Cooperativity or phase transition? Unfolding transition of DNA cationic surfactant complex

We recently reported that single duplex DNA, with the size above the order of several tens kilobase pairs, undergoes a large discrete transition from an elongated coil into a collapsed globule with the addition of a cationic surfactant. In the present article, we describe the manner of the unfolding transition of compact long DNA, or globule DNA, complexed with cationic surfactants, cetyltrimethylammonium bromide (CTAB) and distearyldimethylammonium bromide $({\mathrm{D}}_{\mathrm{18}}\mathrm{DAB}\mathrm{),}$ as is induced by the addition of sodium bromide. The conformational dynamics of individual single duplex T4DNA molecules was directly observed with the use of fluorescence microscopy. We found that on the level of individual DNAs, the salt-induced unfolding transition of the globules is largely discrete, or first-order phase transition for the both complexes with CTAB and ${\mathrm{D}}_{\mathrm{18}}\mathrm{DAB}.$ On the other hand, for the ensemble average of the DNAs, the transition is discrete with CTAB but is continuous (sigmoidal) with ${\mathrm{D}}_{\mathrm{18}}\mathrm{DAB}.$ The discreteness for the coil-globule transition in the ensemble of DNAs complexed with CTAB is attributed to the existence of the phase transition in whole over the bulk solution: the sphere-rod transition in surfactant micelles. On the other hand, for ${\mathrm{D}}_{\mathrm{18}}\mathrm{DAB}$ such phase transition on the micelle structure in the bulk solution seems to be absent. In correspondence to such a large difference on the manner of the transition, x-ray diffraction analysis indicates marked difference on the structure of DNA complexes with CTAB and with ${\mathrm{D}}_{\mathrm{18}}\mathrm{DAB}.$