Study of the behavior of biological molecules in a thin gap by refractive index and force measurements with an...

The automation of the mica surface displacement of a surface force apparatus (SFA) increases the accuracy of the intermolecular force measurements that is obtained without the presence of the experimenter. The automatic device cancels any thermal drift and the stability of the mica surfaces is optimum. The distance between the mica surfaces is measured by Tolansky multiple-beam interferometry (FECO method) which offers a sensitivity of 0.1 nm in the direction perpendicular to the plane of contact area. The mean refractive index of the medium between the mica plates is computed from simultaneous measurements of the wavelengths of two successive fringes of equal chromatic order (FECO). A conversion of an isotropic molecule to another anisotropic molecule can be identified from the variations of the refractive index. In good conditions, we can compute the local concentration of the molecules in the gap of the device, and estimate the Young's modulus of a protein.