The torque transfer coefficient in DNA under torsional stress

In recent years, significant progress in understanding the properties of supercoiled DNA has been obtained due to nanotechniques that made stretching and twisting of single molecules possible. Quantitative interpretation of such experiments requires accurate knowledge of torques inside manipulated DNA. This paper argues that it is not possible to transfer the entire magnitudes of external torques to the twisting stress of the double helix, and that a reducing torque transfer coefficient (TTC<1) should always be assumed. This assertion agrees with simple physical intuition and is supported by the results of all-atom molecular dynamics (MD) simulations. According to MD, the TTCs around 0.8 are observed in nearly optimal conditions. Reaching higher values requires special efforts and it should be difficult in practice. The TTC can be partially responsible for the persistent discrepancies between the twisting rigidity of DNA measured by different methods.