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Extracting the twist rigidity of B-DNA for experimental data
Experimental data were kindly provided by : G. Charvin and V. Croquette
Different groups are now able to take a single molecule of DNA and to attach it, at one end to a glass surface and at the other end to a magnetic bead. Then, thanks to a magnet, they pull on the bead while rotating it around a vertical axis.
Experiments are carried at constant pulling force T. Under large rotation (large n), the DNA molecule wraps around itself in a helical way: this is called the plectonemic regime.
B is the bending rigidity,
C the twist rigidity,
L the total contour length of the molecule,
H = 3.57 nm the helical pitch of the double helix,
R (or rho) is the effective supercoiling radius. The equation involving R,T,B and theta is obtained by fitting results from numerical simulations of the elastic rod model with contact. We give an equation for the linear part of the response curve of extension-rotation diagrams:
This enables us to extract the effective supercoiling radius R, the helical supercoiling angle theta, and the rigidities ratio C/B of DNA from experimental data:
