| Abstract
| - We present the first characterization of the mechanical properties of lysozyme films formed by self-assembly atthe air−water interface using the Cambridge interfacial tensiometer (CIT), an apparatus capable of subjecting proteinfilms to a much higher level of extensional strain than traditional dilatational techniques. CIT analysis, which isinsensitive to surface pressure, provides a direct measure of the extensional stress−strain behavior of an interfacialfilm without the need to assume a mechanical model (e.g., viscoelastic), and without requiring difficult-to-test assumptionsregarding low-strain material linearity. This testing method has revealed that the bulk solution pH from which assemblyof an interfacial lysozyme film occurs influences the mechanical properties of the film more significantly than issuggested by the observed differences in elastic moduli or surface pressure. We have also identified a previouslyundescribed pH dependency in the effect of solution ionic strength on the mechanical strength of the lysozyme filmsformed at the air−water interface. Increasing solution ionic strength was found to increase lysozyme film strengthwhen assembly occurred at pH 7, but it caused a decrease in film strength at pH 11, close to the pI of lysozyme. Thisresult is discussed in terms of the significant contribution made to protein film strength by both electrostatic interactionsand the hydrophobic effect. Washout experiments to remove protein from the bulk phase have shown that a smallpercentage of the interfacially adsorbed lysozyme molecules are reversibly adsorbed. Finally, the washout tests haveprobed the role played by additional adsorption to the fresh interface formed by the application of a large strain tothe lysozyme film and have suggested the movement of reversibly bound lysozyme molecules from a subinterfaciallayer to the interface.
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