Spontaneous formation of pH-sensitive, stable vesicles in aqueous solution of N-[4-n-octyloxybenzoyl]-l-histidine

Abstract

A new amino acid-derived surfactant, sodium N-(4-n-octyloxybenzoyl)-L-histidinate (SOBH), was synthesized. The self-assembly properties and microstructure formation of SOBH was investigated by use of a number of techniques, such as surface tension, fluorescence spectroscopy, conductivity, dynamic light scattering (DLS), confocal fluorescence microscopy (CFM), and transmission electron microscopy (TEM). The amphiphile exists in both anionic and zwitterionic forms depending upon the pH of the solution and has very low critical aggregation concentration (CAC). The results of fluorescence probe studies and TEM as well as CFM pictures showed formation of vesicles in solution of pH > 7.5. The mean size and size distribution of the vesicles was measured by the DLS technique. The interaction of the amphiphile with cholesterol was also examined. In the presence of cholesterol the vesicle size and stability was observed to increase. Effects of salt, surfactant concentration, and temperature on the vesicle formation were investigated with or without varying amounts of cholesterol. Fluorescence anisotropy data indicated that the packing in bilayers becomes tighter by inclusion of cholesterol, which increased the stability of vesicles. The vesicles formed by SOBH were also found to be stable at body temperature.