Non-monotonic dependence of sedimentation coefficient and intrinsic viscosity on superhelix density at positive superhelix densities

The sedimentation coefficient (sp[degrees]) and viscosity of closed circular bacteriophage PM2 DNA, artificially produced so as to possess a low absolute superhelix density, has been measured as a function of ethidium bromide (EB) concentration in a solvent of approximately 0.1 M ionic strength. Under these conditions, which allow the introduction of a substantial number of positive superhelical turns into the molecule with increasing ethidium bromide concentration, the hydrodynamic behavior of the closed circle is not a monotonic function of the superhelix density, and clearly displays a local minimum in the case of the s[degrees] titration curve. As such non-monotonic behavior has previously been demonstrated in the case of negative superhelix densities, it is now clear that the curve of s[degrees] vs superhelix density is non-monotonic and contains local minima on both sides of zero superhelix density.