Biomimetic Identification of Phase-Selective Peptide-Additives
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Abstract
Biomineralisation is a widespread phenomenon, which enables organisms to form highly ordered materials. During the associated processes, certain properties, such as the crystal morphology and -size or the deposited polymorph of a compound, are tightly controlled. Although the mechanisms that govern the polymorph-selective precipitations are intensively investigated, the respective experiments are often hampered by the difficulties in isolating the involved biomolecules from organisms or biominerals. To circumvent the dependence on biologic samples and related problems, this thesis has applied the Phage Display technique to identify peptides capable of biomimetically controlling the formation of the different calcium carbonate polymorphs during crystallisations.
In the first experimental part, phage display was employed for affinity selections on the calcite polymorph of calcium carbonate. The respective screening experiments allowed the identification of several, potentially calcite-affine peptide sequences unreported in the literature. Subsequently, the focus was placed on the preparation of these peptides by automated Solid Phase Peptide Synthesis successfully yielding these peptides or their corresponding derivatives.
The second part covered the investigation of the prepared peptides for their influence on the phase selection. Different methods for the crystallisation of calcium carbonate were adopted, modified and employed in the presence of the prepared peptides. Especially the peptide with the sequence (GQNSTTNYHTLVR) strongly shifted the phase composition towards the vaterite polymorph in a concentration-dependent manner. The crystallisation experiments further allowed to postulate a hypothesis that explained the vaterite-selective influence of the peptide by its function as a crystal growth inhibitor of the thermodynamically stable calcium carbonate-phase calcite. Additionally, several, literature-known peptides were prepared and examined in a further set of crystallisation experiments as a comparison to the peptides identified by phage display. These experiments allowed to identify similarities and differences between the individual peptides and they gave general insights into the behaviour of the respective peptide-additives.
The third part dealt with investigating the proposed effect of the vaterite selective peptide as a crystal growth inhibitor. The intention was to demonstrate and determine the adsorptions of this peptide towards the individual polymorphs of calcium carbonate. To this purpose, a dye-labelled derivative of the vaterite-selective peptide was prepared to assess the adsorptions qualitatively and quantitatively. The tendency of the dye-labelled peptide to form hydrogels prevented the further development of the adsorption test.
In summary, calcite-affine peptides were successfully identified and the effect of these peptides on the phase composition of calcium carbonate was demonstrated through different experimental stages. The newly identified peptides complement known additive structures and facilitate future experiments to gain a deeper insight into the crystallisation- and polymorph control of calcium carbonate.