Self-assembly of ultra-small micelles from amphiphilic lipopeptoids

Supramolecular Peptoid Structure Strengthens Complexation with Polyacrylic Acid Microgels

We have studied the complexation between cationic antimicrobials and polyanionic microgels to create self-defensive surfaces that responsively resist bacterial colonization. An essential property is the stable sequestration of the loaded (complexed) antimicrobial within the microgel under a physiological ionic strength. Here, we assess the complexation strength between poly(acrylic acid) [PAA] microgels and a series of cationic peptoids that display supramolecular structures ranging from an oligomeric monomer to a tetramer. We follow changes in loaded microgel diameter with increasing [Na+] as a measure of the counterion doping level. Consistent with prior findings on colistin/PAA complexation, we find that a monomeric peptoid is fully released at ionic strengths well below physiological conditions, despite its +5 charge. In contrast, progressively higher degrees of peptoid supramolecular structure display progressively greater resistance to salting out, which we attribute to the greater entropic stability associated with the complexation of multimeric peptoid bundles.

Minimal Peptoid Dynamics Inform Self-Assembly Propensity

Peptoids are structural isomers of natural peptides, with side chain attachment at the amide nitrogen, conferring this class of compounds with the ability to access both cis and trans ω torsions as well as an increased diversity of ψ/φ states with respect to peptides. Sampling within these dimensions is controlled through side chain selection, and an expansive set of viable peptoid residues exists. It has been shown recently that "minimal" di- and tripeptoids with aromatic side chains can self-assemble into highly ordered structures, with size and morphological definition varying as a function of sequence pattern (e.g., XFF and FXF, where X = a nonaromatic peptoid monomer). Aromatic groups, such as phenylalanine, are regularly used in the design of minimal peptide assemblers. In recognition of this, and to draw parallels between these compounds classes, we have developed a series of descriptors for intramolecular dynamics of aromatic side chains to discern whether these dynamics, in a preassembly condition, can be related to experimentally observed nanoscale assemblies. To do this, we have built on the atomistic peptoid force field reported by Weiser and Santiso (CGenFF-WS) through the rigorous fitting of partial charges and the collation of Charmm General Force Field (CGenFF) parameters relevant to these systems. Our study finds that the intramolecular dynamics of side chains, for a given sequence, is dependent on the specific combination of backbone ω torsions and that homogeneity of sampling across these states correlates well with the experimentally observed ability to assemble into nanomorphologies with long-range order. Sequence patterning is also shown to affect sampling, in a manner consistent for both tripeptoids and tripeptides. Additionally, sampling similarities between the nanofiber forming tripeptoid, Nf-Nke-Nf in the cc state, and the nanotube forming dipeptide FF, highlight a structural motif which may be relevant to the emergence of extended linear assemblies. To assess these properties, a variety of computational approaches have been employed.

pH-Dependent Solution Micellar Structure of Amphoteric Polypeptoid Block Copolymers with Positionally Controlled Ionizable Sites

While solution micellization of ionic block copolymers (BCP) with randomly distributed ionization sites along the hydrophilic segments has been extensively studied, the roles of positionally controlled ionization sites along the BCP chains in their micellization and resulting micellar structure remain comparatively less understood. Herein, three amphoteric polypeptoid block copolymers carrying two oppositely charged ionizable sites, with one fixed at the hydrophobic terminus and the other varyingly positioned along the hydrophilic segment, have been synthesized by sequential ring-opening polymerization method. The presence of the ionizable site at the hydrophobic segment terminus is expected to promote polymer association toward equilibrium micellar structures in an aqueous solution. The concurrent presence of oppositely charged ionizable sites on the polymer chains allows the polymer association to be electrostatically modulated in a broad pH range (ca. 2-12). Micellization of the amphoteric polypeptoid BCP in dilute aqueous solution and the resulting micellar structure at different solution pHs was investigated by a combination of scattering and microscopic methods. Negative-stain transmission-electron microscopy (TEM), small-angle neutron scattering (SANS), and small-angle X-ray scattering (SAXS) analyses revealed the dominant presence of core-shell-type spherical micelles and occasional rod-like micelles with liquid crystalline (LC) domains in the micellar core. The micellar structures (e.g., aggregation number, radius of gyration, chain packing in the micelle) were found to be dependent on the solution pH and the position of the ionizable site along the chain. This study has highlighted the potential of controlling the position of ionizable sites along the BCP polymer to modulate the electrostatic and LC interactions, thus tailoring the micellar structure at different solution pH values in water.

