Interplay of ion binding and attraction in DNA condensed by multivalent cations

Abstract

We have measured forces generated by multivalent cation-induced DNA condensation using single-molecule magnetic tweezers. In the presence of cobalt hexammine, spermidine, or spermine, stretched DNA exhibits an abrupt configurational change from extended to condensed. This occurs at a well-defined condensation force that is nearly equal to the condensation free energy per unit length. The multivalent cation concentration dependence for this condensation force gives the apparent number of multivalent cations that bind DNA upon condensation. The measurements show that the lower critical concentration for cobalt hexammine as compared to spermidine is due to a difference in ion binding, not a difference in the electrostatic energy of the condensed state as previously thought. We also show that the resolubilization of condensed DNA can be described using a traditional Manning–Oosawa cation adsorption model, provided that cation–anion pairing at high electrolyte concentrations is taken into account. Neither overcharging nor significant alterations in the condensed state are required to describe the resolubilization of condensed DNA. The same model also describes the spermidine3+/Na+ phase diagram measured previously.

DNA bending-induced phase transition of encapsidated genome in phage

“…It was shown that at interaxial DNA–DNA separations of a h ≲ 26 Å, there is a dominant short-range repulsion interaction (11,13,34). In TM buffer, interhelical forces are repulsive at all separations (8).…”

Dressed counterions: Polyvalent and monovalent ions at charged dielectric interfaces

“…Nevertheless one needs to keep in mind that ion-specific effects [26] also make their mark in strongly coupled systems. The SC theory based on purely electrostatic grounds appears to be unable to account for the experimentally observed variation in the strength of the SC effects for equally charged polyvalent counterions [27]. The ion specific effects in the SC theory have remained largely unexplored.…”

Divalent cation shrinks DNA but inhibits its compaction with trivalent cation

“…6 Various cationic chemical species, such as histone proteins, metal cations, and polyamines, have been experimentally shown to induce the compaction of long DNA chains. [6][7][8][9][10][11][12][13][14][15][16] Such conformational transition from elongated coil onto condensed/compact state [17][18][19] has also been actively studied experimentally 13,14,20,21 and theoretically. [22][23][24] Among these chemical species, polyamines are widespread in both prokaryote and eukaryote cells and exert various biological effects.…”