Osmotic effects on the enclosed volume and interlamellar spacing of multilamellar DODAC vesicles

Dispersions of multilamellar dioctadecyldimethylammonium chloride (DODAC) vesicles are known to have a wide range of sizes and lamellarities. Osmotic effects on these dispersions were investigated by means of NMR diffusometry and small angle X-ray scattering (SAXS). A clear decrease in the enclosed volume and d-spacing was observed upon a hyperosmotic stress (sucrose and CaCl2). However, the decrease was lower than theoretically predicted for ideal osmometers. The difference was explained by the electrostatic repulsion between the charged bilayers and the finite bilayer membrane flexibility. No significant differences were observed when the osmotic stress was applied either above or below the transition temperature (Tm) when sucrose was used. Using CaCl2 as the osmotic agent, the reduction of the enclosed volume was smaller when the hyperosmotic pressure was applied above Tm compared to when it was applied below Tm. This can be explained by similar specific shape changes as observed when unilamellar vesicles were osmotically stressed in the presence of a salt above Tm.