Peptide binding to metal oxide nanoparticles

Abstract

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.

Design of Interactions Between Nanomaterials and Proteins: A Highly Affine Peptide Tag to Bare Iron Oxide Nanoparticles for Magnetic Protein Separation

“…Primary nanoparticle aggregates with sizes around 30 nm exist independently at all investigated buffer conditions and in the presence of proteins (Figure F). Particle aggregation of BIONs has already be affirmed by temperature dependent magnetization measurements in a previous investigation . However, the formation of agglomerates is dependent on the buffer conditions and also on the protein present in the system.…”

“…Moreover, a transfer to larger, more complex structures where this sequence can be used as affinity tag is possible. A Glu 6 ‐tagged protein follows the same adsorption behavior as the homohexamer alone and similar fast adsorption kinetics were observed for both molecules . Magnetic separation is an interesting technique in downstream processing which enables the reduction of process steps and thus economizes time and cost.…”

“…The magnetic BIONs were synthesized by a simple low‐cost co‐precipitation method as described by Roth et al The physical properties such as saturation magnetization (83 emu g −1 ), no remanence, an isoelectric point around pH 6.5, the chemical composition Fe 3 O 4 and the crystalline spinel structure were previously described by Schwaminger et al (Figure S2 and S3, Supporting Information) . The nanoparticles demonstrate a sharp size distribution with a mean diameter of 12 nm, a specific surface area of around 89 m 2 g −1 and a low interparticle porosity of 0.18 cm 3 g −1 as determined by nitrogen adsorption isotherms (Figure S2 and S4, Supporting Information).…”

“…This effect can be explained by the high affinity of phosphate and citrate ions to iron oxide which lead to strong complex bonds between di‐ or trivalent anions and the surface cations . These bonds are stronger than the multiple monodentates formed by Glu 6 and therefore able to replace them facilitating a reversible binding of the tagged protein . On the other hand, the monovalent buffer species show an even lower complexing tendency than the protein itself .…”

“…In previous studies we were able to show, that a binding of amino acids to bare iron oxide nanoparticles (BIONs), at ambient conditions close to pH 7, is mainly through the carboxy group and that peptides based on glutamate residues demonstrate a high affinity to BIONs . Moreover, a transfer to larger, more complex structures where this sequence can be used as affinity tag is possible.…”