Monomeric and oligomeric quaternary ammonium derivatives of cholesterol: Concentrated and dilute aqueous solution phase behaviour

Abstract

Five polymerizable and another seven non-polymerizable quaternary ammonium derivatives of cholesterol have been synthesized, and their dilute and concentrated aqueous solution phase behaviour characterized. Electrical conductivity measurements have shown that all the species form small aggregates at concentrations < 10 −4 M, and it seems likely that these have structures similar to those formed by bile acid salts. Surface tension measurements suggest that these molecules do not adsorb spontaneously at an air-water interface. None of the species display thermotropic liquid crystal properties but all form lyotropic liquid crystal phases at high concentration (>0.2 M). The “gel” to liquid crystal transition temperature is typically ∼ + 40°C. The five methacrylate derivatives have been oligomerized to form saturated species with DP n in the range 25–180. These oligomers also form a lamellar phase in water, and the three with the cationic headgroup attached close to the cholesterol nucleus display a transition temperature similar to the monomers. Two oligomers with a C 10 linear hydrocarbon spacer between the headgroup and the nucleus yield an elevated (+20°C) transition temperature, probably because the spacer reliefs steric compression and allows closer packing of the hydrophobes in the “gel” state.