Viscosité superficielle et structure moléculaire des couches de protéines

Abstract

For all the protein monolayers studied we are led to consider different domains inside which the mechanical properties of the film are different. Below a certain surface pressure p n the viscosity is newtonian and the films of protein behave like those of fatty substances. On the other hand, protein films form a gel by compression in the neighbourhood of a surface pressure p g well defined for each protein. Between the surface pressures p n and p g the viscosity of protein films is no longer newtonian. The knowledge of the dimensions and form of molecules allows us to calculate the viscosity coefficient of monolayers. If this calculation corresponding to one molecule is made, extremely high values are found, impossible to compare with the experimental values. We must therefore suppose that the unit of flow is much smaller than one molecule. The theory of Eyring and Moore relates the surface viscosity to the free energy of activation of viscous flow. This energy of activation is a very rapidly increasing function of the diameter of the elementary particles. For all the proteins studied the viscosity coefficient at the point of appearance of the non-newtonian viscosity is of the same order of magnitude; hence the submolecular units have dimension practically independent from the nature of the protein. The direct calculation of the energy of activation arising from the energy of interaction between the elementary functional groups allows us to determine the diameter of the units of flow. We find for the area of these elementary particles about 90 A° 2. We discuss the meaning of this result.