Abstract
The preparation of hydrogels and stable emulsions is important in the formulation of many functional nanostructured soft materials. We investigate the multifunctional self-assembly and bioactivity properties of a novel surfactant-like peptide (SLP) that shows antimicrobial activity, is able to form hydrogels without pH adjustment, and is able to stabilize oil-in-water emulsions. Furthermore, we demonstrate on-demand de-emulsification in response to the protease enzyme elastase. We show that SLP (Ala)9-Arg (A9R) forms β-sheet fibers above a critical aggregation concentration and that water-in-oil emulsions are stabilized by a coating of β-sheet fibers around the emulsion droplets. Furthermore, we demonstrate enzyme-responsive de-emulsification, which has potential in the development of responsive release systems. The peptide shows selective antimicrobial activity against Gram-negative pathogens including Pseudomonas aeruginosa, which causes serious infections. Our results highlight the utility of SLPs in the stabilization of oil/water emulsions and the potential for these to be used to formulate antimicrobial peptide emulsions which are additionally responsive to protease. The peptide A9R has pronounced antibacterial activity against clinically challenging pathogens, and its ability to form β-sheet fibers plays a key role in its diverse structural properties, ranging from hydrogel formation to emulsion stabilization.
Highlights
The global healthcare challenge of emerging antimicrobial resistance is stimulating intense research activity into the development of new antimicrobial agents
These peptides typically contain cationic residues such as arginine or lysine or the aromatic residue tryptophan.[1−10] Surfactant-like peptides (SLPs) are a class of peptide with sequences of uncharged residues capped with charged residues; examples include peptides with alanine repeats capped with charged residues such as lysine, arginine, aspartic acid, or glutamic acid.[11−14] The amphiphilic properties of surfactantlike peptide (SLP) lead to self-assembly into distinct nanostructures in aqueous solution depending on the SLP sequence and the solution conditions.[2,11−29] SLPs are a potentially valuable class of peptide antibacterial agents because the self-assembled structure can lead to high-density presentation of the active motif
Because long alanine repeats are known substrates for the enzyme elastase, we investigate the elastase-induced degradation of the peptide, inspired by our work on related SLP KA6E.36
Summary
The global healthcare challenge of emerging antimicrobial resistance is stimulating intense research activity into the development of new antimicrobial agents. Emulsions were prepared by mixing measured volumes of 1-bromohexadecane (density 1 g mL−1) with aqueous solutions of A9R This oil was chosen to match the density of both phases in order to reduce the possible spontaneous deemulsification process.[46] Preliminary tests were performed to investigate the time stability of emulsions containing a range of different volume ratios of water/1-bromohexadecane emulsions stabilized by 0.05 wt % A9R. Solutions were vortexed for 5 s, and 3 × 20 μL aliquots were taken at times 0, 30, 60, 120, and 1440 min These samples were serially diluted in PBS, and 10 μL of each dilution was plated onto LB agar and incubated at 37 °C overnight before colony counting
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