DNA like$-$charge attraction and overcharging by divalent counterions in the presence of divalent co$-$ions

TL;DR

This study uses Monte Carlo simulations to explore how divalent co-ions influence DNA overcharging and condensation, revealing that divalent co-ions weaken DNA attraction and alter ion distributions compared to monovalent co-ions.

Contribution

It provides new insights into the role of divalent co-ions in DNA electrostatics, highlighting their impact on overcharging and ion screening effects.

Findings

Divalent co-ions increase monovalent salt in DNA condensates.

Presence of divalent co-ions reduces DNA overcharging by multivalent counterions.

Divalent co-ions lead to stronger DNA-DNA repulsion at large distances.

Abstract

Strongly correlated electrostatics of DNA systems has drawn the interest of many groups, especially the condensation and overcharging of DNA by multivalent counterions. By adding counterions of different valencies and shapes, one can enhance or reduce DNA overcharging. In this papers, we focus on the effect of multivalent co-ions, specifically divalent co-ions such as SO$_4^{2-}$. A computational experiment of DNA condensation using Monte$-$Carlo simulation in grand canonical ensemble is carried out where DNA system is in equilibrium with a bulk solution containing a mixture of salt of different valency of co-ions. Compared to system with purely monovalent co-ions, the influence of divalent co-ions shows up in multiple aspects. Divalent co-ions lead to an increase of monovalent salt in the DNA condensate. Because monovalent salts mostly participate in linear screening of electrostatic interactions in the system, more monovalent salt molecules enter the condensate leads to screening out of short-range DNA$-$DNA like charge attraction and weaker DNA condensation free energy. The overcharging of DNA by multivalent counterions is also reduced in the presence of divalent co$-$ions. Strong repulsions between DNA and divalent co-ions and among divalent co-ions themselves leads to a {\em depletion} of negative ions near DNA surface as compared to the case without divalent co-ions. At large distance, the DNA$-$DNA repulsive interaction is stronger in the presence of divalent co$-$ions, suggesting that divalent co$-$ions role is not only that of simple stronger linear screening.