The ability to measure directly the forces between membranes or between macromolecules is creating a new logic for thinking about molecular recognition, assembly, and folding. The outstanding feature of interaction is that as molecules or membranes approach contact, the important work of approach involves removal of organized water solvent from the apposing surfaces. These """"""""hydration forces"""""""" are now recognized to act in materials as diverse as lipid bilayers, proteins, DNA double helices, and stiff polysaccharides. During the current year a first direct measurement of forces between protein molecules (type I collagen triple helices) has been done. It has been shown that the force has all the features characteristic of hydration forces. The temperature dependence of the force is similar to that observed in ordered arrays of DNA molecules. This shows that physical nature of temperature-favored assembly in DNA and proteins might be similar. Temperature-favored assembly is a common feature of many biologically important processes. A theory of temperature-favored assembly induced by attractive hydration forces between hydrophilic molecules has been developed. Measurement of interaction forces between dihexadecyldimethylammonium acetate bilayers has demonstrated that neither thermal-mechanical undulations nor molecular protrusions contribute significantly to hydration forces between lipid bilayers. A dependence of hydration forces between DNA molecules on solution ion composition has been studied. An unusual H -L -H double phase transition induced by variation of water concentration in lipid/water mixtures has been observed and explained. A theoretical model relating hydration forces and phase transitions on the surfaces of interacting molecules has been suggested.
