Simultaneous Targeting of NQO1 and SOD1 Eradicates Breast Cancer Stem Cells via Mitochondrial Futile Redox Cycling.

Triple negative breast cancer (TNBC) contains the highest proportion of cancer stem-like cells (CSCs), which display intrinsic resistance to currently available cancer therapies. This therapeutic resistance is partially mediated by an antioxidant defense coordinated by the transcription factor NRF2 and its downstream targets including NQO1. Here, we identified the antioxidant enzymes NQO1 and SOD1 as therapeutic vulnerabilities of ALDH+ epithelial-like CSCs and CD24-/loCD44+/hi mesenchymal-like CSCs in TNBC. Effective targeting of these CSC states was achieved by utilizing IB-DNQ, a potent and specific NQO1-bioactivatable futile redox cycling molecule, which generated large amounts of reactive oxygen species (ROS) including superoxide and hydrogen peroxide. Furthermore, the CSC killing effect was specifically enhanced by genetic or pharmacological inhibition of SOD1, a copper-containing superoxide dismutase highly expressed in TNBC. Mechanistically, a significant portion of NQO1 resided in the mitochondrial intermembrane space, catalyzing futile redox cycling from IB-DNQ to generate high levels of mitochondrial superoxide, and SOD1 inhibition markedly potentiated this effect resulting in mitochondrial oxidative injury, cytochrome c release, and activation of the caspase 3-mediated apoptotic pathway. Treatment with IB-DNQ alone or together with SOD1 inhibition effectively suppressed tumor growth, metastasis, and tumor-initiating potential in xenograft models of TNBC expressing different levels of NQO1. This futile oxidant-generating strategy, which targets CSCs across the epithelial-mesenchymal continuum, could be a promising therapeutic approach for treating TNBC patients.

Porin-independent accumulation in Pseudomonas enables antibiotic discovery.

Gram-negative antibiotic development has been hindered by a poor understanding of the types of compounds that can accumulate within these bacteria1,2. The presence of efflux pumps and substrate-specific outer-membrane porins in Pseudomonas aeruginosa renders this pathogen particularly challenging3. As a result, there are few antibiotic options for P. aeruginosa infections4 and its many porins have made the prospect of discovering general accumulation guidelines seem unlikely5. Here we assess the whole-cell accumulation of 345 diverse compounds in P. aeruginosa and Escherichia coli. Although certain positively charged compounds permeate both bacterial species, P. aeruginosa is more restrictive compared to E. coli. Computational analysis identified distinct physicochemical properties of small molecules that specifically correlate with P. aeruginosa accumulation, such as formal charge, positive polar surface area and hydrogen bond donor surface area. Mode of uptake studies revealed that most small molecules permeate P. aeruginosa using a porin-independent pathway, thus enabling discovery of general P. aeruginosa accumulation trends with important implications for future antibiotic development. Retrospective antibiotic examples confirmed these trends and these discoveries were then applied to expand the spectrum of activity of a gram-positive-only antibiotic, fusidic acid, into a version that demonstrates a dramatic improvement in antibacterial activity against P. aeruginosa. We anticipate that these discoveries will facilitate the design and development of high-permeating antipseudomonals.

Establishing Quantifiable Guidelines for Antimicrobial α/ß-Peptide Design: A Partial Least-Squares Approach to Improve Antimicrobial Activity and Reduce Mammalian Cell Toxicity.

