Use of the Osmotic Stress method, developed by our group to measure directly forces between membranes or between macromolecules, has spread rapidly this past year to several laboratories in Europe and North America. One result of this recent proliferation in practice has been to advance the idea 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 increasingly recognized to act in materials as diverse as lipid bilayers, proteins, DNA double helices, and stiff polysaccharides. The growing catalog of information about interactions continues to create a new logic for thinking about molecular recognition and folding. During the current year, we have concentrated on forces between proteins, specifically examining native and reconstituted collagen fibers at various temperatures, pH, ionic conditions, and in the presence of several small solutes. It has been shown that salt does not fully penetrate into the space between collagen triple helices. Osmotic pressure applied from outside by the excluded salt is an important component of collagen fiber assembly. Force measurements have demonstrated that temperature-favored assembly of the fibers is driven by water-mediated hydrogen bonding between the apposing polar residues rather than by the hydrophobic effect usually invoked to explain assembly of proteins. One can now think of a competition between repulsive and attractive hydration forces, depending on how well protein surfaces match each other. A dependence of hydration forces between DNA molecules on small solutes, observed present in the solution, has been studied. An unusual re- entrant liquid-gel-liquid phase transition sequence, observed during measurement of forces between didodecylphosphate bilayers, has been explained. The osmotic stress technique has been extended to measure forces between spherical particles in order to create convenient models for the """"""""molecular crowding"""""""" phenomenon important in the regime of the intracellular milieu.