Protein-mimetic peptoid nanoarchitectures for pathogen recognition and neutralization

Recent outbreaks of both new and existing infectious pathogens have threatened healthcare systems around the world. Therefore, it is vital to detect and neutralize pathogens to prevent their spread and treat infected patients. This consideration has led to the development of biosensors and antibiotics inspired by the structure and function of antibodies and antimicrobial peptides (AMPs), which constitute adaptive and innate immunity, efficiently protecting the human body against invading pathogens. Herein, we provide an overview of recent advances in the detection and neutralization of pathogens using protein-mimetic peptoid nanoarchitectures. Peptoids are bio-inspired and sequence-defined polymers composed of repeating N-substituted glycine units. They can spontaneously fold into well-defined three-dimensional nanostructures that encode chemical information depending on their sequences. Loop-functionalized peptoid nanosheets have been constructed by mimicking antibodies containing chemically variable loops as binding motifs for their respective target pathogen. Furthermore, by mimicking the cationic amphipathic features of natural AMPs, helical peptoids and their assemblies have been developed to achieve selective anti-infective activity owing to their intrinsic ability to interact with bacterial membranes and viral envelopes. We believe that this mini-review furnishes in-depth insight into how to construct protein-like nanostructures via the self-assembly of peptoids for application in the detection of pathogens and the treatment of infectious diseases for future healthcare applications.

Advances in hybrid peptide-based self-assembly systems and their applications

Self-assembly of peptides demonstrates a great potential for designing highly ordered, finely tailored supramolecular arrangements enriched with high specificity, improved efficacy and biological activity. Along with natural peptides, hybrid peptide systems composed of natural and chemically diverse unnatural amino acids have been used in various fields, including drug delivery, wound healing, potent inhibition of diseases, and prevention of biomaterial related diseases to name a few. In this review, we provide a brief outline of various methods that have been utilized for obtaining fascinating structures that create an avenue to reproduce a range of functions resulting from these folds. An overview of different self-assembled structures as well as their applications will also be provided. We believe that this review is very relevant to the current scenario and will cover conformations of hybrid peptides and resulting self-assemblies from the late 20^th century through 2022. This review aims to be a comprehensive and reliable account of the hybrid peptide-based self-assembly owing to its enormous influence in understanding and mimicking biological processes.

Self-Assembly of Antimicrobial Peptoids Impacts Their Biological Effects on ESKAPE Bacterial Pathogens

Antimicrobial peptides (AMPs) are promising pharmaceutical candidates for the prevention and treatment of infections caused by multidrug-resistant ESKAPE pathogens, which are responsible for the majority of hospital-acquired infections. Clinical translation of AMPs has been limited, in part by apparent toxicity on systemic dosing and by instability arising from susceptibility to proteolysis. Peptoids (sequence-specific oligo-N-substituted glycines) resist proteolytic digestion and thus are of value as AMP mimics. Only a few natural AMPs such as LL-37 and polymyxin self-assemble in solution; whether antimicrobial peptoids mimic these properties has been unknown. Here, we examine the antibacterial efficacy and dynamic self-assembly in aqueous media of eight peptoid mimics of cationic AMPs designed to self-assemble and two nonassembling controls. These amphipathic peptoids self-assembled in different ways, as determined by small-angle X-ray scattering; some adopt helical bundles, while others form core-shell ellipsoidal or worm-like micelles. Interestingly, many of these peptoid assemblies show promising antibacterial, antibiofilm activity in vitro in media, under host-mimicking conditions and antiabscess activity in vivo. While self-assembly correlated overall with antibacterial efficacy, this correlation was imperfect. Certain self-assembled morphologies seem better-suited for antibacterial activity. In particular, a peptoid exhibiting a high fraction of long, worm-like micelles showed reduced antibacterial, antibiofilm, and antiabscess activity against ESKAPE pathogens compared with peptoids that form ellipsoidal or bundled assemblies. This is the first report of self-assembling peptoid antibacterials with activity against in vivo biofilm-like infections relevant to clinical medicine.

