Peptide-based supramolecular assemblies are a promising class of nanomaterials with important biomedical applications, specifically in drug delivery and tissue regeneration. However, the intrinsic antibacterial capabilities of these assemblies have been largely overlooked. The recent identification of common characteristics shared by antibacterial and self-assembling peptides provides a paradigm shift towards development of antibacterial agents. Here we present the antibacterial activity of self-assembled diphenylalanine, which emerges as the minimal model for antibacterial supramolecular polymers. The diphenylalanine nano-assemblies completely inhibit bacterial growth, trigger upregulation of stress-response regulons, induce substantial disruption to bacterial morphology, and cause membrane permeation and depolarization. We demonstrate the specificity of these membrane interactions and the development of antibacterial materials by integration of the peptide assemblies into tissue scaffolds. This study provides important insights into the significance of the interplay between self-assembly and antimicrobial activity and establishes innovative design principles toward the development of antimicrobial agents and materials.

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Supramolecular assemblies of vesicles composed of ionophores and phospholipids embedded in a matrix of polymerized diacetylene [PDA] lipids are shown to undergo visible color changes in the presence of ions in solution. The blue-to-red color transitions of the vesicles are directly related to binding of the cations to the ionophores, and their association with the lipids. The system detects cations in submillimolar concentrations and demonstrates a significant ionic selectivity, in particular between the physiologically important ions Na+ and K+.

The increased resistance of various bacteria toward available antibiotic drugs has initiated intensive research efforts into identifying new sources of antimicrobial substances. Short antibiotic peptides (10-30 residues) are prevalent in nature as part of the intrinsic defense mechanisms of most organisms and have been proposed as a blueprint for the design of novel antimicrobial agents. Antimicrobial peptides are generally believed to kill bacteria through membrane permeabilization and extensive pore-formation. Assays providing rapid and easy evaluation of interactions between antimicrobial membrane peptides and lipid bilayers could significantly improve screening for substances with effective antibacterial properties, as well as contribute to the elucidation of structural and functional properties of antimicrobial peptides. Here we describe a colorimetric sensor in which particles composed of phospholipids and polymerized polydiacetylene (PDA) lipids were shown to exhibit striking color changes upon interactions with antimicrobial membrane peptides. The color changes in the system occur because of the structural perturbation of the lipids following their interactions with antimicrobial peptides. The assay was also sensitive to the antibacterial properties of structurally and functionally related peptide analogs.

The formation of amyloid fibrils by the human islet amyloid polypeptide is associated with type II diabetes. While it was previously suggested that the formed fibrils are toxic to pancreatic beta-cells due to membrane permeation activity, more recent studies suggested that protofibrillar assemblies have significantly higher potency in permeating lipid bilayers. Here, we specifically studied the membrane interaction activity of soluble and insoluble islet amyloid polypeptide assemblies at high temporal resolution. A colorimetric analysis using lipid/polydiacetylene (PDA) biomimetic vesicles clearly demonstrated the transient formation of soluble assemblies that strongly interact with the lipid vesicles. A peak in the level of membrane binding of the soluble fraction, as reflected by the colorimetric assay, was observed after incubation for approximately 1 h, followed by a decrease in the level of membrane interaction of the assemblies. The transient nature of the membrane-active assemblies was independently confirmed by a fluorescence quenching assay. Ultrastructural analysis using transmission electron microscopy provided morphological evidence of prefibrillar assemblies, supported the transient existence of membrane interacting soluble species, and facilitated observation of the non-membrane-active filaments in the solution. Taken together, our results provide experimental evidence for the formation of transient soluble prefibrillar assemblies which are highly membrane-active. The implications of these observations are discussed in light of designed fibrillization inhibitors.