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.
A library of amphiphilic random copolymers containing cationic and hydrophobic side chains were prepared by copolymerization of amine-functionalized methacrylate monomers with various ratios of an ...alkyl methacrylate. Primary or tertiary amine groups, or quaternary ammonium groups, were utilized as the source of cationic charge in each copolymer series. The antimicrobial and hemolytic activities of these copolymers are reported, enabling a systematic assessment of the effect different amine groups exert on the biological activity of the polymers. It was shown that the copolymer composition of amphiphilic copolymers containing primary or tertiary amine groups can be tuned to achieve potent antimicrobial activity while minimizing red blood cell lysis. On the other hand, the copolymers containing quaternary ammonium groups required a greater amount of hydrophobic comonomer to express activity and showed generally lower selectivity for E. coli versus human red blood cells. Potentiometric titration data revealed the fraction of the primary or tertiary amine groups in the polymers, which are deprotonated (basic) at physiological pH. Measurements of the bactericidal and hemolytic activities in buffers of pH varying from 6 to 8 showed the impact of polymer ionization on biological activity. A decrease in the fraction of amine groups that are cationic, from α = 1.0 to 0.7, caused an enhancement of antimicrobial and hemolytic activity. As this value was decreased further to α = 0.5, loss of activity was observed. The activities of polymers containing quaternary ammonium groups were shown to be pH-independent.
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.
Nickel-catalyzed chain-growth copolymerizations of thiophene and selenophene derivatives afforded well-defined π-conjugated copolymers with narrow molecular weight distributions, defined end-groups, ...and specific sequences. In particular, a π-conjugated copolymer with a novel linear gradient sequence was prepared. Compared to the analogous block and random copolymers, the gradient copolymer displayed unique optical and thermal properties as well as thin-film morphology. Moreover, the gradient copolymer exhibited an intermediate extent of phase separation into thiophene-rich and selenophene-rich domains compared to the block and random copolymers. Because this gradient sequence provides access to new solid-state properties, these materials should be further explored in applications where phase-separated morphology is important (e.g., solar cells, transistors, and light-emitting diodes).
Low molecular weight random copolymers bearing protonated primary amine groups and hydrophobic alkyl groups in the side chains were synthesized and their activities against E. coli, S. aureus, human ...red blood cells, and human epithelial carcinoma cells (HEp-2) were quantified. The mole fraction of alkyl side chains in the copolymers (f alkyl) and the length of the alkyl chains were major determinants of the activities. Against E. coli cells, activity was diminished as f alkyl was increased from 0 to about 0.2, but was then enhanced dramatically as f alkyl was increased further. Activity against S. aureus was diminished continually with increasing f alkyl. The cytotoxicity to human epithelial carcinoma cells also decreased with increasing f alkyl. Conversely, hemolytic activity showed monotonic enhancement with increasing f alkyl. The cationic homopolymer (f alkyl = 0) completely inhibited S. aureus growth at 3 μM (10.2 μg/mL) and completely inhibited metabolic activity in HEp-2 cells at 10 μM (34 μg/mL), although it did not induce any detectable hemolysis up to 645 μM (2000 μg/mL). Polymer-induced dye leakage from liposomes provided a biophysical basis for understanding the factors which modulate the polymer−membrane interactions. Disruption of Zwitterionic POPC vesicles induced by the copolymers was enhanced as f alkyl increased, following trends similar to the hemolytic activity data. The ability of the polymers to permeabilize vesicles of POPE/POPG and DOPG/Lysyl-DOPG/CL displayed trends similar to trends in their activities against E. coli and S. aureus, respectively. This was interpreted as evidence that the antimicrobial mechanism employed by the polymers involves disruption of bacterial cell membranes. An investigation of leakage kinetics revealed that the cationic homopolymer induced a gradual release of contents from POPE/POPG and DOPG/Lysyl-DOPG/CL vesicles, while the more hydrophobic copolymers induced rapid dye efflux. The results are interpreted as evidence that the cationic homopolymer and hydrophobic copolymers in this study exert their antimicrobial action by fundamentally different mechanisms of membrane disruption.
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.
Cationic and amphiphilic polymers are known to exert broad-spectrum antibacterial activity by a putative mechanism of membrane disruption. Typically, nonspecific binding to hydrophobic components of ...the complex biological milieu, such as globular proteins, is considered a deterrent to the successful application of such polymers. To evaluate the extent to which serum deactivates antibacterial polymethacrylates, we compared their minimum inhibitory concentrations in the presence and absence of fetal bovine serum. Surprisingly, we discovered that the addition of fetal bovine serum (FBS) to the assay media in fact enhances the antimicrobial activity of polymers against Gram-positive bacteria S. aureus, whereas the opposite is the case for Gram-negative E. coli. Here, we present these unexpected trends and develop a hypothesis to potentially explain this unusual phenomenon.
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.
PDF
