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Dielectric Control of Counterion-Induced Single-Chain Folding Transition of DNA

Department of Physics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan

Correspondence: Address reprint request to Kenichi Yoshikawa, Tel.: 81-75-753-3812; Fax: 81-75-753-3779; E-mail: yoshikaw{at}scphys.kyoto-u.ac.jp.

In the presence of condensing agents, single chains of giant double-stranded DNA undergo a first-order phase transition between an elongated coil state and a folded compact state. To connect this like-charged attraction phenomenon to counterion condensation, we performed a series of single-chain experiments on aqueous solutions of DNA, where we varied the extent of counterion condensation by varying the relative dielectric constant _r from 80 to 170. Single-chain observations of changes in the conformation of giant DNA were performed by transmission electron microscopy and fluorescence microscopy, with tetravalent spermine (SPM⁴⁺) as a condensing agent. At a fixed dielectric constant, single DNA chains fold into a compact state upon the addition of spermine, whereas at a constant spermine concentration single DNA chains unfold with an increase in _r. In both cases, the transition is largely discrete at the level of single chains. We found that the critical concentration of spermine necessary to induce the single-chain folding transition increases exponentially as the dielectric constant increases, corresponding to 87–88% of the DNA charge neutralized at the onset of the transition. We also observed that the toroidal morphology of compact DNA partially unfolds when _r is increased.