  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.
