Measuring elastic properties of a protein monolayer at water surface by lateral compression

A protein monolayer grown at a water surface decorated with a mild coverage of surfactants, was compressed laterally followed by decompression in a cycle. The surface tension and the thickness of the layer was monitored by using Wilhelmy plate and grazing incidence off-specular X-ray scattering techniques (GIXOS). Under in-plane isothermal compression the thickness of the layer expands. There were two different regimes of expansion- at first it occurs due the conformational changes of proteins under pressure and then buckling of layer into folded layers beyond a critical pressure. On removal of the pressure the layer returns to the original thickness and texture as observed by GIXOS and Brewster Angle Microscope (BAM). To determine elastic constants-Young's modulus and Poisson's ratio-the layer was modeled as a homogeneous and isotropic elastic body. The Young's modulus was found to be of the order of 10 MPa which we account for the elasticity of secondary structures such as alpha-helix and beta-sheets of the protein. The conformational changes are associated with the modification of tertiary structure through the bending and twisting of secondary structures.