Ionization and transfection activity of n-methyl-substituted carbamoyl-cholesterol derivatives

Five novel cationic lipids, the polar head group of which was attached to the cholesterol backbone via a tertiary carbamate linker, were synthesized and their physicochemical properties were compared to their transfection efficiencies. Transfection activity of the primary amine analog was highest among the series, while the quaternary ammonium iodide salt was essentially transfection incompetent. Contrary to DC-Chol, methyl and ethyl carbamoyl derivatives of DC-Chol mediated high levels of transfection in the absence of DOPE. Ionization of the cationic assemblies in 40 mM Tris buffer pH 7.2 exactly correlated with the competitive nature of the inductive and steric effects of the methyl groups on the aliphatic nitrogen of the lipids’ polar moiety. Interestingly, the pH interaction zone of all lipid dispersions at 25°C was extended by ± 2 pH units from the pKa, while the pKa of the cationic lipids determined in mixed vesicles composed of 90 % DOPC and cholesterol was approximately 1.3 to 1.5 times higher than that of pure cationic assemblies. The interaction of cationic lipids with plasmid DNA was correlated with pKa, but not the transfection activity.