A 30-residue polypeptide AQPal14 that was designed to form a three-stranded coiled coil in aqueous solution contains a 4-pyridylalanine residue on its solvent-exposed positions. In the previously reported work, it was shown that the AQPal14 peptide can form directional bonds with the cis-positions of the Pt(ethylenediamine) (Pt(en)) linker and as a result form nanoscale globular and fibrillar assemblies. The current work describes how the reaction of the AQPal14 peptide with a linker having coordination preferences different from Pt(en) can affect the size and morphology of formed nanoassemblies. Specifically, a cobalt (III) protoporphyrin IX (CoPPIX) was chosen for this purpose since it can produces a six-coordinated CoPPIX(AQPal14)2 complex upon binding with AQPal14 in which two trimeric coiled coils of AQPal14 are bound to the axial positions of the porphyrin ring. It was found that conditions at which the AQPal14 peptide reacts with CoPPIX can affect the structure of the formed materials as well. Specifically, the self-assembly of AQPal14 and CoPPIX in unbuffered water leads to the formation of nanoscale globular assemblies having the morphology analogous to the AQPal14-Pt(en) structures. However, when AQPal14 reacts with CoPPIX in 20 mM phosphate buffer, pH 7, the millimeter-scale rod-like structures are formed. Formation of the nanoassemblies with similar morphology after the reaction of AQPal14 with either Pt(en) or CoPPIX suggests that the size and shape of the formed assemblies are not likely dictated by the coordination geometry of the linking compounds, but by the oligomerization state of the peptide. However, it was suggested that the formation of the rod-like structures does not depend on these factors and mostly depends on the crystallization of the phosphate ions in the presence of these compounds.
To study the effect of the oligomerization state on the self-assembling properties and structure of the product materials, a new peptide AQPal14N16 whose sequence is based on the sequence of the original AQPal14 peptide was designed and synthesized. It was expected that a single modification in the AQPal14 sequence would not only change the oligomerization state of the new AQPal14N16 peptide from a trimer to a dimer, but also affect the size and shape parameters of the assembled nanostructures as well. As described in this work, it was found that the change in the oligomerization state does affect the morphology of the self-assembled AQPal14N16-CoPPIX nanoassemblies and also enhance the ability of these nanomaterials to further assemble into microscale fibrous systems.