| The programmed self-assembly of proteins into highly ordered nanostructures creates biomaterials displaying a wide range of physical properties often exceeding those of synthetic polymers. However, precise control of this hierarchical assembly remains a significant challenge. In fact, the uncontrolled aggregation of natural soluble protein (alpha-helix rich) into insoluble fibrillar assemblies (beta-sheet rich) known as amyloid plaques are thought to be responsible for several systemic and neurodegenerative diseases, called "amyloidosis", such as Alzheimer's, Parkinson's, mad-cow disease, and type II diabetes. In additional to the biomedical applications, the amyloid fibers have also gained rapidly growing interests in their potential as advanced nanofiber materials due to the greater-than-steel strength, flexibility, versatility, and ability to self-assemble. Because of the complexity of naturally occurring amyloid proteins, the de novo design of synthetic, short peptides that form amyloid fibers would not only facilitate the understanding of the formation of pathological amyloid fibers in nature but also provide the range of structures and functions needed for engineered nanomaterials.;In our design, a series of 16-mer peptide-dendron hybrids (PDH ), based on an intrinsically alpha-helical, alanine-rich sequence, were constructed to explore how the nature of the dendron interaction affects the conformational properties. The interaction between the dendrons was explored in hybrids by progressively increasing the interdendron spacing from i, i + 4 to i, i + 11. These studies revealed an alpha-helix to beta-sheet conformational transition that occurred in water for PDH i, i + 6 (i6) and i, i + 10 (i10), a hallmark of amyloid formation. i6 further assembled into amyloid fibers in aqueous solution. Interestingly, i10 could be induced to form either soluble nanotubes or insoluble amyloid nanofibers. The two nanostructures could be interconverted by modulating the extent of charge repulsion with changes in pH or in salt concentration. The 6 nm diameter of the nanotubes is among smallest beta-sheet peptide nanotubes that have been observed. The assembled nanostructures of i10 efficiently encapsulate hydrophobic guest molecules in water that can then be released by lowering the pH, suggesting potential to serve as vehicles for drug delivery.*;More recently, we developed a simple method for fabricating n-type semiconductor nanostructures in aqueous solutions via the beta-sheet assembly of dipeptides bearing a naphthalene diimide (NDI) side chain. Depending on the placement of the NDI group, either amyloid-like 1D helical nanofibers or twisted nanoribbons can be formed driven by beta-sheet-type hydrogen bonding along the peptide backbone and pi-pi association of NDI chromophores. Notably, this design exemplifies a shortest beta-sheet forming sequence. Fluorescence lifetime and anisotropy experiments indicate that the nature of the intermolecular packing of NDI chromophores within the nanostructures critically affects intermolecular energy migration due to effective pi-electron delocalization.;Interestingly, even simpler systems using NDI-functionalized lysine derivatives afford a facile self-assembly into well-defined nanotubes. These 1D nanostructures with n-type semiconductivity provide promising building blocks for optoelectronic nanodevices.*;*Please refer to dissertation for diagrams. |
