Structure-activity relationships among random nylon-3 copolymers that mimic antibacterial host-defense peptides

A critical evaluation of random copolymer mimesis of homogeneous antimicrobial peptides

Polymeric synthetic mimics of antimicrobial peptides (SMAMPs) have recently demonstrated similar antimicrobial activity as natural antimicrobial peptides (AMPs) from innate immunity. This is surprising, since polymeric SMAMPs are heterogeneous in terms of chemical structure (random sequence) and conformation (random coil), in contrast to defined amino acid sequence and intrinsic secondary structure. To understand this better, we compare AMPs with a 'minimal' mimic, a well characterized family of polydisperse cationic methacrylate-based random copolymer SMAMPs. Specifically, we focus on a comparison between the quantifiable membrane curvature generating capacity, charge density, and hydrophobicity of the polymeric SMAMPs and AMPs. Synchrotron small angle x-ray scattering (SAXS) results indicate that typical AMPs and these methacrylate SMAMPs generate similar amounts of membrane negative Gaussian curvature (NGC), which is topologically necessary for a variety of membrane-destabilizing processes. Moreover, the curvature generating ability of SMAMPs is more tolerant of changes in the lipid composition than that of natural AMPs with similar chemical groups, consistent with the lower specificity of SMAMPs. We find that, although the amount of NGC generated by these SMAMPs and AMPs are similar, the SMAMPs require significantly higher levels of hydrophobicity and cationic charge to achieve the same level of membrane deformation. We propose an explanation for these differences, which has implications for new synthetic strategies aimed at improved mimesis of AMPs.

Poly-amido-saccharides: synthesis via anionic polymerization of a β-lactam sugar monomer

Enantiopure poly-amido-saccharides (PASs) with a defined molecular weight and narrow dispersity are synthesized using an anionic ring-opening polymerization of a β-lactam sugar monomer. The PASs have a previously unreported main chain structure that is composed of pyranose rings linked through the 1- and 2-positions by an amide with α-stereochemistry. The monomer is synthesized in one-step from benzyl-protected D-glucal and polymerized using mild reaction conditions to give degrees of polymerization ranging from 25 to >120 in high yield. Computational modeling reveals how the monomer's structure and steric bulk affect the thermodynamics and kinetics of polymerization. Protected and deprotected polymers and model compounds are characterized using a variety of methods (NMR, GPC, IR, DLS, etc.). On the basis of circular dichroism, the deprotected polymer possesses a regular secondary structure in aqueous solution, which agrees favorably with the prediction of a helical structure using molecular modeling. Furthermore, we provide evidence suggesting that the polymers bind the lectin concanavalin A at the same site as natural carbohydrates, showing the potential of these polymers to mimic natural polysaccharides. PASs offer the advantages associated with synthetic polymers, such as greater control over structure and derivitization. At the same time, they preserve many of the structural features of natural polysaccharides, such as a stereochemically regular, rigid pyranose backbone, that make natural carbohydrate polymers important materials both for their unique properties and useful applications.

Functionally Diverse Nylon-3 Copolymers from Readily Accessible β-Lactams

A new family of β-lactams is described that enables anionic ring-opening polymerization (AROP) to prepare nylon-3 materials bearing diverse appended functionality, including carboxylic acid, thiol, hydroxyl and secondary amine groups. Nylon-3 copolymers generated with the new β-lactams are shown to display distinctive self-assembly behavior and biological properties.

Cationic spacer arm design strategy for control of antimicrobial activity and conformation of amphiphilic methacrylate random copolymers

Antimicrobial and hemolytic activities of amphiphilic random copolymers were modulated by the structure of the cationic side chain spacer arms, including 2-aminoethylene, 4-aminobutylene, and 6-aminohexylene groups. Cationic amphiphilic random copolymers with ethyl methacrylate (EMA) comonomer were prepared with a range of comonomer fractions, and the library of copolymers was screened for antimicrobial and hemolytic activities. Copolymers with 4-aminobutylene cationic side chains showed an order of magnitude enhancement in their antimicrobial activity relative to those with 2-aminoethylene spacer arms, without causing adverse hemolysis. When the spacer arms were further elongated to hexylene, the copolymers displayed potent antimicrobial and hemolytic activities. The 4-aminobutylene side chain appears to be the optimal spacer arm length for maximal antimicrobial potency and minimal hemolysis, when combined with hydrophobic ethylmethacrylate in a roughly 70/30 ratio. The copolymers displayed relatively rapid bactericidal kinetics and broad-spectrum activity against a panel of Gram-positive and Gram-negative bacteria. The effect of the spacer arms on the polymer conformation in the membrane-bound state was investigated by molecular dynamics simulations. The polymer backbones adopt an extended chain conformation, parallel to the membrane surface. A facially amphiphilic conformation at the membrane surface was observed, with the primary ammonium groups localized at the lipid phoshophate region and the nonpolar side chains of EMA comonomers buried in the hydrophobic membrane environment. This study demonstrates that the antimicrobial activity and molecular conformation of amphiphilic methacrylate random copolymers can be modulated by adjustment of cationic side chain spacer arms.

