  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.
