Self‐assembled structures obtained from organic molecules have shown great potential for applications in a wide range of domains. In this context, short peptides prove to be a particularly versatile class of organic building blocks for self‐assembled materials. These species afford the biocompatibility and polymorphic richness typical of proteins while allowing synthetic availability and robustness typical of smaller molecules. At the nano‐to‐mesoscale, the architectures obtained from peptide units exhibit stability and a large variety of morphologies, the most common of which are nanotubes, nanoribbons, and nanowires. This review describes the formation of peptide‐based self‐assembled structures triggered by different stimuli (e.g., ionic strength, pH, and polarity), and the interactions that drive the assembling processes. It is surveyed how judicious molecular design is exploited to impart favourable assembling properties to afford systems with desired characteristics. A large body of literature provides the experimental and in silico data to predict self‐assembly in a given peptide system and obtain different supramolecular organizations for applications in a wide range of fields, from transport to sensing, from catalysis to drug delivery and tissue regeneration.

Different polymorphic forms of peptide assemblies influence the stereoselectivity of aldol reactions in water medium.

The self-assembly of the amphiphilic lipopeptide PAEPKI-C16 (P = proline, A = alanine, E = glutamic acid, K = lysine, I = isoleucine, and C16 = hexadecyl) was investigated using a combination of microscopy, spectroscopy, and scattering methods and compared to that of C16-IKPEAP with the same (reversed) peptide sequence and the alkyl chain positioned at the N-terminus and lacking a free N-terminal proline residue. The catalytic activity of these peptides was then compared using a model aldol reaction system. For PAEPKI-C16, the cryo-TEM images showed the formation of micrometer-length fibers, which by small-angle X-ray scattering (SAXS) were found to have radii of 2.5–2.6 nm. Spectroscopic analysis shows that these fibers are built from β-sheets. This behavior is in complete contrast to that of C16-IKPEAP, which forms spherical micelles with peptides in a disordered conformation [ Hutchinson Hutchinson J. Phys. Chem. B2019123613]. In PAEPKI-C16, spontaneous alignment of fibers was observed upon increasing pH, which was accompanied by observed birefringence and anisotropy of SAXS patterns. This shows the ability to form a nematic phase, and unprecedented nematic hydrogel formation was also observed for these lipopeptides at sufficiently high concentrations. SAXS shows retention of an ultrafine (1.7 nm core radius) fibrillar network within the hydrogel. PAEPKI-C16 with free N-terminal proline shows enhanced anti:syn diastereoselectivity and better conversion compared to C16-IKPEAP. The cytotoxicity of PAEPKI-C16 was also lower than that of C16-IKPEAP for both fibroblast and cancer cell lines. These results highlight the sensitivity of lipopeptide properties to the presence of a free proline residue. The spontaneous nematic phase formation by PAEPKI-C16 points to the high anisotropy of its ultrafine fibrillar structure, and the formation of such a phase at low concentrations in aqueous solution may be valuable for future applications.

Morphological, spectroscopic, and scattering studies of the self-assembly and aggregation of mixtures of [RF] 4 and P [RF] 4 peptides (where R = arginine; F = phenylalanine; P = proline), in solution and as hydrogels, were performed to obtain information about polymorphism. CD data confirmed a β-sheet secondary structure in aqueous solution, and TEM images revealed nanofibers with diameters of ∼10 nm and micrometer lengths. SAXS curves were fitted using a mass fractal-component and a long cylinder shell form factor for the liquid samples, and only a long cylinder shell form factor for the gels. Increasing the P[RF] 4 content in the systems leads to a reduction in cylinder radius and core scattering density, suggesting an increase in packing of the peptide molecules; however, the opposite effect is observed for the gels, where the scattering density is higher in the shell for the systems containing higher P[RF] 4 content. These compounds show potential as catalysts in the asymmetric aldol reactions, with cyclohexanone and p-nitrobenzaldehyde in aqueous media. A moderate conversion (36.9%) and a good stereoselectivity (69:31) were observed for the system containing only [RF] 4 . With increasing P[RF] 4 content, a considerable decrease of the conversion was observed, suggesting differences in the self-assembly and packing factor. Rheological measurements were performed to determine the shear moduli for the soft gels.