The compactization of a single DNA molecule in polyethylene glycol (PEG) solution was investigated both theoretically and experimentally. A theory is proposed taking into account the polyelectrolyte effect and redistribution of PEG within DNA coils. This approach makes it possible to describe the dependence of critical value, c, of PEG concentration at the point of DNA collapse on the degree of PEG polymerization, P, and on the concentration of low-molecular salt, ns. Observation of single DNA molecule in solution of PEG has been carried out by means of fluorescence microscopy which allows one to observe the conformation of individual DNA directly. Direct evidence that the coil–globule transition of DNA occurs as first order phase transition was obtained. It was confirmed that the critical concentration of PEG decreases with an increase of the degree of PEG polymerization and salt concentration. The width of the coexistence region of coil and globule was found to be dependent on salt concentration and degree of polymerization of PEG. It was found that DNA undergoes re-entrant globule–coil transition in concentrated solution of high-molecular weight PEG. These experimental results correspond well to the theoretical predictions.

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Recently, it has become clear that single, long duplex DNAs exhibit a large discrete transition between elongated coil and compacted globule states. To obtain further insight into this phenomenon, in the present study we observed individual DNA chains in an aqueous environment by fluorescence microscopy. The long-axis lengths of individual T4DNA (166 kbp) were calibrated to obtain a size distribution. The main purpose of the present study was to determine the effect of the valence of cations on the coil−globule transition. We used the following multivalent cations to induce the compaction of long DNA chains:  1,3-diaminopropane (bivalent), spermidine (trivalent), and spermine (tetravalent). Our results showed that the collapse of isolated DNA chains induced by either bivalent or multivalent cations is discrete. The critical concentration of cation for inducing the transition was lowest for the tetravalent cation and highest for the bivalent cation. We also compare the properties of the transition observed experimentally with a theoretical calculation including the effects on condensation of multivalent cations and ion-exchange reaction.

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