Superparamagnetic nanoparticles have recently gained much attention due to their broad range of applicability including medical in vivo technologies, sensors, and as supports for catalysts. As magnetic affinity materials, they can be utilized for the development of new purification strategies for pharmaceuticals and other target molecules from crude lysates. Here, a short peptide tag based on a glutamate sequence is introduced and the adsorption of pure protein as well as protein from crude cell lysate at different conditions is demonstrated. Fused to a model protein this tag can be used to recognize and purify this protein from a fermentation broth by bare iron oxide nanoparticles (BIONs). Binding of up to 0.2 g protein per g nanoparticles can be achieved and recovered easily by switching to a citrate buffered system. For a deeper understanding of the separation process, the aggregation and agglomeration of the nanoparticle protein systems were monitored for binding and elution steps. Furthermore, an upscaling of the process to the liter scale and the separation of a green fluorescent protein (GFP) containing the affinity tag to purities of 70% from Escherichia coli fermentation broth was possible in a one step process by means of high gradient magnetic separation (HGMS).

Magnetic metal oxide nanoparticles demonstrate great applicability in several fields such as biotechnology, medicine and catalysis. A stable, magnetic and low-cost material, nanoscale magnetite, is an interesting adsorbent for protein purification. Downstream processing can account for up to 80% of the total production costs in biotechnological production. As such, the development of new innovative separation methods can be regarded as highly profitable. While short peptide sequences can be used as specific affinity tags for functionalised adsorber surfaces, they need expensive affinity ligands on the particle surface for adsorption. In order to identify peptide tags for several non-functionalised inorganic surfaces, different binding conditions to iron oxide nanoparticles are evaluated. Therefore, magnetite nanoparticles in a range of 5-20 nm were synthesised with a co-precipitation method. Zeta potential measurements indicated an amphiphilic surface with an isoelectric point in the neutral pH region. Glutamic acid-based homo-peptides were used as affinity peptides for the magnetite nanoparticles. We demonstrate a dependence of the binding affinity of the peptides on pH and buffer ions in two different experimental set-ups. The nature of surface coordination for glutamic acid-based peptides can be demonstrated with different spectroscopic approaches such as infrared spectroscopy (IR), Raman spectroscopy and circular dichroism spectroscopy (CD). We want to emphasise the importance of physicochemical properties such as surface energy, polarity, morphology and charge. These parameters, which are dependent on the environmental conditions, play a crucial role in peptide interactions with iron oxide surfaces. The understanding of the adsorption of simple biomolecules on nanoscale metal oxide surfaces also represents the key to the even more complex interactions of proteins at the bio-nano interface. From the identification of interaction patterns and an understanding of the adsorption of these peptides, the up-scaling to tagged model proteins facilitates the possibility of an industrial magnetic separation process and might therefore reduce time and costs in purification processes.