Complex formation between DNA and cationic surfactant

Complex formation between short DNA fragments and dodecyltrimethylammonium bromide (DoTAB) has been studied using dynamic (DLS), static light scattering (SLS), high performance capillary electrophoresis (HPCE) and a DoTAB-specific electrode. The combination of diffusion coefficient obtained from DLS, mobility from HPCE and binding isotherm allow us to deduce information about the structure of complexes as a function of bound surfactant. The binding of cationic surfactant to short DNA fragments was shown to proceed in two stages. In the first stage of binding, surfactant ions exchange with counterions “condensed” on the surface of DNA. At this stage the effective charge on DNA does not change as shown by the combination of mobility and diffusion coefficients of the complexes. The observed high cooperativity of binding can be explained by hydrophobic interactions between the surfactant molecules. In the second stage, surfactant molecules bind to DNA without exchange of condensed counterions, which brings about a dramatic change in the effective charge of DNA with phase separation occurring at high DNA concentration.