DIRECT MEASUREMENT OF FORCES BETWEEN MEMBRANES OF 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 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, DNA double helices, and stiff linear polysaccharides. Until now, only force or energy vs. surface separation between molecules or membranes has been measured. During the current year, the "entropy" and "enthalpy" of release water, as a function of the distance between macromolecules, has been successfully extracted. The first phase of a systematic study of forces between polysaccharides has also been completed. Again, for these materials as for nucleic acids and lipids, one sees exponentially decaying "hydration" forces between molecules approaching contact. A theory of lipid bilayer forces that claims to relate the "dipole ]potential" set up by the lipid polar groups to the perturbation of water near the lipid interface is being tested. A combined osmotic stress, x-ray diffraction, NMR and ion transport study shows that the correlation demanded by the theory does not exist. Bilayers of synthetic lipids, thought from the work of others not to show strong hydration forces, have now shown a very strong version of this force when observed by our direct, osmotic stress method. Finally, a surprisingly easy transition between lamellar and non-lamellar packing of lipids whose occurrence may be related to easy rearrangement of cell membrane lipids in secretion and fusion is being investigated.