DNA twist stability changes with magnesium(2+) concentration
  For an understanding of DNA elasticity at high mechanical loads (F > 30 pN), its helical nature needs to be taken into account, in the form of coupling between the twist and stretch degrees of freedom. The prevailing model for DNA elasticity, the worm-like chain, was previously extended to include this twist-stretch coupling, giving rise to the twistable worm-like chain. Motivated by DNA's charged nature, and the known effects of ionic charges on the molecule's persistence length and stretch modulus, we explored the impact of buffer ionic conditions on twist-stretch coupling. After developing a robust fitting approach for force-extension data, we find that DNA's helical twist is stabilized at high concentrations of the magnesium divalent cation. 
Onno D. Broekmans, Graeme A. King, Greg J. Stephens, Gijs J.L. Wuite
physics.bio-ph