Design and synthesis of self-degradable antibacterial polymers by simultaneous chain- and step-growth radical copolymerization

Self-degradable antimicrobial copolymers bearing cationic side chains and main-chain ester linkages were synthesized using the simultaneous chain- and step-growth radical polymerization of t-butyl acrylate and 3-butenyl 2-chloropropionate, followed by the transformation of t-butyl groups into primary ammonium salts. We prepared a series of copolymers with different structural features in terms of molecular weight, monomer composition, amine functionality, and side chain structures to examine the effect of polymer properties on their antimicrobial and hemolytic activities. The acrylate copolymers containing primary amine side chains displayed moderate antimicrobial activity against E. coli but were relatively hemolytic. The acrylate copolymer with quaternary ammonium groups and the acrylamide copolymers showed low or no antimicrobial and hemolytic activities. An acrylate copolymer with primary amine side chains degraded to lower molecular weight oligomers with lower antimicrobial activity in aqueous solution. This degradation was due to amidation of the ester groups of the polymer chains by the nucleophilic addition of primary amine groups in the side chains resulting in cleavage of the polymer main chain. The degradation mechanism was studied in detail by model reactions between amine compounds and precursor copolymers.

Polymer chain length effects on fibroblast attachment on nylon-3-modified surfaces

Nylon-3 polymers have a polyamide backbone reminiscent of that found in proteins (β- vs α-amino acid residues, respectively), which makes these materials interesting for biological applications. Because of the versatility of the ring-opening polymerization process and the variety of β-lactam starting materials available, the structure of nylon-3 copolymers is highly amenable to alteration. A previous study showed that relatively subtle changes in the structure or ratio of hydrophobic and cationic subunits that comprise these polymers can result in significant changes in the ability of nylon-3-bearing surfaces to support cell adhesion and spreading. In the present study, we have exploited the highly tailorable nature of these polymers to synthesize new versions possessing a wide range of chain lengths, with the intent of optimizing these materials for use as cell-supportive substrates. We find that longer nylon-3 chains lead to better fibroblast attachment on modified surfaces and that at the optimal chain lengths less hydrophobic subunits are superior. The best polymers we identified are comparable to an RGD-containing peptide in supporting fibroblast attachment. The results described here will help to focus future efforts aimed at refining nylon-3 copolymer substrates for specific tissue engineering applications.

From a marine neuropeptide to antimicrobial pseudopeptides containing aza-β(3)-amino acids: structure and activity

Incorporation of aza-β(3)-amino acids into an endogenous neuropeptide from mollusks (ALSGDAFLRF-NH(2)) with weak antimicrobial activity allows the design of new AMPs sequences. Depending on the nature of the substitution, this can render the pseudopeptides inactive or lead to a drastic enhancement of the antimicrobial activity without high cytotoxicity. Structural studies of the pseudopeptides carried out by NMR and circular dichroism show the impact of aza-β(3)-amino acids on peptide structure. The first three-dimensional structures of pseudopeptides containing aza-β(3)-amino acids in aqueous micellar SDS were determined and demonstrate that the hydrazino turn can be formed in aqueous solution. Thus, AMP activity can be modulated through structural modifications induced by the nature and the position of such amino acid analogues in the peptide sequences.

C-terminal functionalization of nylon-3 polymers: effects of C-terminal groups on antibacterial and hemolytic activities

Nylon-3 polymers contain β-amino-acid-derived subunits and can be viewed as higher homologues of poly(α-amino acids). This structural relationship raises the possibility that nylon-3 polymers offer a platform for development of new materials with a variety of biological activities, a prospect that has recently begun to receive experimental support. Nylon-3 homo- and copolymers can be prepared via anionic ring-opening polymerization of β-lactams, and use of an N-acyl-β-lactam as coinitiator in the polymerization reaction allows placement of a specific functional group, borne by the N-acyl-β-lactam, at the N-terminus of each polymer chain. Controlling the unit at the C-termini of nylon-3 polymer chains, however, has been problematic. Here we describe a strategy for specifying C-terminal functionality that is based on the polymerization mechanism. After the anionic ring-opening polymerization is complete, we introduce a new β-lactam, approximately 1 equiv relative to the expected number of polymer chains. Because the polymer chains bear a reactive imide group at their C-termini, this new β-lactam should become attached at this position. If the terminating β-lactam bears a distinctive functional group, that functionality should be affixed to most or all C-termini in the reaction mixture. We use the new technique to compare the impact of N- and C-terminal placement of a critical hydrophobic fragment on the biological activity profile of nylon-3 copolymers. The synthetic advance described here should prove to be generally useful for tailoring the properties of nylon-3 materials.

