The binding of cationic surfactants by DNA

Isotherms for the binding of dodecyltrimethylammonium (DTA+) and tetradecyltrimethylammonium (TTA+) ions by DNA in aqueous solution at 30 degrees C are reported. The binding isotherms were determined using a potentiometric technique with cationic surfactant-selective electrodes. The DNA concentrations used are 5 X 10(-4) and 10(-3) equiv./kg, surfactant concentrations varying from 3 X 10(-6) M to the critical micelle concentration. The influence of added NaCl (0.01 M) on the binding process is studied. The binding process is shown to be highly cooperative. Applying the binding theory of Schwarz and of Satake and Yang, binding constants and cooperativity parameters can be calculated. The binding constant K is found to be 1.2kT larger for TTA+ than for DTA+ in salt-free solution, and 1.4kT larger for TTA+ than for DTA+ in 0.01 M NaCl. The cooperativity parameter mu is about 1.1kT larger for TTA+ in salt-free solution, and 1.2kT larger in 0.01 M NaCl. It is concluded that the hydrophobic part of the bound surfactant is not completely immersed in the hydrophobic DNA core, but also interacts with other surfactant molecules. This situation is compared to the case of micelle formation.