Peptide self-assembly, ubiquitous in biology, is one of the most promising ‘bottom up’ approach for the generation of synthetic supramolecular architectures. The type and size of the peptide nanostructures e.g., nanotubes, vesicles or micelles depend upon the amino acid sequence, peptide concentration, geometrical constraints and the type and charge of electrolyte used to induce self-assembly. These factors affect peptide alignment, the packing density and the strength of the intermolecular bonds between the monomers which in turn result in hierarchical supramolecular structures of different morphologies and properties.
Here in, we describe the nanostructures generated from self-assembly of a tetrapeptide H2N-Ala-Ala-Ala-Ala-CONH2 (α-Ala)4. The nanospheres formed are of 100 – 170 nm diameters was confirmed by transmission and scanning electron microscopy. These experimental findings were correlated with theoretically simulated self-assembled structure of (α-Ala)4 obtained using semi-empirical energy minimization upto the RMS gradient of 0.1 kcal / Å-mol. The conformational change occurred during self-assembly were probed by circular dichroism and infra-red spectroscopic studies before and after the self-assembly. Moreover, the stability of nanospheres was checked over the range of temperature 25°C to 80°C and was also checked in acidic to basic medium over the pH range of 5 to 8. In fact, the secondary structure of (α-Ala)4 was also evaluated by computational conformational analysis using molecular mechanics MM+ force field and further energy minimized using Ab initio 6-31G** basis set. Since short polyalanine oligopeptides were reported to form monomeric α-helices in aqueous solution, we found the backbone containing four residues of α-alanine coiled in such a manner that forms a 10-membered hydrogen bonded ring.
The peptide based nanospheres were further explored for interaction with antiparkinson’s drug L-DOPA by employing absorption and emission spectroscopic techniques. These nanostructures can be considered as models for further development as delivery agents for different biomolecules.