Antimicrobial peptides (AMPs) are promising candidates to combat pathogens that are resistant to conventional antimicrobial drugs because they operate through mechanisms that involve membrane disruption. However, the use of AMPs in clinical settings has been limited, at least in part, by their susceptibility to proteolytic degradation and their lack of selectivity toward pathogenic microbes vs mammalian cells. We recently reported on the design of α- and ß-peptide oligomers structurally templated upon the naturally occurring α-helical AMP aurein 1.2. These α/ß-peptide oligomers are more proteolytically stable than aurein 1.2 and have several other attributes that render them attractive as alternatives to conventional AMPs. This study describes the influence of peptide physicochemical properties on the broad-spectrum activity of aurein 1.2-based α/ß-peptide mimics against nine bacterial, fungal, and mammalian cell lines. We used a partial least-squares regression (PLSR)-supervised machine learning model to quantify and visualize relationships between experimentally determined physicochemical properties (e.g., hydrophobicity, charge, and helicity) and experimentally measured cell-type-specific activities of 21 peptides in a 149-member α/ß-peptide library. Using this approach, we identified several peptides that were predicted to exhibit enhanced broad-spectrum selectivity, a measure that evaluates antimicrobial activity relative to mammalian cell toxicity compared to aurein 1.2. Experimental validation demonstrated high model predictive performance, and characterization of compounds with the highest broad-spectrum selectivity revealed peptide hydrophobicity, helicity, and helical rigidity to be strong predictors of broad-spectrum selectivity. The most selective peptide identified from the model prediction has more than a 13-fold improvement in broad-spectrum selectivity than that of aurein 1.2, demonstrating the ability of using PLSR models to identify quantitative structure-function relationships for nonstandard amino acid-containing peptides. Overall, this work establishes quantifiable guidelines for the rational design of helical antimicrobial α/ß-peptides and identifies promising new α/ß-peptides with significantly reduced mammalian toxicities and improved antifungal and antibacterial activities relative to aurein 1.2.

Cationic Homopolymers Inhibit Spore and Vegetative Cell Growth of Clostridioides difficile.

A wide range of synthetic polymers have been explored for antimicrobial activity. These materials usually contain both cationic and hydrophobic subunits because these two characteristics are prominent among host-defense peptides. Here, we describe a series of nylon-3 polymers containing only cationic subunits and their evaluation against the gastrointestinal, spore-forming pathogen Clostridioides difficile. Despite their highly hydrophilic nature, these homopolymers showed efficacy against both the vegetative and spore forms of the bacterium, including an impact on C. difficile spore germination. The polymer designated P34 demonstrated the greatest efficacy against C. difficile strains, along with low propensities to lyse human red blood cells or intestinal epithelial cells. To gain insight into the mechanism of P34 action, we evaluated several cell-surface mutant strains of C. difficile to determine the impacts on growth, viability, and cell morphology. The results suggest that P34 interacts with the cell wall, resulting in severe cell bending and death in a concentration-dependent manner. The unexpected finding that nylon-3 polymers composed entirely of cationic subunits display significant activities toward C. difficile should expand the range of other polymers considered for antibacterial applications.

Compound Uptake into E. coli Can Be Facilitated by N-Alkyl Guanidiniums and Pyridiniums.

Multidrug-resistant Gram-negative bacterial infections are on the rise, and with no FDA approvals for new classes of broad-spectrum antibiotics in over 50 years, these infections constitute a major threat to human health. A significant challenge is the inability of most compounds to accumulate in Gram-negative bacteria. Recently developed predictive guidelines show that appending a primary amine to an appropriately shaped compound can enhance Gram-negative accumulation. Here, we report that other positively charged nitrogen functional groups, namely, N-alkyl guanidiniums and pyridiniums, can also facilitate compound uptake into Gram-negative bacteria. The accumulation of a set of 60 nonantibiotic compounds, consisting of 20 primary amines and their corresponding guanidiniums and pyridiniums, was assessed in Escherichia coli. We also installed these alternate functional groups onto antibiotic scaffolds and assessed their accumulation and antibacterial activity in Gram-negative bacteria. The results suggest that other positively-charged, nitrogen-containing functional groups should be considered when designing antibiotics with Gram-negative activity.

Small-Molecule Morphogenesis Modulators Enhance the Ability of 14-Helical ß-Peptides To Prevent Candida albicans Biofilm Formation.

