Why is the compact state of DNA preferred at higher temperature? Folding transition of a single DNA chain in the presence of a multivalent cation

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

Corresponding author: yoshikaw@scphys.kyoto-u.ac.jp

Received: 15 December 2004
Accepted: 27 May 2005

Abstract

Upon the addition of multivalent cations, a giant DNA chain exhibits a large discrete transition from an elongated coil into a folded compact state. We performed single-chain observation of long DNAs in the presence of a tetravalent cation (spermine), at various temperatures and monovalent salt concentrations. We confirmed that the compact state is preferred at higher temperatures and at lower monovalent salt concentrations. This result is interpreted in terms of an increase in the net translational entropy of small ions due to ionic exchange between higher and lower valence ions.