Interaction between Dodecyltrimethylammonium Chloride and DNA

Binding of dodecyltrimethylammonium cation to calf thymus DNA was studied by using the surfactant-selective electrode as a function of added electrolyte concentration at various temperatures. Binding is highly cooperative and biphasic. An abruptly occurring binding attains a slight shoulder within a very narrow equilibrium concentration range followed by a further augmentation in binding, as the equilibrium concentration is increased. Temperature caused relatively small shift in binding isotherm, but a distinct inversion point was found around 27 °C. Above this temperature the isotherm shifts to the left and then to the right below it. Analogous binding studies as above were repeated with heat-denatured DNA, which binds the surfactant with higher intrinsic affinity but less cooperatively than the intact DNA. A fluorescent probe, pyrene, was employed to assess the microenvironmental polarity of pyrene in DNA before and after surfactant binding. The surfactant–DNA complex has a hydrophobic region akin to ordinary micelle.