Candida albicans is an opportunistic fungal pathogen responsible for mucosal candidiasis and systemic candidemia in humans. Often, these infections are associated with the formation of drug-resistant biofilms on the surfaces of tissues or medical devices. Increased incidence of C. albicans resistance to current antifungals has heightened the need for new strategies to prevent or eliminate biofilm-related fungal infections. In prior studies, we designed 14-helical ß-peptides to mimic the structural properties of natural antimicrobial α-peptides (AMPs) in an effort to develop active and selective antifungal compounds. These amphiphilic, cationic, helical ß-peptides exhibited antifungal activity against planktonic C. albicans cells and inhibited biofilm formation in vitro and in vivo Recent studies have suggested the use of antivirulence agents in combination with antifungals. In this study, we investigated the use of compounds that target C. albicans polymorphism, such as 1-dodecanol, isoamyl alcohol, and farnesol, to attempt to improve ß-peptide efficacy for preventing C. albicans biofilms. Isoamyl alcohol, which prevents hyphal formation, reduced the minimum biofilm prevention concentrations (MBPCs) of ß-peptides by up to 128-fold. Combinations of isoamyl alcohol and antifungal ß-peptides resulted in less than 10% hemolysis at the antifungal MBPCs. Overall, our results suggest potential benefits of combination therapies comprised of morphogenesis modulators and antifungal AMP peptidomimetics for preventing C. albicans biofilm formation.

Preventing S. aureus biofilm formation on titanium surfaces by the release of antimicrobial ß-peptides from polyelectrolyte multilayers.

Staphylococcus aureus infections represent the major cause of titanium based-orthopaedic implant failure. Current treatments for S. aureus infections involve the systemic delivery of antibiotics and additional surgeries, increasing health-care costs and affecting patient's quality of life. As a step toward the development of new strategies that can prevent these infections, we build upon previous work demonstrating that the colonization of catheters by the fungal pathogen Candida albicans can be prevented by coating them with thin polymer multilayers composed of chitosan (CH) and hyaluronic acid (HA) designed to release a ß-amino acid-based peptidomimetic of antimicrobial peptides (AMPs). We demonstrate here that this ß-peptide is also potent against S. aureus (MBPC = 4 µg/mL) and characterize its selectivity toward S. aureus biofilms. We demonstrate further that ß-peptide-containing CH/HA thin-films can be fabricated on the surfaces of rough planar titanium substrates in ways that allow mammalian cell attachment and permit the long-term release of ß-peptide. ß-Peptide loading on CH/HA thin-films was then adjusted to achieve release of ß-peptide quantities that selectively prevent S. aureus biofilms on titanium substrates in vitro for up to 24 days and remained antimicrobial after being challenged sequentially five times with S. aureus inocula, while causing no significant MC3T3-E1 preosteoblast cytotoxicity compared to uncoated and film-coated controls lacking ß-peptide. We conclude that these ß-peptide-containing films offer a novel and promising localized delivery approach for preventing orthopaedic implant infections. The facile fabrication and loading of ß-peptide-containing films reported here provides opportunities for coating other medical devices prone to biofilm-associated infections. STATEMENT OF SIGNIFICANCE: Titanium (Ti) and its alloys are used widely in orthopaedic devices due to their mechanical strength and long-term biocompatibility. However, these devices are susceptible to bacterial colonization and the subsequent formation of biofilms. Here we report a chitosan and hyaluronic acid polyelectrolyte multilayer-based approach for the localized delivery of helical, cationic, globally amphiphilic ß-peptide mimetics of antimicrobial peptides to inhibit S. aureus colonization and biofilm formation. Our results reveal that controlled release of this ß-peptide can selectively kill S. aureus cells without exhibiting toxicity toward MC3T3-E1 preosteoblast cells. Further development of this polymer-based coating could result in new strategies for preventing orthopaedic implant-related infections, improving outcomes of these titanium implants.

14-Helical ß-Peptides Elicit Toxicity against C. albicans by Forming Pores in the Cell Membrane and Subsequently Disrupting Intracellular Organelles.

