Simple simulations of DNA condensation

Molecular dynamics simulations of a simple, bead-spring model of semiflexib le polyelectrolytes such as DNA are performed. All charges are explicitly t reated. Starting from extended, noncondensed conformations, condensed struc tures form in the simulations with tetravalent or trivalent counterions. No condensates form or are stable for divalent counterions. The mechanism by which condensates form is described. Briefly, condensation occurs because e lectrostatic interactions dominate entropy, and the favored coulombic struc ture is a charge-ordered state. Condensation is a generic phenomenon and oc curs for a variety of polyelectrolyte parameters. Toroids and rods are the condensate structures. Toroids form preferentially when the molecular stiff ness is sufficiently strong.