Non-hemolytic α-AApeptides as antimicrobial peptidomimetics

We report a new class of peptide mimetics, α-AApeptides, that display broad-spectrum activity against both Gram-negative and Gram-positive bacteria and fungi. With non-hemolytic activity, resistance to protease hydrolysis, and easy sequence programmability, α-AApeptides may emerge as a novel class of antibiotics.

Beyond natural antimicrobial peptides: multimeric peptides and other peptidomimetic approaches

Naturally occurring antimicrobial peptides (AMPs) present several drawbacks that strongly limit their development into therapeutically valuable antibiotics. These include susceptibility to protease degradation and high costs of manufacture. To overcome these problems, researchers have tried to develop mimics or peptidomimetics endowed with better properties, while retaining the basic features of membrane-active natural AMPs such as cationic charge and amphipathic design. Protein epitope mimetics, multimeric (dendrimeric) peptides, oligoacyllysines, ceragenins, synthetic lipidated peptides, peptoids and other foldamers are some of the routes explored so far. The synthetic approach has led to compounds that have already entered clinical evaluation for the treatment of specific conditions, such as Staphylococcus (MRSA) infections. Should these trials be successful, an important proof-of-concept would be established, showing that synthetic oligomers rather than naturally occurring molecules could bring peptide-based antibiotics to clinical practice and the drug market for local and systemic treatment of medical conditions associated with multi-drug resistant pathogens.

Antibacterial studies of cationic polymers with alternating, random, and uniform backbones

Antibacterial polymers have potential as pharmaceuticals and as coatings for implantation devices. The design of these materials will be optimized when we have a complete understanding of the structural features that impart activity toward target organisms and those that are benign with respect to the mammalian host. In this work, four series of polymers in which cationic and hydrophobic groups were distributed along the backbone were tested against six different bacterial species (both Gram-positive and Gram-negative) and for host cytotoxicities (red blood cell lysis). The most effective of the polymers studied are regularly spaced, featuring a 6-8 carbon stretch along the backbone between side chains that present positively charged groups. They cause potassium efflux, disorder the bacterial cytoplasmic membrane, and disrupt the membrane potential. These polymers, available from alternating ring-opening metathesis polymerization (AROMP), offer proof of principle for the importance of regular spacing in antibacterial polymers and for the synthesis of additional functional materials based on regularly spaced scaffolds.

Recent synthetic transport systems

This critical review covers progress with synthetic transport systems, particularly ion channels and pores, between January 2006 and December 2009 in a comprehensive manner. This is the third part of a series launched in the year 2000, covering a rich collection of structural and functional motifs that should appeal to a broad audience of non-specialists, including to organic, biological, supramolecular and polymer chemists. Impressive breakthroughs have been achieved over the past four years in part because of a fruitful expansion toward new types of interactions, including metal-organic, π-π, aromatic electron donor-acceptor, anion-π or anion-macrodipole interactions as well as dynamic covalent bonds (169 references).

Role of cationic group structure in membrane binding and disruption by amphiphilic copolymers

Cationic, amphiphilic polymers are currently being used as antimicrobial agents that disrupt biomembranes, although their mechanisms remain poorly understood. Herein, membrane association and disruption by amphiphilic polymers bearing primary, tertiary, or quaternary ammonium salt groups reveal the role of cationic group structure in the polymer-membrane interaction. The dissociation constants of polymers to liposomes of POPC were obtained by a fluorometric assay, exploiting the environmental sensitivity of dansyl moieties in the polymer end groups. Dye leakage from liposomes and solid-state NMR provided further insights into the polymer-induced membrane disruption. Interestingly, the polymers with primary amine groups induced reorganization of the bilayer structure to align lipid headgroups perpendicular to the membrane. The results showed that polymers bearing primary amines exceed the tertiary and quaternary ammonium counterparts in membrane binding and disrupting abilities. This is likely due to enhanced complexation of primary amines to the phosphate groups in the lipids, through a combination of hydrogen bonding and electrostatic interactions.

