Assembly Of An Peptide Amphiphile Based On Hydrolyzed Silkworm Silk Fibroin And Its Application

Peptide assembly has attracted many attentions in recent years. Various nanostructures, such as micelles, nanotubes, nanofibers, nanoribbon and nanovesicles, can be prepared through peptide assembly driven by the non-covalent interactions (including hydrogen bonds, hydrophobic interactions, electrostatic interactions and π-π interactions etc.). Traditionally, these peptides are obtained via chemical synthesis or genetic engineering, which are rather expensive and time-consuming.In this thesis, we reported a novel and simple way to obtain peptide, i.e. hydrolysis of Bombyx mori silkworm silk fibroin (SF) and then purification. Since the silkworm silk fibroin are an abundant natural resources, our method suggests a more practical way to prepare peptides at low cost. Then GAGAGAGY, an octapeptide separated from the hydrolysate, was coupled with lauric acid to construct peptide amphiphiles (PAs). This PAs showed an pH induced hierarchical assembly from molecules to macroscopic materials. Moreover, the PAs was utilized to in situ synthesize gold nanoparticles (AuNPs) and direct their assembly.In the first part, chymotrypsin was used to hydrolyze Bombyx mori silkworm silk fibroin. After HPLC purification, one of the peptides based on SF, an octapeptide, GAGAGAGY was obtained from the hydrolysate. Although this peptide didn’t show any assembly behavior in aqueous solution, its derivative, C12-GAGAGAGY, showed an interesting pH induced assembly after coupling with lauric acid, i.e. cylindrical nanofibers in alkaline condition and twisted nanoribbons in acidic condition, which was due to the dissociation of the carboxyl group in the PAs. In alkaline condition, the carboxyl group was dissociated, resulted in the electrostatic repulsion between PAs. In combination with the electrostatic repulsion, the hydrophobic interaction between alkyl chains drove the molecules to form micelles. On the other hand, the intermolecular H-bonds in the peptide regions broke the spherical symmetry, resulting in cylindrical nanofibers. On the other sides,-COO-was neutralized in acidic condition, resulted in the formation of β-sheet layers. Due to hydrophobic interactions, these layers tended to stack and entwine, finally resulted in the formation of twisted nanoribbons.In the second part, C12-GAGAGAGY was utilized to in situ synthesize AuNPs and control their assembly. Due to the incorporation of tyrosine, C12-GAGAGAGY was able to reduce Au3+to AuNPs and further stabilize them at pH11without any external reducing or capping reagents. Besides, these AuNPs showed reversible assembly/disassembly by varying pH, due to the pH-sensitive assembly of its template, C12-GAGAGAGY. This template was cylindrical nanofibers at pH11and stacking nanoribbons at pH4, and AuNPs well dispersed among the networks formed by cylindrical nanofibers of C12-GAGAGAGY at pH11while aggregated on both sides of stacking nanoribbons at pH4. Interestingly, unlike traditional predefined templates, the pH-induced assembly of C12-GAGAGAGY and assembly/disassembly of AuNPs occurred simultaneously. These results present us the potential to utilize such smart peptide amphiphile templates for the fabrication of1-D inorganic nanostructure with promising applications in nano-scale optical device.Inspired by the stacking of (3-sheet, the hierarchical assembly of C12-GAGAGAGY was investigated at high concentration in the third part. It was found that the PAs assembled into well dispersed cylindrical nanofibers at pH11. However, nanoribbons appeared when the pH value was decreased to around8. Moreover, these nanoribbons showed parallel aggregation to form bundles with hierarchical structure, due to the hydrogen bonds formed by the carboxyl group located at the both sides of the nanoribbons. When the pH was further decreased, a hydrogel with local anisotropic networks was formed, resulted from the further parallel aggregation of the nanoribbons. More interestingly, a hydrogel fiber was obtained when the solution at pH8was directly injected into the HC1solution, and nanoribbons was well aligned along the long axis of such hydrogel fiber.