Static and Dynamic Properties of Bio-Mimetic Systems

We used total internal reflection microscopy (TIRM) to measure the static interaction between colloidal probe spheres and a flat glass wall. The technique was applied to two fundamentally different systems. (i) The colloidal probe spheres and/or the glass wall were covered with a so called S-layer protein, Sgs-EGFP. In this case the protein-protein interaction was investigated. (ii) Bare colloidal spheres were suspended in a solution of rod shaped fd-virus next to a bare glass wall. Here we were mainly interested in the depletion interaction between the sphere and the wall induced by the rods. In the latter case we also analyzed the dynamic information, which is inherent to TIRM raw data, to obtain further insight into the mechanism underlying the sphere-wall interaction.Protein-protein interactions: Measurements at increasing electrolyte content of the suspending buffer showed that the sphere is stable against irreversible sticking to the surface due to van der Waals attraction at significantly higher ionic strength, if the sphere and the surface are protein coated, as compared to the case of bare surfaces. Qualitative data analysis implies that there is an additional repulsive interaction on top of the DLVO potential, which is effective over a range of tens of nano-meters. This is at least one order of magnitude larger than the range of so-called hydration forces, which are usually considered responsible for protein stability beyond DLVO-interaction.Depletion interaction induced by fd-virus: We measured depletion potentials by TIRM over a wide range of probe spheres sizes and rod concentrations to explore the limits of the fundamental approximations used in the classical Asakura-Oosawa theory (AO), which are the treatment of the depletant as an ideal gas and Derjaguin approximation, demanding that the sphere radius is much larger than the rod length. The experimental data follow the AO predictions at concentrations and size ratios, at which this is expected to fail. At even higher fd-concentrations, we observe deviations from the ideal gas behavior, which are much larger and of opposite sign than predicted earlier. By analyzing the dynamic information inherent to the raw data, we found evidence that this observation is caused by the dynamics of the rod network which is inevitably formed at fd-concentrations above the overlap density. In a first step, we used the initial slope of the intensity correlation functions to determine spatially averaged particle diffusion coefficients, which show a dependence on the fd-concentration which is intriguingly similar to the concentration dependence of the amplitude of the depletion potential. Therefore, we assume that the large amplitude of the apparent attractive potential at high fd-concentrations is not anymore due to depletion forces but rather to the particle being mechanically trapped in the network of rods.To gain further insight into the systems dynamics, we determined spatially resolved dynamic data. We found that the particle’s drift velocity due to the external force field can be determined with excellent accuracy, while it appears to be generally much more difficult to measure near-wall diffusion coefficients by TIRM. This finding might open a new route to use TIRM as tool to measure local viscosities at extremely low shear rates by a passive micro-rheology approach