| Abstract
| - The relation between mechanical film properties of various adsorbed protein layers at the air/waterinterface and intrinsic stability of the corresponding proteins is discussed. Mechanical film properties weredetermined by surface deformation in shear and dilation. In shear, fracture stress, σf, and fracture strain,γf, were determined, as well as the relaxation behavior after macroscopic fracture. The dilatationalmeasurements were performed in a Langmuir trough equipped with an infra-red reflection absorptionspectroscopy (IRRAS) accessory. During compression and relaxation of the surface, the surface pressure,Π, and adsorbed amount, Γ (determined from the IRRAS spectra), were determined simultaneously. Inaddition, IRRAS spectra revealed information on conformational changes in terms of secondary structure.Possible correlations between macroscopic film properties and intrinsic stability of the proteins weredetermined and discussed in terms of molecular dimensions of single proteins and interfacial protein films.Molecular properties involved the area per protein molecule at Π ≈ 0 mN/m (A0), A0/M (M = molecularweight) and the maximum slope of the Π−Γ curves (dΠ/dΓ). The differences observed in mechanical propertiesand relaxation behavior indicate that the behavior of a protein film subjected to large deformation mayvary widely from predominantly viscous (yielding) to more elastic (fracture). This transition is also observedin gradual changes in A0/M. It appeared that in general protein layers with high A0/M have a high γf andbehave more fluidlike, whereas solidlike behavior is characterized by low A0/M and low γf. Additionally,proteins with a low A0/M value have a low adaptability in changing their conformation upon adsorptionat the air/water interface. Both results support the conclusion that the hardness (internal cohesion) ofprotein molecules determines predominantly the mechanical behavior of adsorbed protein layers.
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