Dielectric control of counter-ion-induced single-chain folding transition of DNA

¹ Kyoto University

^* To whom correspondence should be addressed. E-mail: yoshikaw{at}scphys.kyoto-u.ac.jp.

Submitted on January 12, 2005
Revised on January 31, 2005
Accepted on 22 February 2005

	Abstract

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. In order to connect this like-charged attraction phenomenon to counter-ion condensation, we performed a series of single-chain experiments on aqueous solutions of DNA, where we varied the extent of counter-ion 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.

Key Words: Counter-ion condensation, DNA compaction, Electron microscopy, Fluorescence microscopy, Like-charged attraction, Single molecule