How does alcohol dissolve the complex of DNA with a cationic surfactant?

The single-chain observation of isolated giant DNAs complexed with a cationic surfactant, CTAB, was performed using fluorescence microscopy. The DNA-CTAB complex exhibits a re-entrant transition, collapsed globule ← elongated coil ← collapsed globule, with an increase in the alcohol concentration. The existence of DNA in its coil state at an intermediate concentration of alcohol implies that this environment is a good solvent for the DNA chains. On the other hand, the presence of the globule state at both low and high alcohol concentrations indicates that this is a poor solvent for the complex. Regardless of this fact, the globule generated at a high alcohol concentration is unexpectedly soluble; i.e., this is a good solvent for the complex with respect to the solvability, but it is a bad solvent with respect to the polymer conformation. This unique property of the complex is attributable to the effect of micelle formation, where surfactant molecules cover the entire globule and lower the surface energy of the collapsed state. This conclusion is supported by additional experiments on the conformational change in DNA with alcohol in the absence of CTAB and on observations with CD and UV spectroscopy for the complex with different alcohol concentrations.