Synthetic peptidomimetics of antimicrobial peptides (AMPs) are promising antimicrobial drug candidates because they promote membrane disruption and exhibit greater structural and proteolytic stability than natural AMPs. We previously reported selective antifungal 14-helical ß-peptides, but the mechanism of antifungal toxicity of ß-peptides remains unknown. To provide insight into the mechanism, we studied antifungal ß-peptide binding to artificial membranes and living Candida albicans cells. We investigated the ability of ß-peptides to interact with and permeate small unilamellar vesicle models of fungal membranes. The partition coefficient supported a pore-mediated mechanism characterized by the existence of a critical ß-peptide concentration separating low- and high-partition coefficient regimes. Live cell intracellular tracking of ß-peptides showed that ß-peptides translocated into the cytoplasm, and then disrupted the nucleus and vacuole sequentially, leading to cell death. This understanding of the mechanisms of antifungal activity will facilitate design and development of peptidomimetic AMPs, including 14-helical ß-peptides, for antifungal applications.

Incorporation of ß-Amino Acids Enhances the Antifungal Activity and Selectivity of the Helical Antimicrobial Peptide Aurein 1.2.

Antimicrobial peptides (AMPs) are attractive antifungal drug candidates because they kill microbes via membrane disruption and are thus unlikely to provoke development of resistance. Low selectivity for fungal vs human cells and instability in physiological environments have limited the development of AMPs as therapeutics, but peptidomimetic AMPs can overcome these obstacles and also provide useful insight into AMP structure-function relationships. Here, we describe antifungal peptidomimetic α/ß-peptides templated on the natural α-peptidic AMP aurein 1.2. These α/ß-aurein analogs fold into i → i + 4 H-bonded helices that present arrays of side chain functionality in a manner virtually identical to that of aurein 1.2. By varying charge, hydrophobicity, conformational stability, and α/ß-amino acid organization, we designed active and selective α/ß-peptide aurein analogs that exhibit minimum inhibitory concentrations (MIC) against the opportunistic pathogen Candida albicans that are 4-fold lower than that of aurein 1.2 and elicit less than 5% hemolysis at the MIC. These α/ß-aurein analogs are promising candidates for development as antifungal therapeutics and as tools to elucidate mechanisms of AMP activity and specificity.

Antifungal activity of a ß-peptide in synthetic urine media: Toward materials-based approaches to reducing catheter-associated urinary tract fungal infections.

UNLABELLED: Catheter-associated urinary tract infections (CAUTI) are the most common type of hospital-acquired infection, with more than 30 million catheters placed annually in the US and a 10-30% incidence of infection. Candida albicans forms fungal biofilms on the surfaces of urinary catheters and is the leading cause of fungal urinary tract infections. As a step toward new strategies that could prevent or reduce the occurrence of C. albicans-based CAUTI, we investigated the ability of antifungal ß-peptide-based mimetics of antimicrobial peptides (AMPs) to kill C. albicans and prevent biofilm formation in synthetic urine. Many α-peptide-based AMPs exhibit antifungal activities, but are unstable in high ionic strength media and are easily degraded by proteases-features that limit their use in urinary catheter applications. Here, we demonstrate that ß-peptides designed to mimic the amphiphilic helical structures of AMPs retain 100% of their structural stability and exhibit antifungal and anti-biofilm activity against C. albicans in a synthetic medium that mimics the composition of urine. We demonstrate further that these agents can be loaded into and released from polymer-based multilayer coatings applied to polyurethane, polyethylene, and silicone tubing commonly used as urinary catheters. Our results reveal catheters coated with ß-peptide-loaded multilayers to kill planktonic fungal cells for up to 21days of intermittent challenges with C. albicans and prevent biofilm formation on catheter walls for at least 48h. These new materials and approaches could lead to advances that reduce the occurrence of fungal CAUTI. STATEMENT OF SIGNIFICANCE: Catheter-associated urinary tract infections are the most common type of hospital-acquired infection. The human pathogen Candida albicans is the leading cause of fungal urinary tract infections, and forms difficult to remove 'biofilms' on the surfaces of urinary catheters. We investigated synthetic ß-peptide mimics of natural antimicrobial peptides as an approach to kill C. albicans and prevent biofilm formation in media that mimics the composition of urine. Our results reveal these mimics to retain structural stability and activity against C. albicans in synthetic urine. We also report polymer-based approaches to the local release of these agents within urinary catheter tubes. With further development, these materials-based approaches could lead to advances that reduce the occurrence of fungal urinary tract infections.

