Thermodynamics of cationic lipid binding to DNA and DNA condensation: roles of electrostatics and hydrophobicity.

Alkylammonium binding to DNA was studied by isothermal titration calorimetry. Experimental data, obtained as functions of alkyl chain length, salt concentration, DNA concentration, and temperature, provided a detailed thermodynamic description of lipid-DNA binding reactions leading to DNA condensation. Lipid binding, counterion displacement, and DNA condensation were highly cooperative processes, driven by a large increase in entropy and opposed by a relatively small endothermic enthalpy at room temperature. Large negative heat capacity change indicated a contribution from hydrophobic interactions between aliphatic tails. An approximation of lipid-DNA binding as dominated by two factors-ionic and hydrophobic interactions-yielded a model that was consistent with experimental data. Chemical group contributions to the energetics of binding were determined and could be used to predict energetics of other lipid binding to DNA. Electrostatic and hydrophobic contributions to Gibbs free energy, enthalpy, entropy, and heat capacity could be distinguished by applying additivity principles. Binding of lipids with two, three, and four aliphatic tails was investigated and compared to single-tailed lipid binding. Structurally, the model suggests that lipid cationic headgroups and aliphatic tails distribute evenly and lay down on DNA surface without the formation of micelles.