Calorimetric Studies On The Interaction Between DNA And Cationic Surfactants

Six cationic surfactants with different alkyl chains and head groups, N-dodecyl-hydroxyethyl-dimethyl ammonium bromide, N-dodecyl-dihydroxyehtyl-methyl ammonium bromide, N-tetradecyl-hydroxyethyl-dimethyl ammonium bromide, N-tetradecyl-dihydroxyethyl-methyl ammonium bromide, N-cetyl-hydroxyethyl-dimethyl ammonium bromide and N-cetyl-dihydroxyethyl-methyl ammonium bromide, have been synthesized and characterized by using IR, NMR and elemental analysis (EA).The critical micelle concentrations (CMC) and the enthalpies of micellization,â–³H_mic(?), for N-dodecyl-trimethyl ammonium bromide, N-cetyl- trimethyl ammonium bromide and the above six surfactants in pure water have been measured in the temperature range from 298.15 K to 323.15 K by using the isothermal titration calorimetry (ITC). It has been found that the CMC value decreases with the increase of the alkyl chain length in the surfactants, while becomes large with the increase of the temperature. For surfactants with the same alkyl chain and different head groups, the more hydroxyethyls the surfactant has, the smaller CMC it has. The Gibbs free energy change,â–³G_mic(?), and the entropy change,â–³S_mic(?), for micellization are obtained using the phase-separation model from the CMC andâ–³H_mic(?) values. Among the thermodynamic functions, the values ofâ–³H_mic(?) andâ–³S_mic(?) decrease with the increase of the temperature, whereas that ofâ–³G_mic(?) nearly keep constant. The enthalpy-entropy compensation effect can be observed for the micellization of these surfactants. Within the investigated temperature range, the entropy change,â–³S_mic(?), is found to be the dominant factor responsible for the micellization. The micelle formation is an enthalpy-driven process.The interactions of the eight cationic surfactants with the calf thymus DNA have also been studied with the isothermal titration calorimetry at 303.15 K and 310.15 K. The experimental results show that the surfactants interacted with DNA, forming two types of c0itical aggregate concentration, CAC₁ and CAC₂, before the formation of the normal micelles. The surfactant—DNA interaction is governed by both of the electrostatic and the hydrophobic interactions. The DNA double helix gets condensed after saturation with surfactants, and the transition from the coil to the globe of the DNA is induced.