DIRECT MEASUREMENT OF FORCES BETWEEN MEMBRANES OR MACROMOLECULES

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The ability to measure directly the forces between membranes or between macromolecules is creating a new logic for thinking about molecular recognition, 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 we have measured intermolecular forces in native and reconstituted collagen fibers at various temperatures, ph, ionic condiitons, 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 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. An osmotic stress technique for measuring forces between spherical particles has been developed.