Competitive Effects of 2+ and 3+ Cations on DNA Compaction

TL;DR

This study investigates how divalent and trivalent cations influence DNA structure, revealing competitive effects that challenge traditional electrostatic models and highlighting the role of ion entropy in DNA compaction.

Contribution

It provides new insights into the competitive interactions of 2+ and 3+ cations on DNA, emphasizing entropy changes over classical shielding effects.

Findings

Divalent cations inhibit DNA compaction induced by trivalent cations.

Divalent cations cause DNA shrinkage in the absence of trivalent cations.

Experimental results challenge the Debye-Huckel additivity model.

Abstract

By using single-DNA observation with fluorescence microscopy, we observed the effects of divalent and trivalent cations on the higher-order structure of giant DNA (T4 DNA with 166 kbp). It was found that divalent cations, such as Mg(2+) and Ca(2+), inhibit DNA compaction induced by a trivalent cation, spermidine (SPD(3+)). On the other hand, in the absence of SPD(3+), divalent cations cause the shrinkage of DNA. These experimental observations are inconsistent with the well-established Debye-Huckel scheme regarding the shielding effect of counter ions, which is given as the additivity of contributions of cations with different valences. We interpreted the competition between 2+ and 3+ cations in terms of the change in the translational entropy of the counter ions before and after the folding transition of DNA. For the compaction with SPD(3+), we considered the increase in translational entropy due to the ion-exchange of the intrinsic monovalent cations condensing on a highly-charged polyelectrolyte, double-stranded DNA, by the 3+ cations. In contrast, the presence of 2+ cation decreases the gain of entropy owe to the ion-exchange between monovalent and 3+ ions.