Superhelical DNA studied by solution scattering and computer models.

Abstract

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 curves of two different superhelical DNAs were determined by static light scattering. The measured radii of gyration do not change significantly with salt concentration. Small-angle neutron scattering, together with calculations from a Monte Carlo model, allows to determine the superhelix diameter. Measured and simulated scattering curves agreed almost quantitatively. Experimentally 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 simulated conformations decreased from 18.0 +/- 1.5 nm at 10 mM to 9.4 +/- 1.5 nm at 100 mM NaCl. At higher salt concentrations up to 1.5 M NaCl, the diameter stays constant at 9 nm.