Coil-globule transition of DNA molecules induced by cationic surfactants: a dynamic light scattering study

The compaction and aggregation of DNA induced by cationic surfactants was studied by dynamic light scattering (DLS). Furthermore, the effect on surfactant-compacted DNA of the addition of nonionic amphiphiles and salt was studied. When using sufficiently low amounts of DNA and cetyltrimethylammonium bromide (CTAB), compacted DNA molecules could be monitored by the appearance of a band characterized by lower hydrodynamic radius and by the decrease in the intensity of the peak corresponding to extended DNA molecules. Notably, we observed a region where compacted molecules coexist with extended ones; these two populations were found to be stable with time. For higher concentrations of CTAB, only compacted molecules were observed and the size of the particles increased with time indicating aggregation. The number of globules present in the coexistence region increased linearly with the surfactant concentrations, as given by the area of the band corresponding to this population, which indicates a double-cooperativity of the binding. The DLS experiments were in good agreement with previous fluorescence microscopy studies, with certain advantages over this technique since there is no need to add fluorescence dyes and antioxidants. Furthermore, it allows the study of molecules which are too small to be visualized by fluorescence microscopy.