Enhancing physical stability of positively charged catanionic vesicles in the presence of calcium chloride via cholesterol-induced fluidic bilayer characteristic

An ion pair amphiphile (IPA), hexadecyltrimethylammonium-dodecylsulfate (HTMA-DS), and a double-chained cationic surfactant, dimethyldimyristylammonium bromide (DTDAB), could form positively charged catanionic vesicles with a potential application in gene delivery. To improve the gene delivery efficiency, the addition of CaCl₂ into cationic liposomal systems has been proposed in the literature. In this study, detrimental effect of calcium chloride on the physical stability of the positively charged HTMA-DS/DTDAB catanionic vesicles was demonstrated by the size and zeta potential analyses of the vesicles. It was noted that the reduced electrostatic interaction between the catanionic vesicles could not fully explain the lowered physical stability of the vesicles in the presence of CaCl₂. Apparently, the molecular packing/interaction in the vesicular bilayers played an important role in the vesicle physical stability. To modify the molecular packing/interaction in the vesicular bilayers, cholesterol was adopted as an additive to form catanionic vesicles with HTMA-DS/DTDAB. It was found that the physical stability of the catanionic vesicles was significantly improved with the presence of cholesterol in the vesicular bilayers even in the presence of 50 mM CaCl₂. An infrared analysis suggested that with the incorporation of cholesterol into HTMA-DS/DTDAB vesicular bilayers, the alkyl chain motion was enhanced, and the molecular packing became less ordered. The cholesterol-induced fluidic bilayer characteristic allowed the vesicular bilayers to be adjusted to a stable status, resulting in improved physical stability of the catanionic vesicles even in the presence of CaCl₂ with a high concentration.