Phase behavior of single DNA in mixed solvents

Compaction of single large dsDNA chains in aqueous solution in the presence of primary alcohols, acetone, and ethylene glycol has been studied experim entally with the use of a fluorescence microscopy technique. It is found th at in the presence of all studied organic solvents single DNA molecules exh ibit a discrete phase transition from an elongated coiled. to a compacted g lobular conformation. interestingly, DNA phase transition occurred at vario us weight fractions of organic solvents in aqueous solution, but at similar dielectric constants of mixed solvent for all studied primary alcohols and acetone. On the other hand, the dielectric constant of ethylene glycol-wat er mixtures corresponding to the collapsing transition in single DNA differ ed from that for the other studied systems. The explanation of this phenome non comes through consideration of the existence of ethylene glycol conform ers with various polarities in aqueous solution. Thus, the dielectric permi ttivity of the solvent is a key factor that determines the conformational b ehavior of DNA in solution. The compaction of a single DNA molecule when th e dielectric permittivity constant is lowered is thought to be clue to the increased importance of ion-ion correlation. Monte Carlo simulations for a single polyelectrolyte chain also show that the dimensions of the chain dim inish when the electrostatic coupling is increased, i.e., by decreasing the dielectric constant. The experimental result can be rationalized with a si mple free energy model balancing the counterion entropy and the ion-ion cor relation energy.