The diameter of the DNA superhelix decreases with salt concentration: SANSmeasurements and Monte Carlo simulations

We have measured the static form factor of superhelical DNA by SANS in dilu te aqueous solution as a function of salt concentration. Theoretical static form factors were calculated by Monte Carlo simulations. Simulated and mea sured form factors are in very good agreement and can be interpreted in ter ms of the superhelix diameter, which decreases from (16.0 +/- 0.9) nm at 10 mM to (9.0 +/- 0.7) nm at 100 mM Na+ concentration. This result contradict s a lateral collapse of the DNA superhelix in physiological salt concentrat ions as recently found by cryo-electron microscopy.