze-etching and X-ray diffraction

The structure of bimolecular lecithin leaflets (or bilayers) has been studied in lecithin-water and lecithin-glycerol-water systems using freeze-etch and X-ray diffraction techniques. The X-ray diffraction experiments indicate that the thickness of these bilayers increases both with increasing lecithin concentration and with the addition of glycerol. Due to the glycerol-induced thickening of the bilayers the limiting value of about 5 nm is reached at a lower lecithin concentration (70% lipid) in glycerol-treated systems than in specimens prepared with pure water (757; lipid). It is suggested that the addition of glycerol increases molecular order within the bilayers by 'cross-linking' the polar end-groups of the lecithin molecules. The resultant straightening of the molecules increases the bilayer thickness for all concentrations below 7504 lecithin. The freeze-etch studies show that the fracturing properties of the bilayers at -100 degrees C change both when the concentration of lecithin increases from 1% to 80% and when glycerol is added to the preparations. The appearance of all the exposed membrane faces can be explained by a bilayer arrangement of the lecithin molecules. At concentrations below 75% lipid in lecithin-water systems and concentrations below 70% lipid in glycerol-treated specimens the membrane faces are usually covered with plaque structures, which represent island-like remnants of the partly split away upper-lying layer of the bilayers (Staehelin, 1968a). Above these concentrations most bilayers are completely split during the fracturing process. The freeze-etch results also support the idea that glycerol strengthens the cohesive forces in the surface layers of the bilayer membranes and indicate that plaque formation depends on the presence of 'free' water between the bilayers. A parallel freeze-etch investigation of the hydration of bulk lecithin has shown that it could be a two-step process. In the first stage the lecithin molecules appear to be arranged into bilayers while the bound water layer is formed. In the second stage the free water seems to penetrate as a sheet in between the bound water layers of the lipid bilayers and forces them apart. Possible biological implications of these studies are briefly discussed.