DNA condensation in two dimensions

We have found that divalent electrolyte counterions common in biological ce lls (Ca2+, Mg2+, and Mn2+) can condense anionic DNA molecules confined to t wo-dimensional cationic surfaces. DNA-condensing agents in vivo include cat ionic histones and polyamines spermidine and spermine with sufficiently hig h valence (Z) 3 or larger. In vitro studies show that electrostatic forces between DNA chains in bulk aqueous solution containing divalent counterions remain purely repulsive, and DNA condensation requires counterion valence Z greater than or equal to 3. In striking contrast to bulk behavior, synchr otron x-ray diffraction and optical absorption experiments show that above a critical divalent counterion concentration the electrostatic forces betwe en DNA chains adsorbed on surfaces of cationic membranes reverse from repul sive to attractive and lead to a chain collapse transition into a condensed phase of DNA tethered by divalent counterions. This demonstrates the impor tance of spatial dimensionality to intermolecular interactions where nonspe cific counterion-induced electrostatic attractions between the like-charged polyelectrolytes overwhelm the electrostatic repulsions on a surface for Z = 2. This new phase, with a one-dimensional counterion liquid trapped betw een DNA chains at a density of 0.63 counterions per DNA bp, represents the most compact state of DNA on a surface in vitro and suggests applications i n high-density storage of genetic information and organo-metallic materials processing.