Self-assembling peptides are a category of peptides which undergo spontaneous assembling into ordered nanostructures. Originally described in 1993, these designer peptides have attracted interest in the field of nanotechnology for their potential for application in areas such as biomedical nanotechnology, tissue cell culturing, molecular electronics, and more.
Effectively self-assembling peptides act as building blocks for various material and device applications. The essence of this technology is to replicate what nature does: to use molecular recognition processes to form ordered assemblies of building blocks capable of conducting biochemical activities.
Background
Peptides can perform as excellent building blocks for a wide range of materials as they can be designed to combine with a range of other building blocks such as lipids, sugars, nucleic acids, metallic nanocrystals, and so on; this gives the peptides an edge over carbon nanotubes, which are another popular nanomaterial, as the carbon structure is unreactive. They also exhibit biocompatibility and molecular recognition; the latter is particularly useful as it enables specific selectivity for building ordered nanostructures. Additionally, peptides have superb resistance to extreme conditions of high/low temperatures, detergents and denaturants.
The ability of peptides to perform self-assembly allows them to be used as fabrication tools, which is currently and will continue to grow as a fundamental part of nanomaterials production. The self-assembling of peptides is facilitated through the molecules’ structural and chemical compatibility with each other. The structures formed demonstrate physical and chemical stability.
A great advantage of using self-assembling peptides to build nanostructures in a bottom-up approach is that specific features can be incorporated; the peptides can be modified and functionalized. This approach means the final structures are made from the self-integration of small, simple building blocks. This approach is needed for nanoscale structure, as the top-down method of miniaturizing devices using sophisticated lithography and etching techniques has reached a physical limit. Moreover, the top-down approach applies mainly to silicon-based technology and cannot be used for biological developments.
The peptide structure is organized hierarchically into four levels. The primary structure of a peptide is the sequence of the amino acids on the peptide chain. Amino acids are monomer molecules that carry a carboxyl and an amine functional groups; a spectrum of other chemical groups are attached to different amino acids, such as thiols and alcohols. This facilitates the wide range of chemical interactions and, therefore, molecular recognitions that peptides are capable of, for designer self-assembling peptides, both natural and non-natural amino acids are used. They link together in a controlled manner to form short peptides, which link to form long polypeptide chains.
Along these chains, the alternating amine (NH) and carbonyl (CO) groups are highly polar, and they readily form hydrogen bonds with each other. These hydrogen bonds bind peptide chains together to give rise to secondary structures. Stable secondary structures include the alpha-helices and beta-sheets. Unstable secondary structures are random loops, turns, and coils that are formed. The secondary structure that is formed depends on the primary structure; different sequences of the amino acids exhibit different preferences. Secondary structures usually fold into with a variety of loops and turns into a tertiary structure. What differentiates the secondary structure from the tertiary structure is that the latter includes non-covalent interactions. The quaternary structure combines two or more different chains of polypeptide to form what is known as a protein sub-unit.
The self-assembly process of the peptide chains is dynamic—reassembly occurs repeatedly in a self-healing manner. The type of interactions that occur to reassemble peptide structures include van der Waals forces, ionic bonds, hydrogen bonds, and hydrophobic forces. These forces also facilitate the molecular recognition function that the peptides encompasses. These interactions works on a basis of preference dependent on energy properties and specificity.
A range of different nanostructures can be formed. Nanotubes are defined as an elongated nano-object with a definite inner hole. Nanofibrils are solid on the inside, as opposed to hollow nanotubes.