  The symmetries of the DNA double helix require a new term in its linear response to stress: the coupling between twist and stretch. Recent experiments with torsionally-constrained single molecules give the first direct measurement of this new material parameter. We extract its value from a recent experiment. We also present a very simple microscopic theory predicting a value comparable to the one observed. Finally we sketch the effect of constrained twist on entropic elasticity of DNA arising from the connection between Link, Twist, and Writhe. 
New Measurements of DNA Twist Elasticity