Power struggles in peptide-amphiphile nanostructures

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Inspired by the ubiquitous functions fulfilled by native proteins, the self-assembly of peptide amphiphiles (PAs) holds much promise for the creation of functional nanostructures. Typically, PAs are constructed by conjugating blocks of very different character: a hydrophilic peptide segment with a hydrophobic element (an alkyl chain, lipid, polymer or polypeptide). The resulting amphiphilicity governs the self-assembly process in aqueous solutions. This self-assembly process is guided by attractive forces (for example hydrophobic interactions, hydrogen bonding, electrostatic attraction) and repulsive forces (for example electrostatic repulsion, mechanical forces). The balance between these forces is responsible for the secondary structure of the peptide segment, and furthermore the size and shape of the assemblies that are formed. A result of PA self-assembly is that the properties of the peptide segment can be altered, as it is a general observation that peptides are more likely to exhibit a well-defined secondary structure at an interface (e.g. the corona of a micelle) than they are in solution. This characteristic of peptides can be exploited to give nanostructures with well-defined properties. The art of controlled PA self-assembly consists of carefully combining all the inter- and intramolecular forces to arrive at a material which is both structurally well-defined and has controllable functionalities. In this tutorial review the forces that act within PA nanostructures are discussed, that is, the effect of the hydrophobic block and peptide secondary structure on each other as well as on the aggregate as a whole. At the end of the review, a short section is devoted to the applications of these PA nanostructures.