Peptoid Mimics of Helical, Cationic Antimicrobial Peptides: Studies of Structure-Activity Relationships, Synergy, and Mechanism

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The growing worldwide threat of antibiotic-resistant pathogens has necessitated a constant search for new classes of antibiotics. Antimicrobial peptides (AMPs) are integral components of innate immunity in virtually every living organism, and, due to their proven efficacy over millions of years of evolution, are considered promising leads for new antibiotic therapies. However, most peptides are rapidly degraded by proteases; thus, they typically have very short therapeutic life spans, making them problematic for use as pharmaceuticals. This can be circumvented using non-natural peptidomimetics like poly-<em>N</em>-substituted glycines (peptoids), which are chemically similar to peptides, but are protease-resistant. Previously, members of our laboratory created antimicrobial peptoids ("ampetoids") that mimic the polycationic and amphipathic structures of AMPs and possess antimicrobial activities and selectivities comparable to AMPs. However, mimicry of AMP structure and function does not guarantee that these peptoids also mimic mechanism. The specifics of how AMPs kill bacteria are still under active study. Most are thought to disrupt the plasma membrane, causing leakage and depolarization, though others are known to target intracellular processes without disrupting membranes. Here, we report that the structure-activity relationships and synergistic interactions of ampetoids are highly similar to those of AMPs, suggesting that these two classes of oligomers employ analogous bacterial killing mechanisms. We also explored ampetoid-lipid interactions using biophysical techniques and scanning electron microscopy. These data reveal that membrane permeabilization does not correlate with activity, and suggest that membrane interactions play only a partial role in the mechanisms of these molecules--even for AMPs classified as membrane-disruptive. Lastly, transmission electron micrographs reveal that ampetoids and AMPs alike cause dramatic alterations to bacterial cytoplasm, correlated with death. Clear interactions with ribosomes and DNA lead us to hypothesize that AMPs and ampetoids kill bacteria through an "intracellular biomolecular phase transition" mechanism, in which cytoplasmic polyanions are crosslinked and aggregated to the point at which cellular function is irreparably disrupted. That is, these oligomers do not so much interfere with a particular cellular process as they damage membranes and cytoplasm non-specifically. Since we tested several peptides alongside peptoids, this work yields new insights into the mechanisms of AMPs and ampetoids alike.

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  • 09/13/2018
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