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

We have measured the static form factor of superhelical DNA by SANS in dilute aqueous solution as a function of salt concentration. Theoretical static form factors were calculated by Monte Carlo simulations. Simulated and measured form factors are in very good agreement and can be interpreted in terms 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 contradicts a lateral collapse of the DNA superhelix in physiological salt concentrations as recently found by cryo-electron microscopy.