Extracting DNA Twist Rigidity from Experimental Supercoiling Data

We use an elastic rod model with contact to study the extension versus rotation diagrams of single supercoiled DNA molecules. We reproduce quantitatively the supercoiling response of overtwisted DNA and, using experimental data, we obtain an estimate of the effective supercoiling radius and of the twist rigidity of $B$-DNA. We find that the twist rigidity of DNA seems to vary widely with the nature and concentration of the salt buffer in which it is immersed.

Figure 1

The magnetic tweezer experiment.

Figure 2

Fitting the linear part of the response curve.

Figure 3

Experimental response curves with a 48 kbp DNA molecule in a 10 mM phosphate buffer. The different curves correspond to experiments carried out at fixed force (from bottom to top, 0.25, 0.33, 0.44, 0.57, 0.74, 1.1, 1.31, 2.2, and 2.95 pN) [25].

Figure 4

Experimental data [26] taken with a 11 kbp DNA molecule in a 150 mM phosphate and 5 mM magnesium ($\mathrm{M}\mathrm{g}2+$) buffer. Forces are, from bottom to top, 0.45, 1.45, and 4.3 pN. The curves are numerical results of simulations of an elastic rod with contact with the values of $\rho$ and $K_3/K_0$ of Table .