Deriving Novel Quaternary Ammonium Compound Disinfectant Scaffolds from a Natural Product: Mechanistic Insights of the Quaternization of Ianthelliformisamine C

Abstract

In the search for novel quaternary ammonium compound (QAC) disinfectants that can evade bacterial resistance, we turned to natural products as a source of inspiration. Herein we used natural product ianthelliformisamine C as a scaffold to design a small library of QACs. We first synthesized ianthelliformisamine C via an amide coupling that allowed for facile purification without the need for protecting groups. We then alkylated and quaternized the internal amines to yield four novel QACs, but all but one demonstrated no antibacterial activity against the tested strains. Using a combination of membrane depolarization and permeabilization assays, we were able to demonstrate that ianthelliformisamine C and the active QAC analog enact cell death via membrane permeabilization, contrary to prior reports on ianthelliformisamine C's mechanism of action.

Keywords: P. aeruginosa; antibiotics; mechanism of action; natural products; quaternary ammonium compounds.

Figure 1.

Structures of ianthelliformisamine C and pulmonarin B and the general synthetic strategy to access the desired QAC analogs.

Figure 2.

Membrane integrity assays. DMSO was used as a negative control and BAC was used as a positive control. (A) Membrane depolarization in S. aureus monitored with DiSC(5) fluorescence. (B) Membrane permeabilization in S. aureus monitored with propidium iodide fluorescence. (C) Outer membrane permeabilization in P. aeruginosa monitored with nitrocefin hydrolysis absorbance, using polymyxin B (PMB) as an additional positive control. (D) Inner membrane permeability of P. aeruginosa monitored with propidium iodide fluorescence.

Figure 3.

Structures of the natural product 1 and analog synthesized by Pieri and coworkers used for MoA studies (15).

Figure 4.

Nitrocefin hydrolysis monitored over time as a proxy for outer membrane permeabilization at 2x the MIC of each compound.

Scheme 1.

Total synthesis of ianthelliformisamine C and alkyl and QAC analogs.