No abstract

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.

Antimicrobial peptides (AMPs) can target bacterial membranes and kill bacteria through membrane structural damage and cytoplasmic leakage. A group of surfactant-like cationic AMPs was developed from substitutions to selective amino acids in the general formula of G­(IIKK)3I-NH2, (called G3, a de novo AMP), to explore the correlation between AMP hydrophobicity and bioactivity. A threshold surface pressure over 12 mN/m was required to cause measurable antimicrobial activity and this corresponded to a critical AMP concentration. Greater surface activity exhibited stronger antimicrobial activity but had the drawback of worsening hemolytic activity. Small unilamellar vesicles (SUVs) with specific lipid compositions were used to model bacterial and host mammalian cell membranes by mimicking the main structural determinants of the charge and composition. Leakage from the SUVs of encapsulated carboxyfluorescein measured by fluorescence spectroscopy indicated a negative correlation between hydrophobicity and model membrane selectivity, consistent with measurements of the zeta potential that demonstrated the extent of AMP binding onto model SUV lipid bilayers. Experiments with model lipid membranes thus explained the trend of minimum inhibitory concentrations and selectivity measured from real cell systems and demonstrated the dominant influence of hydrophobicity. This work provides useful guidance for the improvement of the potency of AMPs via structural design, whilst taking due consideration of cytotoxicity.

Due to their structural simplicity and robust self-assembled nanostructures, short peptides prove to be an ideal system to explore the physical processes of self-assembly, hydrogels, semi-flexible polymers, quenched disorder and reptation. Rational design in peptide sequences facilitates cost-effective manufacturing, but the huge number of possible peptides has imposed obstacles for their characterisation to establish functional connections to the primary, secondary and tertiary structures. This review aims to cover recent advances in the self-assembly of designed short peptides, with a focus on physical driving forces, design rules, characterisation methods and exemplar applications. Super-resolution microscopy in combination with modern image analysis have been applied to quantify the structure and dynamics of peptide hydrogels whilst SANS and ssNMR continue to provide valuable information on structures over complementary lengths. Short peptides are attractive in biomedicine and nanotechnology, e.g., as antimicrobials, anticancer agents, vehicles for controlled drug release, peptide bioelectronics and responsive cell culture materials.

Antimicrobial peptides are promising alternatives to traditional antibiotics. A group of selfassembling lipopeptides was formed by attaching an acyl chain to the N-terminus of α-helix forming peptides with the sequence C x -G(IIKK) y I-NH 2 (C x G y , x = 4-12 and y = 2). C x G y selfassemble into nanofibers above their critical aggregation concentrations (CACs). With increasing x, the CACs decrease and the hydrophobic interactions increase, promoting secondary structure transitions within the nanofibers. Antimicrobial activity, determined by the minimum inhibition concentration (MIC), also decreases with increasing x, but the MICs are significantly smaller than the CACs, suggesting effective bacterial membrane disrupting power. Unlike conventional antibiotics, both C 8 G 2 and C 12 G 2 can kill S. aureus and E. coli after only minutes of exposure. C 12 G 2 nanofibers have considerably faster killing dynamics and lower cytotoxicity than their non-aggregated monomers. Antimicrobial activity of peptide aggregates has to date been underexploited and it is found to be a very promising mechanism for peptide design. Detailed evidence for the molecular mechanisms involved are provided, based on super-resolution fluorescence microscopy, ss-NMR, AFM, neutron scattering/reflectivity, CD and Brewster angle microscopy.