Correction: Hydrophobicity of Antifungal ß-Peptides Is Associated with Their Cytotoxic Effect on In Vitro Human Colon Caco-2 and Liver HepG2 Cells.

[This corrects the article DOI: 10.1371/journal.pone.0149271.].

Hydrophobicity of Antifungal ß-Peptides Is Associated with Their Cytotoxic Effect on In Vitro Human Colon Caco-2 and Liver HepG2 Cells.

The widespread distribution of fungal infections, with their high morbidity and mortality rate, is a global public health problem. The increase in the population of immunocompromised patients combined with the selectivity of currents treatments and the emergence of drug-resistant fungal strains are among the most imperative reasons to develop novel antifungal formulations. Antimicrobial ß-peptides are peptidomimetics of natural antimicrobial peptides (AMPs), which have been proposed as developmental platforms to enhance the AMPs selectivity and biostability. Their tunability allows the design of sequences with remarkable activity against a wide spectrum of microorganisms such as the human pathogenic Candida spp., both in planktonic and biofilm morphology. However, the ß-peptide's effect on surrounding host cells remains greatly understudied. Assessments have mainly relied on the extent of hemolysis that a candidate peptide is able to cause. This work investigated the in vitro cytotoxicity of various ß-peptides in the Caco-2 and HepG2 mammalian cell lines. Results indicated that the cytotoxic effect of the ß-peptides was influenced by cell type and was also correlated to structural features of the peptide such as hydrophobicity. We found that the selectivity of the most hydrophobic ß-peptide was 2-3 times higher than that of the least hydrophobic one, for both cell types according to the selectivity index parameter (IC50/MIC). The IC50 of Caco-2 and HepG2 increased with hydrophobicity, which indicates the importance of testing putative therapeutics on different cell types. We report evidence of peptide-cell membrane interactions in Caco-2 and HepG2 using a widely studied ß-peptide against C. albicans.

Intraluminal Release of an Antifungal ß-Peptide Enhances the Antifungal and Anti-Biofilm Activities of Multilayer-Coated Catheters in a Rat Model of Venous Catheter Infection.

Candida albicans is the most prevalent cause of hospital-acquired fungal infections and forms biofilms on indwelling medical devices that are notoriously difficult to treat or remove. We recently demonstrated that the colonization of C. albicans on the surfaces of catheter tube segments can be reduced in vitro by coating them with polyelectrolyte multilayers (PEMs) that release a potent antifungal ß-peptide. Here, we report on the impact of polymer structure and film composition on both the inherent and ß-peptide-mediated ability of PEM-coated catheters to prevent or reduce the formation of C. albicans biofilms in vitro and in vivo using a rat model of central venous catheter infection. Coatings fabricated using polysaccharide-based components [hyaluronic acid (HA) and chitosan (CH)] and coatings fabricated using polypeptide-based components [poly-l-lysine (PLL) and poly-l-glutamic acid (PGA)] both served as reservoirs for the loading and sustained release of ß-peptide, but differed substantially in loading and release profiles and in their inherent antifungal properties (e.g., the ability to prevent colonization and biofilm growth in the absence of ß-peptide). In particular, CH/HA films exhibited inherent antifungal and antibiofilm behaviors in vitro and in vivo, a result we attribute to the incorporation of CH, a weak polycation demonstrated to exhibit antimicrobial properties in other contexts. The antifungal properties of both types of films were improved substantially when ß-peptide was incorporated. Catheter segments coated with ß-peptide-loaded CH/HA and PLL/PGA films were both strongly antifungal against planktonic C. albicans and the formation of surface-associated biofilms in vitro and in vivo. Our results demonstrate that PEM coatings provide a useful platform for the design of new antifungal materials, and suggest opportunities to design multifunctional or dual-action platforms to prevent or reduce the severity of fungal infections in applied biomedical contexts or other areas in which fungal biofilms are endemic.

