Gesa Helms

• In this thesis the interactions between chromatographic beads, surfaces, and cells were studied. The main goal was to establish a method to study such interactions by the direct measurements of forces with an atomic force microscope. The study is motivated by expanded bed adsorption (EBA), where bioproducts are purified from an unclarified fermentation broth by their adsorption on chromatographic beads in a fluidized bed. The unspecific deposition of biomass onto the adsorbent matrix can severely affect the process performance, leading to a poor system hydrodynamics which then decreases the success of this unit operation. To quantify the bead-biomass interactions different chromatographic beads were attached to AFM cantilevers, and force spectroscopy experiments were performed with these colloidal probes. For establishing the method, measurements were done on technical surfaces first, later the system was applied to yeast cells. The experiments are conducted under varying conditions to study the influence of ionic strength and hydrophobicity on various chromatographic bead – surface/ biomass combinations. The measurements were compared to bead-biomass interactions modeled by using XDLVO theory. So far only qualitative comparisons between force measurements and XDLVO calculations were done. For the probing an electrostatic repulsion between a positively charged bead and a positively charged poly-L-lysine surface a good agreement was found. The measurements between hydrophobic beads and hydrophobic surfaces showed an increase of the interactions with increasing ammonium sulfate concentration, which is in good agreement with the Hofmeister series. To analyze the force curves obtained on whole cells, possible elastic contributions, e.g. from deforming cellular membranes, have to be decoupled from the interaction forces. The project aims for a better understanding of the interaction forces in chromatography and might help to improve the process quality of EBA.