Hierarchical assemblies of polypeptoids for rational design of advanced functional nanomaterials

Polypeptoids (poly-N-substituent glycines) are a class of highly tailorable peptidomimetic polymers. Polypeptoids have identical backbones as polypeptides (poly-C-substituent glycines), but sidechains of polypeptoids are appended to backbone nitrogen rather than α-carbon of polypeptides. As a result, peptoid backbone lacks of chirality and hydrogen bond donors. This unique structure gives polypeptoids a combined merit of both high stability as synthetic polymers and biocompatibility as biopolymers. In addition, peptoid sequences can be engineered precisely to assemble specific crystalline patterns such as spheres, fibers, ribbons, tubes, and sheets, which shows promising potentials of polypeptoids for different applications such as antimicrobials, catalysts, drug delivery, and templating inorganic materials. In this review, we summarize recent investigations into hierarchical self-assembly pathways and molecular structures of peptoid crystals that are of interest as templates for fabricating functional materials for potential biomedical, biochemical, and bioengineering applications. This review provides a summary of recent experimental and computational studies of polypeptoid assembly in solution and solid-liquid interfaces, current achievements in the field, and discusses future challenges and opportunities for the rational design of self-assembled polypeptoid nanomaterials.

Hierarchical Nanomaterials Assembled from Peptoids and Other Sequence-Defined Synthetic Polymers

In nature, the self-assembly of sequence-specific biopolymers into hierarchical structures plays an essential role in the construction of functional biomaterials. To develop synthetic materials that can mimic and surpass the function of these natural counterparts, various sequence-defined bio- and biomimetic polymers have been developed and exploited as building blocks for hierarchical self-assembly. This review summarizes the recent advances in the molecular self-assembly of hierarchical nanomaterials based on peptoids (or poly-N-substituted glycines) and other sequence-defined synthetic polymers. Modern techniques to monitor the assembly mechanisms and characterize the physicochemical properties of these self-assembly systems are highlighted. In addition, discussions about their potential applications in biomedical sciences and renewable energy are also included. This review aims to highlight essential features of sequence-defined synthetic polymers (e.g., high stability and protein-like high-information content) and how these unique features enable the construction of robust biomimetic functional materials with high programmability and predictability, with an emphasis on peptoids and their self-assembled nanomaterials.

Lipidation of Antimicrobial Peptides as a Design Strategy for Future Alternatives to Antibiotics

Multi-drug-resistant bacteria are becoming more prevalent, and treating these bacteria is becoming a global concern. One alternative approach to combat bacterial resistance is to use antimicrobial (AMPs) or host-defense peptides (HDPs) because they possess broad-spectrum activity, function in a variety of ways, and lead to minimal resistance. However, the therapeutic efficacy of HDPs is limited by a number of factors, including systemic toxicity, rapid degradation, and low bioavailability. One approach to circumvent these issues is to use lipidation, i.e., the attachment of one or more fatty acid chains to the amine groups of the N-terminus or a lysine residue of an HDP. In this review, we examined lipidated analogs of 66 different HDPs reported in the literature to determine: (i) whether there is a link between acyl chain length and antibacterial activity; (ii) whether the charge and (iii) the hydrophobicity of the HDP play a role; and (iv) whether acyl chain length and toxicity are related. Overall, the analysis suggests that lipidated HDPs with improved activity over the nonlipidated counterpart had acyl chain lengths of 8–12 carbons. Moreover, active lipidated peptides attached to short HDPs tended to have longer acyl chain lengths. Neither the charge of the parent HDP nor the percent hydrophobicity of the peptide had an apparent significant impact on the antibacterial activity. Finally, the relationship between acyl chain length and toxicity was difficult to determine due to the fact that toxicity is quantified in different ways. The impact of these trends, as well as combined strategies such as the incorporation of d- and non-natural amino acids or alternative approaches, will be discussed in light of how lipidation may play a role in the future development of antimicrobial peptide-based alternatives to current therapeutics.

Direct N-substituted N-thiocarboxyanhydride polymerization towards polypeptoids bearing unprotected carboxyl groups

Synthesis of poly(α-amino acid)s bearing carboxyl groups is a critical pathway to prepare biomaterials to simulate functional proteins. The traditional approaches call for carboxyl-protected monomers to prevent degradation of monomers or wrong linkage. In this contribution, we synthesize N-carboxypentyl glycine N-thiocarboxyanhydride (CPG-NTA) and iminodiacetic acid N-thiocarboxyanhydride (IDA-NTA) without protection. Initiated by amines, CPG-NTA directly polymerizes into polyCPG bearing unprotected carboxyl groups with controlled molecular weight (2.8-9.3 kg mol^-1) and low dispersities (1.08-1.12). Block and random copolymerizations of CPG-NTA with N-ethyl glycine N-thiocarboxyanhydride (NEG-NTA) demonstrate its versatile construction of complicated polypeptoids. On the contrary, IDA-NTA transforms amines into cyclic IDA dimer-capped species with carboxyl end group in decent yields (>89%) regio-selectively. Density functional theory calculation elucidates that IDA repeating unit is prone to cyclize to be the six-membered ring product with low ΔG. The polymer is a good adhesive reagent to various materials with adhesive strength of 33-229 kPa.

