FOLDING OF LONG DNA CHAINS IN THE PRESENCE OF DISTEARYLDIMETHYLAMMONIUM BROMIDE AND UNFOLDING INDUCED BY NEUTRAL LIPOSOMES

The interaction between large duplex T4DNA (166 kbp) and a synthetic d ialkyl cationic lipid, distearyldimethylammonium bromide (D(18)DAB), w as studied using fluorescence microscopy for single-chain observation. The dependence of the higher-order structure of single T4DNAs on the surfactant concentration was evaluated, starting at extremely low valu es (1.0 x 10(-8) M). We found that individual T4DNA chains undergo a m arked discrete transition between elongated coil and compact globule s tates, and that there is a very wide region of coexistence (about two orders of magnitude of the surfactant concentration) for the coiled an d globular T4DNAs. We propose a simple theoretical model for assessing the relationship between the binding equilibrium and the coil-globule transition. In addition, we found that liposomes composed of neutral phospholipids induce unfolding of DNA compacted by a cationic surfacta nt.