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

Abstract

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 α-helix and β-sheets of the protein. The conformational changes are associated with the modification of tertiary structure through the bending and twisting of secondary structures.