Chain-End Modifications and Sequence Arrangements of Antimicrobial Peptoids for Mediating Activity and Nano-Assembly

Poly(N-substituted glycine) "peptoids" are an interesting class of peptidomimics that can resist proteolysis and mimic naturally found antimicrobial peptides (AMPs), which exhibit wide spectrum activity against bacteria. This work investigates the possibility of modifying peptoid AMP mimics (AMPMs) with aliphatic lipid "tails" to generate "lipopeptoids" that can assemble into micellar nanostructures, and evaluates their antimicrobial activities. Two families of AMPMs with different distributions of hydrophobic and cationic residues were employed-one with a uniform repeating amphiphilicity, the other with a surfactant-like head-to-tail amphiphilicity. To further evaluate the interplay between self-assembly and activity, the lipopeptoids were variously modified at the AMPM chain ends with a diethylene glycol (EG2) and/or a cationic group (Nlys-Nlys dipeptoid) to adjust amphiphilicity and chain flexibility. Self-assembly was investigated by critical aggregation concentration (CAC) fluorescence assays and dynamic light scattering (DLS). The structure of a key species was also verified by small-angle X-ray scattering (SAXS) and cryo-electron microscopy (cryo-EM). To screen for antibacterial properties, we measured the minimum inhibitory concentrations (MIC) against S. aureus, E. coli, and P. aeruginosa. We found that certain combinations of lipid tail and AMPM sequences exhibit increased antibacterial activity (i.e., decreased MICs). Perhaps counter-intuitively, we were particularly interested in increased MICs in combination with low CACs. Concealing antimicrobial interactions due to packing of AMPMs in nano-assemblies could pave the way to AMPMs that may be "inert" even if unintentionally released and prevent microbes from gaining resistance to the lipopeptoids. Overall, incorporation of EG2 significantly improved lipopeptoids packing while the hydrophobic tail length was found to have a major influence over the MIC. One particular sequence, which we named C15-EG2-(kss)4, exhibited a very low CAC of 34 μM (0.0075 wt.%) and a significantly increased MIC above values for the unmodified AMPM. With the sequence design trends uncovered from this study, future work will focus on discovering more species such as C15-EG2-(kss)4 and on investigating release mechanisms and the potency of the released lipopeptoids.

Altering the X-ray Scattering Contrast of Triton X-100 Micelles and Its Trapping in a Supercooled Solvent

The structure of core-shell micelles formed by nonionic surfactant Triton X-100 (TX-100) in a supercooled glucose-urea melt is investigated by contrast variation small-angle X-ray scattering (SAXS), small angle neutron scattering (SANS), and HR-TEM. Cooling a molten mixture of glucose-urea (weight ratio of 3:2) to room temperature yields a supercooled solvent without crystallization that can be used for trapping micelles of TX-100. By use of a combination of water and glucose-urea mixture at different proportions as solvent for micellization, the scattering length density (SLD) of the solvent can be tuned to match the shell contrast of the micelles. A systematic analysis of SAXS and SANS data with different SLD of solvent permits a quantitative evaluation of electron density profile of micelles in different matrices. The core of TX-100 micelles shows significant swelling in glucose-urea melt, as compared to that in water. The dimension and morphology of micelles were evaluated by scattering techniques and HR-TEM. Dynamic light scattering (DLS) studies suggest that, unlike micelles in water, the diffusion of micelles in supercooled glucose-urea melt decreased by several orders of magnitude.

