Direct measurement of temperature-dependent solvation forces between DNA double helices.

The assembly of double stranded DNA helices with divalent manganese ion is favored by increasing temperature. Direct force measurements, obtained from the osmotic stress technique coupled with x-ray diffraction, show that the force characteristics of spontaneously precipitated Mn(2+)-DNA closely resemble those observed previously by us for other counterion condensed DNA assemblies. At temperatures below the critical one for spontaneous assembly, we have quantitated the changes in entropy and manganese ion binding associated with the transition from repulsive to attractive interactions between helices mediated by osmotic stress. The release of structured water surrounding the DNA helix to the bulk solution is the most probable source of increased entropy after assembly. Increasing the water entropy of the bulk solution by changing the manganese salt anion from CI- to ClO4- predictably and quantitatively increases the transition entropy. This is further evidence for the dominating role of water in the close interaction of polar surfaces.