  DNA deformations play crucial roles in many biological processes and material
applications. During DNA deformation, DNA structural parameters often exhibit
non-trivial and counterintuitive couplings, such as the twist-stretch and
twist-bending couplings. Here, we reveal an unexpectedly strong negative
twist-diameter coupling through the synergy of magnetic-tweezers experiments,
atomistic molecular dynamics simulations, and theoretical calculations. In
experiments, the DNA twist angle always increases with the concentration of
NaCl, KCl, or RbCl. Our simulations quantitatively reproduce salt-induced twist
changes and reveal the underlying physical mechanism: the reduction of DNA
diameter under a high salt concentration leads to the increase in DNA twist
angle through a strong negative twist-diameter coupling. The twist-diameter
coupling is mediated by two dihedral angles in DNA structure and the coupling
constant is 4.5 kBT/(deg nm) for one base-pair. Based on this coupling
constant, we predict the temperature-dependence of DNA twist -0.0102 deg/K per
bp, which agrees with our and previous experimental results. Our analysis
suggests that the twist-diameter coupling is a common driving force for salt-
and temperature-induced DNA twist changes.
