Marked discreteness on the coil-globule transition of single duplex DNA

From the study of single molecular observation of individual DNA chains by use of fluorescence microscopy, it has become clear that the transition between elongated coil state and compacted globule state is highly discrete on the level of individual chains. On the other hand, the transition appears continuous on the level of the ensemble of DNAs. We report the experimental evidence on this unique property of the DNA chain, using T4 DNA, 166 kilo base pairs (kbp), with the addition of Co(NH 3 ) 6 3+ . Theoretical consideration has been performed on the bimodality in the free energy of single DNA chains, indicating that the character of the coil-globule transition changes largely depending on the valency of the coexistence cationic species.