A fundamental understanding of biological process involves a knowledge of the attractive and repulsive colloidal forces that determine interaction between amphiphitic aggregates (bilayers, vesicles, microtubules, micelles, etc.) as well as polyelectrolyte aggregates (bilayers, vesicles, microtubules, micelles, etc.) as well as polyelectrolyte macromolecules (DNA, etc.). These forces are complex functions of distance and changing surface morphology. They include short range hydration forces, van der Walls attractive forces, long range attractive hydrophobic forces, electrostatic repulsive forces and forces associated with counterion and bilayer fluctuations. The surface forces apparatus permits these colloidal forces to be measures with a resolution of plus or minus 1 angstrom from contact to layer separation. Within the past few years there has been significant advances in our understanding of these colloid forces that control biological processes. There are new theoretical insights and more sophisticated experimental techniques. The long term goal of this research is to couple the emerging theoretical insights and the ability to directly measure amphophilic interaction forces. These studies will also involve osmotic stress measurements. The experiments address a wide range of issues including: (1) a comparison of the measurements using the surface forces apparatus and the osmotic stress techniques on the same system; (2) the role of Helfrich thermal-mechanical fluctuations in determining the structure and stability of bilayers; (3) the nature of long range hydrophobic attractive interactions which may play a role in controlling biological fusion processes; (4) how surface heterogeneity and molecular spacing of phospholipid-neutral lipid mixtures affect bilayer interactions; (5) role of headgroup motion in stabilizing zwitterionic bilayers; (6) whether correlated motions of counterion can lend to net attraction between charged bilayers; and (7) characterization of monolayers and biolayers using scanning tunneling and atomic force microscopy. The experiments are focused on providing information directly applicable to biological amphiphilic assemblies.
