First-order phase transition in large single duplex DNA induced by a nonionic surfactant.

The effect of the nonionic surfactant Triton X-100 on the conformational behavior of large T4DNA was studied. Through single-molecule observation using fluorescence microscopy, we found that T4DNA macromolecules exhibit a discrete coil-globule transition with an increase in the Triton X-100 concentration. At low surfactant concentrations, all of the DNAs exhibited an elongated coil state, whereas only compacted globular DNAs were observed at high molar fractions of Triton X-100. The formation of DNA globules was not detected at relatively low Triton X-100 concentrations, even above the CMC; DNA collapse occurred in 50-90% solutions of Triton X-100. The increase in osmotic pressure in concentrated Triton X-100 solutions is considered to be the driving force for the compaction of single T4DNAs.