Superhelical DNA studied by solution scattering and computer models

We present here recent results on the structure of superhelical DNA and its changes with salt concentration between 0.01 and 1.5 M NaCl. Scattering cu rves of two different superhelical DNAs were determined by static light sca ttering. The measured radii of gyration do not change significantly with sa lt concentration. Small-angle neutron scattering, together with calculation s from a Monte Carlo model, allows to determine the superhelix diameter. Me asured and simulated scattering curves agreed almost quantitatively. Experi mentally we find that the diameter decreases from 16.0 +/- 0.9 nm at 10 mM to 9.0 +/- 0.7 nm at 100 mM NaCl. The superhelix diameter from the simulate d conformations decreased from 18.0 +/- 1.5 nm at 10 mM to 9.4 +/- 1.5 nm a t 100 mM NaCl. At higher salt concentrations up to 1.5 M NaCl, the diameter stays constant at 9 nm.