# Optimizing Intracellular Transport of Antimicrobial Metallohelices Delivers Selective Nanomolar Potency in E. coli

## TL;DR

Researchers optimized antimicrobial metallohelices to achieve selective and potent activity against E. coli at nanomolar concentrations.

## Contribution

A highly potent and selective antimicrobial compound was discovered in an inactive metallohelix series.

## Key findings

- A potent compound with ~500 nM activity emerged from the previously inactive 'triplex' series.
- Antimicrobial activity is strongly linked to passive diffusion into bacterial cells.
- The most active enantiomer inhibits E. coli growth at 250 nM, while its mirror image is inactive.

## Abstract

To investigate large discrepancies in antimicrobial potency
between
cationic amphipathic metallohelix architectures, 22 new optically
pure candidates were synthesized via self-assembly. A total of 34
compounds were tested against S. aureus, E. coli, and, for the most active,
against a panel of ESKAPE pathogens. While addition of substituents
reduced activity in a 3-fold symmetric “flexicate” series,
a potent compound (∼500 nM) with promising selectivity against
a challenging E. coli microbe emerged
in the hitherto inactive “triplex” series. This and
other key compounds were studied by using techniques focused on transport
and localization in Gram-positive and Gram-negative bacteria. Zeta-potential
measurements at model membranes revealed affinities that mirror the
antimicrobial activity. Extensive temperature- and concentration-dependent
intracellular accumulation studies via isotopic labeling revealed
that antimicrobial activity (within each architecture) is strongly
dependent on the ability to enter the cell via passive diffusion.
Mechanistic differences across metallohelix classes are confirmed
by checkerboard activity assays and confocal microscopy studies via
Click-labeled alkyne derivatives. The most active (and bactericidal)
enantiomer achieves a growth-inhibiting concentration across the microbial
population (apparently not restricted to dividing cells) at ca. 250
nM applied dose. Extraordinarily, given this very high potency, the
mirror image of this compound is essentially inactive.