Self-Assembly of Minimal Peptoid Sequences

Peptoids are biofunctional N-substituted glycine peptidomimics. Their self-assembly is of fundamental interest because they demonstrate alternatives to conventional peptide structures based on backbone chirality and beta-sheet hydrogen bonding. The search for self-assembling, water-soluble "minimal" sequences, be they peptide or peptidomimic, is a further challenge. Such sequences are highly desired for their compatibility with biomacromolecules and convenient synthesis for broader application. We report the self-assembly of a set of trimeric, water-soluble α-peptoids that exhibit a relatively low critical aggregation concentration (CAC ∼ 0.3 wt %). Cryo-EM and angle-resolved DLS show different sequence-dependent morphologies, namely uniform ca. 6 nm wide nanofibers, sheets, and clusters of globular assemblies. Absorbance and fluorescence spectroscopies indicate unique phenyl environments for π-interactions in the highly ordered nanofibers. Assembly of our peptoids takes place when the sequences are fully ionized, representing a departure from superficially similar amyloid-type hydrogen-bonded peptide nanostructures and expanding the horizons of assembly for sequence-specific bio- and biomimetic macromolecules.

Peptoid drug discovery and optimization via surface X-ray scattering

Synthetic polymers mimicking antimicrobial peptides have drawn considerable interest as potential therapeutics. N-substituted glycines, or peptoids, are recognized by their in vivo stability and ease of synthesis. Peptoids are thought to act primarily on the negatively charged lipids that are abundant in bacterial cell membranes. A mechanistic understanding of lipid-peptoid interaction at the molecular level will provide insights for rational design and optimization of peptoids. Here, we highlight recent studies that utilize synchrotron liquid surface X-ray scattering to characterize the underlying peptoid interactions with bacterial and eukaryotic membranes. Cellular membranes are highly complex, and difficult to characterize at the molecular level. Model systems including Langmuir monolayers, are used in these studies to reduce system complexity. The general workflow of these systems and the corresponding data analysis techniques are presented alongside recent findings. These studies investigate the role of peptoid physicochemical characteristics on membrane activity. Specifically, the roles of cationic charge, conformational constraint via macrocyclization, and hydrophobicity are shown to correlate their membrane interactions to biological activities in vitro. These structure-activity relationships have led to new insights into the mechanism of action by peptoid antimicrobials, and suggest optimization strategies for future therapeutics based on peptoids.

Vesicles from the self-assembly of the ultra-small fatty acids with amino acids under aqueous conditions

The properties of vesicles formed from the self-assembly of amphiphilic molecules can mimic the functionality of the natural lipid membranes. In this study, the self-assembly process of the amphiphilic structures formed by the interaction between ultra-small fatty acids [FAs, Cn (n = 4-8)] and amino acids (AAs) to generate vesicles under aqueous conditions were investigated in detail, along with the corresponding dynamic vesiculation mechanisms. Our results showed that the molar ratio of FAs/AAs and the chain length of FAs largely affected the structural characteristics and dispersion of vesicles. The detailed information about the entire size distributions and morphology of obtained vesicles were explored by the cryogenic transmission electron microscopy (Cryo-EM). Fourier transform infrared (FT-IR) spectra and quantum calculations suggested that the intermolecular hydrogen bond and electrostatic interactions between ultra-small molecules (FAs and AAs) during the aggregation processes were responsible for the formation of vesicles, where the hydrogen-bonding effect was dominant. Our findings shed new light on the effective and simple preparation of biological vesicles via ultra-small molecules self-assembly in aqueous solutions, which may have potential applications in vesicle physiology and drug delivery systems, and also get a mature understanding of the fundamental intermolecular interactions in life process.

MRI micelles self-assembled from synthetic gadolinium-based nano building blocks

Synthetic self-assembled stealth Gd-micelles are a new blood pool contrast agent for magnetic resonance imaging.

Self-assembly of bioactive peptides, peptide conjugates, and peptide mimetic materials

Molecular self-assembly is a multi-disciplinary field of research, with potential chemical and biological applications. One of the main driving forces of self-assembly is molecular amphiphilicity, which can drive formation of complex and stable nanostructures. Self-assembling peptide and peptide conjugates have attracted great attention due to their biocompatibility, biodegradability and biofunctionality. Understanding assembly enables the better design of peptide amphiphiles which may form useful and functional nanostructures. This review covers self-assembly of amphiphilic peptides and peptide mimetic materials, as well as their potential applications.

Insight into the polymerization mechanism of photoinduced step transfer-addition & radical-termination (START) polymerizations

The intrinsic polymerization mechanism of photoinduced step transfer-addition & radical-termination (START) has been revealed based on the successful construction of a catalytic system (Ru(bpy)₃Cl₂/RA) and a solvent system (DMC/MeCN).