Besides interacting by long-range physical forces, large molecules or membranes often interact by bumping or steric forces due to the mechanical motion of bending or rotating. This past year we solved the difficult problem of formulating such motion and estimating its consequent forces during the simultaneous action of the underlying long-range physical force. We now are making a new kind of correlation between molecular interactions and molecular conformation. Measurements on forces between DNA double helices reveal very strong attractive forces apparently mediated by restructuring water solvent between molecules and regulated by ion binding to the molecular surface. This attraction in many ways resembles the """"""""hydrophobic force"""""""" usually thought to occur between nonpolar surfaces. It appears to us that in the many cases where it has been invoked to explain macromolecular properties that this force can be due to polar molecules and that it can be the strong, specific, and modulated interaction necessary to explain molecular assembly. Investigations on lipid systems have now been extended to measuring the work of forming aqueous cavities. We are in the process of relating these energies to those involved in the formation of ionic channels in membrane transport.
