Testing Models For DNA Elasticity on Short Length Scales by Simulating DNA Supercoiling under Tension

The worm-like-chain (WLC) model is widely used to describe the energetics of DNA bending. However, for length scales much shorter than the persistence length the validity of a purely harmonic bending potential has been questioned by recent experiments. So-called sub-elastic chain (SEC) models were proposed that predict a lower elastic energy of highly bent DNA conformations. Until now, no unambiguous verification of these models has been obtained since probing the elasticity of DNA on short length scales remains challenging. Here we address this issue by modeling single molecule supercoiling experiments of DNA under tension using coarse-grained Monte Carlo simulations. Twisting of stretched DNA is accompanied by an abrupt decrease of the DNA extension at a critical supercoil density due to buckling of the molecule. This transition is caused by an energetic offset due to the strongly bent end-loop of the forming superhelical structure. While simulations based on WLC bending energetics could quantitatively reproduce the buckling measured in magnetic tweezers experiments, the buckling almost disappears for the tested linear SEC model. Thus, our data support the validity of a harmonic bending potential even for strongly bent DNA down to bending radii of 3.5 nm.