Building β-peptide H10/12 foldamer helices with six-membered cyclic side-chains: fine-tuning of folding and self-assembly

The ability of the β-peptidic H10/12 helix to tolerate side-chains containing six-membered alicyclic rings was studied. cis-2-Aminocyclohex-3-ene carboxylic acid (cis-ACHEC) residues afforded H10/12 helix formation with alternating backbone configuration. Conformational polymorphism was observed for the alternating cis-ACHC hexamer, where chemical exchange takes place between the major left-handed H10/12 helix and a minor folded conformation. The hydrophobically driven self-assembly was achieved for the cis-ACHC-containing helix which was observed as vesicles ~100 nm in diameter.

Structural determinants of antimicrobial activity in polymers which mimic host defense peptides

Antimicrobial polymers, designed to mimic the salient structural features of host defense peptides, are an emerging class of materials with potential for applications to combat infectious disease. Because the putative mode of action relies on physiochemical parameters of peptides such as hydrophobicity and cationic charge, rather than specific receptor-mediated interactions, the activity of the polymers can be modulated by tuning key structural parameters. While a wide diversity of chemical structures have been reported as antimicrobial polymers, a precise understanding of the structural factors which control their activity is a subject of current investigations. In this mini-review, we will outline the design principles that have been developed so far to fine tune the activity of these antimicrobial agents. The roles played by specific structural features such as cationic charge, hydrophobicity, and molecular weight will be discussed. Future directions of the field and potential challenges will be proposed.

Biophysical mimicry of lung surfactant protein B by random nylon-3 copolymers

Non-natural oligomers have recently shown promise as functional analogues of lung surfactant proteins B and C (SP-B and SP-C), two helical and amphiphilic proteins that are critical for normal respiration. The generation of non-natural mimics of SP-B and SP-C has previously been restricted to step-by-step, sequence-specific synthesis, which results in discrete oligomers that are intended to manifest specific structural attributes. Here we present an alternative approach to SP-B mimicry that is based on sequence-random copolymers containing cationic and lipophilic subunits. These materials, members of the nylon-3 family, are prepared by ring-opening polymerization of beta-lactams. The best of the nylon-3 polymers display promising in vitro surfactant activities in a mixed lipid film. Pulsating bubble surfactometry data indicate that films containing the most surface-active polymers attain adsorptive and dynamic-cycling properties that surpass those of discrete peptides intended to mimic SP-B. Attachment of an N-terminal octadecanoyl unit to the nylon-3 copolymers, inspired by the post-translational modifications found in SP-C, affords further improvements by reducing the percent surface area compression to reach low minimum surface tension. Cytotoxic effects of the copolymers are diminished relative to that of an SP-B-derived peptide and a peptoid-based mimic. The current study provides evidence that sequence-random copolymers can mimic the in vitro surface-active behavior of lung surfactant proteins in a mixed lipid film. These findings raise the possibility that random copolymers might be useful for developing a lung surfactant replacement, which is an attractive prospect given that such polymers are easier to prepare than are sequence-specific oligomers.

Experimental conditions that enhance potency of an antibacterial oligo-acyl-lysyl

Oligo-acyl-lysyls (OAKs) are synthetic mimics of host defense peptides known to exert antibacterial activity both in cultures and in animal models of disease. Here, we investigated how environmental conditions (temperature, pH, and ionic strength) affect the antibacterial properties of an octamer derivative, C(12)K-7alpha(8). Data obtained with representative bacteria, including the Gram-negative bacterium Escherichia coli and the Gram-positive bacteria Listeria monocytogenes and Staphylococcus aureus, showed that OAK's potency was proportionally affected by pH changes and subsided essentially throughout a wide range of salt concentrations and temperature values, whereas antistaphyloccocal activity was relatively more vulnerable. It was rather the mode of action that was most susceptible to the environmental changes. Thus, OAK's bactericidal effect was limited to a growth-inhibitory effect under acidic pH, low temperatures, or high salt concentrations, whereas basic pH or high temperatures have enhanced the bactericidal kinetics. Properties of binding to model phospholipid membranes provided evidence that correlated the differential modes of action with variable binding affinities. Interestingly, combination of the optimal incubation conditions resulted in a remarkable increase in potency, as expressed by a 16- to 32-fold reduction in the MIC value and by much faster bactericidal rates (>99% death induced within minutes versus hours) compared with the standard incubation conditions. Collectively, the data suggest that OAKs might be useful in developing design strategies for robust antimicrobial peptides that are able to affect a pathogen's viability under a large spectrum of incubation conditions.