Antifungal Activity of 14-Helical ß-Peptides against Planktonic Cells and Biofilms of Candida Species.

Candida albicans is the most prevalent cause of fungal infections and treatment is further complicated by the formation of drug resistant biofilms, often on the surfaces of implanted medical devices. In recent years, the incidence of fungal infections by other pathogenic Candida species such as C. glabrata, C. parapsilosis and C. tropicalis has increased. Amphiphilic, helical ß-peptide structural mimetics of natural antimicrobial α-peptides have been shown to exhibit specific planktonic antifungal and anti-biofilm formation activity against C. albicans in vitro. Here, we demonstrate that ß-peptides are also active against clinically isolated and drug resistant strains of C. albicans and against other opportunistic Candida spp. Different Candida species were susceptible to ß-peptides to varying degrees, with C. tropicalis being the most and C. glabrata being the least susceptible. ß-peptide hydrophobicity directly correlated with antifungal activity against all the Candida clinical strains and species tested. While ß-peptides were largely ineffective at disrupting existing Candida biofilms, hydrophobic ß-peptides were able to prevent the formation of C. albicans, C. glabrata, C. parapsilosis and C. tropicalis biofilms. The broad-spectrum antifungal activity of ß-peptides against planktonic cells and in preventing biofilm formation suggests the promise of this class of molecules as therapeutics.

Hydrophobicity and helicity regulate the antifungal activity of 14-helical ß-peptides.

Candida albicans is one of the most prevalent fungal pathogens, causing both mucosal candidiasis and invasive candidemia. Antimicrobial peptides (AMPs), part of the human innate immune system, have been shown to exhibit antifungal activity but have not been effective as pharmaceuticals because of low activity and selectivity in physiologically relevant environments. Nevertheless, studies on α-peptide AMPs have revealed key features that can be designed into more stable structures, such as the 14-helix of ß-peptide-based oligomers. Here, we report on the ways in which two of those features, hydrophobicity and helicity, govern the activity and selectivity of 14-helical ß-peptides against C. albicans and human red blood cells. Our results reveal both antifungal activity and hemolysis to correlate to hydrophobicity, with intermediate levels of hydrophobicity leading to high antifungal activity and high selectivity toward C. albicans. Helical structure-forming propensity further influenced this window of selective antifungal activity, with more stable helical structures eliciting specificity for C. albicans over a broader range of hydrophobicity. Our findings also reveal cooperativity between hydrophobicity and helicity in regulating antifungal activity and specificity. The results of this study provide critical insight into the ways in which hydrophobicity and helicity govern the activity and specificity of AMPs and identify criteria that may be useful for the design of potent and selective antifungal agents.

Polymer multilayers loaded with antifungal ß-peptides kill planktonic Candida albicans and reduce formation of fungal biofilms on the surfaces of flexible catheter tubes.

