Beyond Amphiphilic Balance: Changing Subunit Stereochemistry Alters the Pore-Forming Activity of Nylon-3 Polymers.

Lei Liu,Leslie A Rank,Kevin C Courtney,Bernard Weisblum,Edwin R Chapman,Sean W Huth,Samuel H Gellman

doi:10.1021/jacs.0c12731

Abstract

Amphiphilic nylon-3 polymers have been reported to mimic the biological activities of natural antimicrobial peptides, with high potency against bacteria and minimal toxicity toward eukaryotic cells. Amphiphilic balance, determined by the proportions of hydrophilic and lipophilic subunits, is considered one of the most important features for achieving this activity profile for nylon-3 polymers and many other antimicrobial polymers. Insufficient hydrophobicity often correlates with weak activities against bacteria, whereas excessive hydrophobicity correlates with high toxicity toward eukaryotic cells. To ask whether factors beyond amphiphilic balance influence polymer activities, we synthesized and evaluated new nylon-3 polymers with two stereoisomeric subunits, each bearing an ethyl side chain and an aminomethyl side chain. Subunits that differ only in stereochemistry are predicted to contribute equally to amphiphilic balance, but we observed that the stereochemical difference correlates with significant changes in biological activity profile. Antibacterial activities were not strongly affected by subunit stereochemistry, but the ability to disrupt eukaryotic cell membranes varied considerably. Experiments with planar lipid bilayers and synthetic liposomes suggested that eukaryotic membrane disruption results from polymer-mediated formation of large pores. Collectively, our results suggest that factors other than amphiphilic balance influence the membrane activity profile of synthetic polymers. Subunits that differ in stereochemistry are likely to have distinct conformational propensities, which could potentially lead to differences in the average shapes of polymer chains, even when the subunits are heterochiral. These findings highlight a dimension of polymer design that should be considered more broadly in efforts to improve specificity and efficacy of antimicrobial polymers.

Highlights

Eukaryotes deploy a wide array of antimicrobial peptides (AMPs) to control or prevent the growth of prokaryotes.[1]Collectively, these AMPs manifest diverse conformations, including helix and sheet secondary structures, and some adopt discrete tertiary structures
The data presented here show that altering the relative configuration of the ethyl and aminomethyl side chains within an ME-cis or ME-trans subunit exerts a substantial effect on the biological activities of nylon-3 polymers
Because the diastereomeric ME subunits presumably make similar or identical contributions to the polymer amphiphilic balance, our findings show that factors beyond amphiphilic balance can play a significant role in determining the biological activity profiles of synthetic polymers

Summary

Introduction

Eukaryotes deploy a wide array of antimicrobial peptides (AMPs) to control or prevent the growth of prokaryotes.[1]. These AMPs manifest diverse conformations, including helix and sheet secondary structures, and some adopt discrete tertiary structures (usually enforced by internal disulfides). Many AMPs, do not appear to have a strong conformational preference.[2,3] The heterogeneity in composition, sequence, and shape among AMPs has led many research groups to consider synthetic polymers as possible alternatives.[4−14] These efforts have been motivated by the practical consideration that polymers are generally less costly to synthesize relative to sequence-specific peptides. Most AMPs appear to compromise bacterial membrane integrity, a property that could lead directly to antibacterial effects and/or enable AMPs to reach

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Conclusion