Comparison of Facially Amphiphilic versus Segregated Monomers in the Design of Antibacterial Copolymers

Abstract

A direct comparison of two strategies for designing antimicrobial polymers is presented. Previously, we published several reports on the use of facially amphiphilic (FA) monomers which led to polynorbornenes with excellent antimicrobial activities and selectivities. Our polymers obtained by copolymerization of structurally similar segregated monomers, in which cationic and non-polar moieties reside on separate repeat units, led to polymers with less pronounced activities. A wide range of polymer amphiphilicities was surveyed by pairing a cationic oxanor-bornene with eleven different non-polar monomers and varying the comonomer feed ratios. Their properties were tested using antimicrobial assays and copolymers possessing intermediate hydrophobicities were the most active. Polymer-induced leakage of dye-filled liposomes and microscopy of polymer-treated bacteria support a membrane-based mode of action. From these results there appears to be profound differences in how a polymer made from FA monomers interacts with the phospholipid bilayer compared with copolymers from segregated monomers. We conclude that a well-defined spatial relationship of the whole polymer is crucial to obtain synthetic mimics of antimicrobial peptides (SMAMPs): charged and non-polar moieties need to be balanced locally, for example, at the monomer level, and not just globally. We advocate the use of FA monomers for better control of biological properties. It is expected that this principle will be usefully applied to other backbones such as the polyacrylates, polystyrenes, and non-natural polyamides.

Antimicrobial Polymers: Mimicking Amino Acid Functionali ty, Sequence Control and Three-dimensional Structure of Host-defen se Peptides

“…In contrast to AMPs the presence of multiple sizes of polymers can influence the biological performance drastically. This is nicely demonstrated for nylon-3 copolymers where a small change in dispersity (at constant molar mass) shows tremendous increase in haemolysis (Ð = 1.06, HC 50 = 800 µg mL -1 ; Ð = 1.15, HC 50 = 12.5 µg mL -1 ). 27 It was confirmed by dialysis experiments that the higher molecular weight fractions cause the toxicity.…”

“…40 A similar trend was found by Hedrick et al for poly(carbonate)s where an increased spacer length led to lower MIC and HC 50 values. 39 They also found improved properties when combining long and short spacer arm length resulting in polymers with slightly decreased antimicrobial activity (in comparison to a polymer with only long spacers) but a drastically increased HC 50 values. This concept was further investigated by Yang and coworkers, combining a C 6 spacer, primary amine monomer with a C 2 spacer, secondary amine monomer in different ratios to obtain optimal selectivity.…”

“…When these polymers were assessed for their activities against E. coli, the acrylic lysine mimicking degradable copolymers provided the lowest MIC values at 16-104 µg/mL, compared to the peptide mgainin-2, which gave an MIC of 125 µg/mL. Lower molecular weight polymers within this discrete class also proved to be more potent, however, they displayed low HC 50 values, indicating that their hemolytic activity would prevent their development as clinical candidates. Interestingly, a change to the acrylamide back bone resulted in a significant loss of activity, with MIC's observed at > 500 µg/mL, which was attributed to the hydrophilic nature of the acrylamide, preventing the proposed mechanism of insertion of hydrophobic polymer regions into the bacterial cell membrane.…”

“…Further investigation of the distribution of functional groups along the polymer backbone was undertaken by Gabriel et al 50 The study was directed towards the comparison of norbornene polymers obtained by ROMP from facially amphiphilic (FA) monomers, which contain a cationic group opposite a hydrophobic alkyl chain within the monomer unit, to those from mono-functional monomers (Fig. 26C).…”

“…The FA structure might enhance the process of phase separation, thus boosting the disruption of bacterial membranes. 50 The rigidity of the norbornene backbone might emphasize the effect of the facial amphiphilicity of the monomers. A more flexible backbone could allow the polymer chains to adapt in presence of a membrane, thus rendering the initial distribution of the functionalities along the polymer backbone less important.…”