Effect of Hofmeister Series Anions on the Thermotropic Phase Behavior of Bioactive O-Acylcholines

O-Acylcholines (OACs), which are true cationic lipids due to the quaternary ammonium functionality in the headgroup, exhibit interesting biological activities and medicinal properties. In the present study, a homologous series of OACs with even chain lengths (n = 12-20) have been synthesized, and their thermotropic and chaotropic phase transitions have been characterized. The role of various anions (Cl-, Br-, I-, NO3-, SO42-, ClO3-, ClO4-) on the phase behavior of O-stearoylcholine was investigated by calorimetric, spectroscopic, and turbidimetric approaches. The results obtained revealed that in aqueous dispersion O-stearoylcholine undergoes a cooperative phase transition from a gel phase to a micellar structure and that the transition temperature increases when the counterions are changed in the Hofmeister series. Single-crystal X-ray diffraction studies showed that O-stearoylcholine iodide forms an interdigitated bilayer structure, with the polymethylene chain adopting an all-trans conformation. The Hofmeister effect and phase behavior were explained using the concepts of matching water affinities, water penetration into the bilayer, and electrostatic repulsion. It was also observed that one counterion per molecule is sufficient to strongly modulate the phase properties of the lipid/surfactant.