How to develop globular proteins into adhesives

To make globular proteins suitable for application in adhesives, the specif ic 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 create d. Such a network layer is essential to save the adhesive bond under an app lied force, because it can distribute the concentration of stresses generat ed 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 fo und with 98% formic acid as a solvent. In formic acid, beta-lactoglobulin l ooses its amphoteric character (it is protonated, probably for approximate to 20%). In addition, formic acid is less polar than water and thus a bette r solvent for the apolar parts of the protein. The swelling and unfolding b ehaviour of beta-lactoglobulin is studied by viscosity and CD-spectroscopy measurements. For the interpretation of the results we apply the Kuhn forma lism that the conformation of a protein can be described in terms of a stat istical 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 specif ic 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 app roximate to 2.2 nm, pointing at a relatively flexible chain, The minimum ne t 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 stre ngths of approximate to 2.9 +/- 1.1 N mm(-2), indeed. (C) 2000 Elsevier Sci ence B.V. All rights reserved.