Hydration forces are the recently uncovered interactions between DNA or lipid bilayer surfaces that dominate the energies between these surfaces at separation distances of 20 angstroms and less. These forces appear to be due to the structuring of water between surfaces and can be either strongly attractive or strongly repulsive depending on the surface hydration. We can directly measure these forces by combining the osmotic stress technique with x-ray diffraction to measure the separation of the surfaces. In analyzing the energetics of these forces, we have concentrated on the osmotic pressure induced assembly of Mn2+-DNA. The force curves show an abrupt transition at a critical osmotic pressure, that depends on temperature and Mn2+ concentration, between repulsive and attractive hydration forces, mediated by the presumed rearrangement on Mn2+ on the surface of DNA. The transition is entropically driven presumably by the release of bound water. We have now examined the effect on the transition of different anions that structure bulk water differently. The transition occurs more readily with ClO4- than with Cl- and is more difficult with SO4/2- than with Cl-. We have quantitated the entropy changes and found that the differences are due to the additional entropy gained or lost by releasing structured water around DNA helices into the bulk. This is the first clear, direct indication that water structuring is the source of hydration forces.
