Influence of cationic surfactants on DNA conformation

In previous work the binding of the cationic surfactant dodecyltrimethylammonium bromide to DNA was studied. The original work has been extended to include data for the decyltrimethy-lammonium ion. Additionally, while previously considerations of the complex structure were done within the framework of the dynamics of rigid rods, in the present case we have extended this analysis to include a wide range of three-dimensional configurations. Furthermore, the secondary structure of the DNA within the complex is taken into account. Examination of secondary structural changes on surfactant binding indicates that there are no significant changes in the DNA secondary structure. Consideration of the hydrodynamic properties of the surfactant—DNA complex and extension of the experimental data to include decyltrimethylammonium along with application of hydrody-namic modelling allowed us to exclude highly bent or folded complex conformations. The magnitude of the DNA diffusion coefficient decrease on surfactant binding was measured for surfactant molecules of two different tail lengths. The data showed that a rod covered in a single surfactant layer can provide a simple explanation for the difference in the magnitude of the decrease between the two surfactants. In order to account for the observed ratio of 0.8 surfactants per DNA phosphate observed on completion of the first-stage binding, the surfactant headgroups should be located close to the DNA surface, within the condensation volume. This would leave the tail groups projecting outwards, with lateral hydrophobic association between the tails.