Candida albicans is the most common fungal pathogen responsible for hospital-acquired infections. Most C. albicans infections are associated with the implantation of medical devices that act as points of entry for the pathogen and as substrates for the growth of fungal biofilms that are notoriously difficult to eliminate by systemic administration of conventional antifungal agents. In this study, we report a fill-and-purge approach to the layer-by-layer fabrication of biocompatible, nanoscale 'polyelectrolyte multilayers' (PEMs) on the luminal surfaces of flexible catheters, and an investigation of this platform for the localized, intraluminal release of a cationic ß-peptide-based antifungal agent. We demonstrate that polyethylene catheter tubes with luminal surfaces coated with multilayers ~700nm thick fabricated from poly-l-glutamic acid (PGA) and poly-l-lysine (PLL) can be loaded, post-fabrication, by infusion with ß-peptide, and that this approach promotes extended intraluminal release of this agent (over ~4months) when incubated in physiological media. The ß-peptide remained potent against intraluminal inoculation of the catheters with C. albicans and substantially reduced the formation of C. albicans biofilms on the inner surfaces of film-coated catheters. Finally, we report that these ß-peptide-loaded coatings exhibit antifungal activity under conditions that simulate intermittent catheter use and microbial challenge for at least three weeks. We conclude that ß-peptide-loaded PEMs offer a novel and promising approach to kill C. albicans and prevent fungal biofilm formation on surfaces, with the potential to substantially reduce the incidence of device-associated infections in indwelling catheters. ß-Peptides comprise a promising new class of antifungal agents that could help address problems associated with the use of conventional antifungal agents. The versatility of the layer-by-layer approach used here thus suggests additional opportunities to exploit these new agents in other biomedical and personal care applications in which fungal infections are endemic.

Carbohydrate microarrays for enzymatic reactions and quantification of binding affinities for glycan-protein interactions.

Glycans are involved in a variety of physiological and pathological processes through interactions with proteins. Thus, the molecular basis of glycan-protein interactions provides valuable information on understanding biological phenomena and exploiting more effective carbohydrate-based therapeutic agents and diagnostic tools. Carbohydrate microarray technology has become a powerful tool for evaluating glycan-mediated biological events in a high-throughput manner. This technology is mostly applied for rapid analysis of glycans-protein interactions in the field of functional glycomics. In order to expand application areas of glycan microarrays, we have used carbohydrate microarrays for measurement of binding affinities between glycans and proteins and profiling of glycosyltransferase activities. The glycan microarrays used for these studies are constructed by immobilizing maleimide or hydrazide-conjugated glycans on the thiol or hydrazide-derivatized glass slides, respectively. This protocol describes the fabrication of carbohydrate microarrays and their applications to enzymatic reactions and determination of quantitative binding affinities.

Sheet-like assemblies of charged amphiphilic α/ß-peptides at the air-water interface.

There is growing interest in the design of molecules that undergo predictable self-assembly. Bioinspired oligomers with well-defined conformational propensities are attractive from this perspective, since they can be constructed from diverse building blocks, and self-assembly can be directed by the identities and sequence of the subunits. Here we describe the structure of monolayers formed at the air-water interface by amphiphilic α/ß-peptides with 1:1 alternation of α- and ß-amino acid residues along the backbone. Two of the α/ß-peptides, one a dianion and the other a dication, were used to determine differences between self-assemblies of the net negatively and positively charged oligomers. Two additional α/ß-peptides, both zwitterionic, were designed to favor assembly in a 1:1 molar ratio mixture with parallel orientation of neighboring strands. Monolayers formed by these α/ß-peptides at the air-water interface were characterized by surface pressure-area isotherms, grazing incidence X-ray diffraction (GIXD), atomic force microscopy and ATR-FTIR. GIXD data indicate that the α/ß-peptide assemblies exhibited diffraction features similar to those of ß-sheet-forming α-peptides. The diffraction data allowed the construction of a detailed model of an antiparallel α/ß-peptide sheet with a unique pleated structure. One of the α/ß-peptide assemblies displayed high stability, unparalleled among previously studied assemblies of α-peptides. ATR-FTIR data suggest that the 1:1 mixture of zwitterionic α/ß-peptides assembled in a parallel arrangement resembling that of a typical parallel ß-sheet secondary structure formed by α-peptides. This study establishes guidelines for design of amphiphilic α/ß-peptides that assemble in a predictable manner at an air-water interface, with control of interstrand orientation through manipulation of Coulombic interactions along the backbone.