How to develop globular proteins into adhesives

To make globular proteins suitable for application in adhesives, the specific bonds and interactions which shape their structure have to broken. Only then, a layer of relatively large, flexible and interwoven polymer chains, which are firmly attached to the solid surface by adsorption, can be created. Such a network layer is essential to save the adhesive bond under an applied force, because it can distribute the concentration of stresses generated at the interface into the bulk. Unfolding and swelling of a protein can be achieved by changing the solvent quality. For the globular whey protein beta-lactoglobulin, the optimal conditions for unfolding and swelling is found with 98% formic acid as a solvent. In formic acid, beta-lactoglobulin looses its amphoteric character (it is protonated, probably for approximate to 20%). In addition, formic acid is less polar than water and thus a better solvent for the apolar parts of the protein. The swelling and unfolding behaviour of beta-lactoglobulin is studied by viscosity and CD-spectroscopy measurements. For the interpretation of the results we apply the Kuhn formalism that the conformation of a protein can be described in terms of a statistical chain which consists of segments of an average persistence length (P) over bar. The statistical segment length (P) over bar, which varies with the experimental conditions, is directly related to the adsorption energy required for a strong adhesion between coil and surface. It determines the depletion energy kT (P) over bar(-2) m(-2) which must be overcome by specific attraction between side groups of the protein chain and the surface. For beta-lactoglobulin in 98% formic acid, we find a (P) over bar value of approximate to 2.2 nm, pointing at a relatively flexible chain, The minimum net adsorption energy kT (P) over bar(-2) is then approximate to 1 mJ m(-2) a relatively small value to be exceeded. Preliminary results of destructive adhesion tests on beech wood lap-shear Joints reveal promising tensile strengths of approximate to 2.9 +/- 1.1 N mm(-2), indeed. (C) 2000 Elsevier Science B.V. All rights reserved.