Phase Behavior of Single DNA in Mixed Solvents

Abstract

Compaction of single large dsDNA chains in aqueous solution in the presence of primary alcohols, acetone, and ethylene glycol has been studied experimentally with the use of a fluorescence microscopy technique. It is found that in the presence of all studied organic solvents single DNA molecules exhibit a discrete phase transition from an elongated coiled to a compacted globular conformation. Interestingly, DNA phase transition occurred at various 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−water mixtures corresponding to the collapsing transition in single DNA differed from that for the other studied systems. The explanation of this phenomenon comes through consideration of the existence of ethylene glycol conformers with various polarities in aqueous solution. Thus, the dielectric permittivity of the solvent is a key factor that determines the conformational behavior of DNA in solution. The compaction of a single DNA molecule when the dielectric permittivity constant is lowered is thought to be due to the increased importance of ion−ion correlation. Monte Carlo simulations for a single polyelectrolyte chain also show that the dimensions of the chain diminish when the electrostatic coupling is increased, i.e., by decreasing the dielectric constant. The experimental result can be rationalized with a simple free energy model balancing the counterion entropy and the ion−ion correlation energy.