MEASUREMENTS OF CONFORMATIONAL-CHANGES DURING ADHESION OF LIPID AND PROTEIN (POLYLYSINE AND S-LAYER) SURFACES

The adhesion forces between various surfaces were measured using the ' 'surface forces apparatus'' technique. This technique allows for the t hickness of surface layers and the adhesion force between them to be d irectly measured in controlled vapor or liquid environments. Three typ es of biological surfaces were prepared by depositing various lipid-pr otein monolayers (with thicknesses ranging from 1 to 4 nm) on the iner t, molecularly smooth mica surface: (i) hydrophobic lipid monolayers; (ii) amphiphilic polyelectrolyte surfaces of adsorbed polylysine; and (iii) deposited bacterial S-layer proteins. The adhesion, swelling, an d wetting properties of these surfaces was measured as a function of r elative humidity and time. Initial adhesion is due mainly to the van d er Waals forces arising from nonpolar (hydrophobic) contacts. Followin g adhesive contact, significant molecular rearrangements can occur whi ch alter their hydrophobic-hydrophilic balance and increase their adhe sion with time. Increased adhesion is generally enhanced by (i) increa sed relative humidity (or degree of hydration); (ii) increased contact time; and (iii) increased rates of separation. The results are likely to be applicable to the adhesion of many other biosurfaces, and show that the hydrophobicity of a lipid or protein surface is not an intrin sic property of that surface but depends on its environment (e.g., on whether it is in aqueous solution or exposed to the atmosphere), and o n the relative humidity of the atmosphere. It also depends on whether the surface is in adhesive contact with another surface and-when consi dering dynamic (nonequilibrium) conditions-on the time and previous hi story of its interaction with that surface.