Fig. 1.

Dependence of the torsional modulus of a generic, naturally straight DNA homopolymer on the relative spring constant k/k₀ used to restrain the chain ends at the equilibrium end-to-end distance. Here k₀ is the force constant, defined by (6), which keeps neighboring base pairs at their van der Waals’ separation distance $({\theta }_6^0=3.4\mathrm{Å})$. Three coupling interactions (f₂₃, f₂₅, f₃₆)—Roll-Twist, Twist-Slide, and Twist-Rise—are incorporated into the internal energy function.