The observation of hydration forces as the dominant interaction between macromolecules at close approach has been extended to another class of important biopolymers, polysaccharides. The two helical carbohydrates studied were xanthan and schizophyllan. In both cases, the observed forces between helices are characteristic of hydration forces as defined by previous work on DNA and lipid bilayers, a force that varies exponentially with distance between surfaces with a 3A decay length, that is independent of ionic strength and composition. Polysaccharides offer a wide variety of surface groups for study. By measuring forces between a large number of these surfaces, we can begin to assign surface hydration parameters to chemical groups. These parameters will determine the strength and polarity (attraction or repulsion) of hydration interactions between any two surfaces. This is a necessary step toward developing a predictive hydration force framework. The utility of xanthan for these measurements is that a number of surface groups can be chemically removed without distrubing structure and changes in hydration forces mesured. The polymer schizophyllan is important for another reason. This carbohydrate is a triple helix composed entirely of glucose, with no formal charge in the structure. The observation of hydration forces without added salt precludes that these forces are in any way due to screened Coulombic interactions. In a second project, the effect of configurational entropy on intermolecular forces is being evaluated. A decrease in macromolecular freedom is an important component of all assemby reactions and the interplay of this with intermolecular forces is as yet not well understood for molecules with internal degrees of freedom. At present, we are observing changes in DNA configurational freedom for arrays of helices under osmotic stress. It appears very strongly that this internal motion significantly affects the interactions between DNA helices.
