Mixed thermoreversible gels were successfully fabricated by the addition of a thermosensitive polymer, poly­(N-isopropylacrylamide) (PNIPAM), to fibrillar nanostructures self-assembled from a short peptide I3K. When the temperature was increased above the lower critical solution temperature of the PNIPAM, the molecules collapsed to form condensed globular particles, which acted as cross-links to connect different peptide nanofibrils and freeze their movements, resulting in the formation of a hydrogel. Since these processes were physically driven, such hydrogels could be reversibly switched between the sol and gel states as a function of temperature. As a model peptide, I3K was formulated with PNIPAM to produce a thermoreversible sol–gel system with a transition temperature of ∼33 °C, which is just below the body temperature. The antibacterial peptide of G­(IIKK)3I-NH2 could be conveniently encapsulated in the hydrogel by the addition of the solution at lower temperatures in the sol phase and then increasing the temperature to be above 33 °C for gelation. The hydrogel gave a sustained and controlled linear release of G­(IIKK)3I-NH2 over time. Using the peptide nanofibrils as three-dimensional scaffolds, such thermoresponsive hydrogels mimic the extracellular matrix and could potentially be used as injectable hydrogels for minimally invasive drug delivery or tissue engineering.

Self-assembling peptides have the ability to spontaneously aggregate into large ordered structures. The reversibility of the peptide hydrogen bonded supramolecular assembly make them tunable to a host of different applications, although it leaves them highly dynamic and prone to disassembly at the low concentration needed for biological applications. Here we demonstrate that a secondary hydrophobic interaction, near the peptide core, can stabilise the highly dynamic peptide bonds, without losing the vital solubility of the systems in aqueous conditions. This hierarchical self-assembly process can be used to stabilise a range of different β-sheet hydrogen bonded architectures.

Hydrogels offer great potential for many biomedical and technological applications. For clinical uses, hydrogels that act as scaffold materials for cell culture, regenerative medicine, and drug delivery are required to have bactericidal properties. The amphiphilic peptide A9K2 was designed to effectively inhibit bacterial growth via a mechanism of membrane permeabilization. The present study demonstrated that addition of fetal bovine serum (FBS) or plasma amine oxidase (PAO) induced a sol-gel transition in A9K2 aqueous solutions. The transformation of A9K2 molecules catalyzed by lysyl oxidase (LO) in FBS or PAO accounted for the hydrogelation. Importantly, the enzymatic A9K2 hydrogel displayed high antibacterial ability against both Gram-negative and Gram-positive bacterial strains while showing extremely low mammalian cell cytotoxicity, thus demonstrating good biocompatibility. Under established coculture conditions, the peptide hydrogel showed excellent selectivity by favoring the adherence and spreading of mammalian cells, while killing pathogenic bacteria, thus avoiding bacterial contamination. These advantages endow the enzymatic A9K2 hydrogel with great potential for biomedical applications.

Peptide self-assembly is a hierarchical process, often starting with the formation of α-helices, β-sheets or β-hairpins. However, how the secondary structures undergo further assembly to form higher-order architectures remains largely unexplored. The polar zipper originally proposed by Perutz is formed between neighboring β-strands of poly-glutamine via their side-chain hydrogen bonding and helps to stabilize the sheet. By rational design of short amphiphilic peptides and their self-assembly, here we demonstrate the formation of polar zippers between neighboring β-sheets rather than between β-strands within a sheet, which in turn intermesh the β-sheets into wide and flat ribbons. Such a super-secondary structural template based on well-defined hydrogen bonds could offer an agile route for the construction of distinctive nanostructures and nanomaterials beyond β-sheets.

Super-resolution fluorescence microscopy, specifically stochastic reconstruction microscopy (STORM), and atomic force microscopy (AFM) were used to image the self-assembly processes of the peptide surfactant IK. The peptide surfactants self-assembled into giant helical fibrils with diameters between 5 and 10 nm with significant helical twisting. The resolution of the STORM images was 30 nm, calculated using the Fourier ring correlation method. STORM compares favorably with AFM for the calculation of contour lengths (∼6 μm) and persistence lengths (10.1 ± 1.2 μm) due to its increased field of view (50 μm), and its ability to image bulk morphologies away from surfaces under ambient solution conditions. Two-color STORM experiments were performed to investigate the dynamic process of self-assembly after mixing of two separately labeled samples, and the results revealed the formation of long nanofibers via end-to-end connections of short ones. No evidence was found for significant monomer exchange between the samples, and the self-assembled structures were very stable and long-lived.