Reentrant phase transitions of DNA-surfactant complexes

Complexes of double-stranded DNA with the cationic surfactant cetyltrimethylammonium bromide have been studied using small angle x-ray diffraction at varying concentrations of DNA and the cosurfactant hexanol. At low DNA concentrations, an intercalated hexagonal ${(H}_I^c)\to \mathrm{lamellar}$ ${(L}_{\alpha }^c)\to \mathrm{inverted}$ hexagonal ${(H}_{\mathrm{II}}^c)$ transformation is found on increasing hexanol content. The $H_{\mathrm{II}}^c$ structure is converted into $L_{\alpha }^c$ on adding more DNA. Further increase in hexanol content leads to a phase separation in the surfactant solution, and a reentrant $L_{\alpha }^c\to H_{\mathrm{II}}^c\to L_{\alpha }^c$ transition is observed as DNA concentration is increased. Such structural transformations of DNA-surfactant complexes, driven by DNA concentration, have not been reported until now.