Mechanistic Insights into Lipooligourea-Lipid Membrane Interactions

Abstract

Understanding how synthetic peptidomimetics interact with bacterial membranes is key to developing next-generation antimicrobials. In this study, we investigate the membrane-disruptive behavior of C10-OU4, a cationic lipooligourea foldamer that mimics the amphiphilic architecture of antimicrobial lipopeptides. Using a multitechnique approach─Langmuir monolayer analysis, quartz crystal microbalance with dissipation monitoring (QCM-D), and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR)─we probe the concentration-dependent interactions of C10-OU4 with lipid membranes that model Gram-positive bacterial membranes. At low concentrations (1 μM), C10-OU4 adsorbs to the membrane surface, inducing minor structural perturbations limited to the polar headgroup region. Increasing the concentration to 5 μM results in significant acyl chain disorder, partial membrane solubilization, and likely, micelle-like aggregate formation, as evidenced by QCM-D frequency shifts and ATR-FTIR data. At 10 μM, near the minimal inhibitory concentration, membrane disintegration becomes extensive, with the lipooligourea adopting orientations suggestive of random or tilted insertion geometries. These findings support a multimodal mechanism of action that transitions from surface association to full bilayer disruption in a concentration-dependent manner. The combined use of structural and dynamic measurements provides detailed insight into the physicochemical principles underlying lipooligourea-membrane interactions, offering a foundation for the rational design of membrane-active foldamer antibiotics.

1. Structure of Lipooligourea C10-OU4

1

Surface pressure (Π) vs area per molecule isotherms for DPPG/POPG/CL monolayers recorded in the absence (black curves) and presence (red curve) of lipooligourea C10-OU4 (1 μM) dissolved in the subphase. The subphase consisted of an aqueous 0.01 M PBS solution. The insets depict the changes in the compression modulus as a function of surface pressure.

2

Time-dependent surface pressure changes of a DPPG/POPG/CL monolayer compressed to an initial pressure of 35 mN/m, under constant-area conditions at the air–water interface. The black curve represents the monolayer without any additional treatment, while the red curve corresponds to the monolayer after the injection of C10-OU4 into the subphase. Total concentration of C10-OU4 was 1 μM. The subphase consisted of an aqueous 0.01 M PBS solution.

3

Representative QCM-D data showing changes in frequency (Δf/n) and energy dissipation (ΔD) for quartz crystals coated with a DPPG/POPG/CL lipid bilayer upon exposure to the lipooligourea C10-OU4 at concentrations of 1 (A, B), 5 (C, D), and 10 μM (E, F). All measurements were conducted in 0.01 M phosphate-buffered saline (PBS) at room temperature.

4

C–H stretching region of ATR-FTIR spectra recorded for DPPG/POPG/CL bilayer deposited onto the Si prism before (A) and after ∼60 min of exposure to 1 μM C10-OU4 (B); 5 μM C10-OU4 (C); and 10 μM C10-OU4 (D). The spectra were recorded in 0.01 M PBS dissolved in D₂O. The red line corresponds to the spectra recorded with s-polarized light, while the black line corresponds to the spectra recorded with p-polarized light. A bare prism was used as a reference to assess global changes within hydrocarbon region.

5

ATR-FTIR spectra in amide/urea region recorded for DPPG/POPG/CL bilayer deposited onto the Si prisms after ∼60 min of exposure to 1 (A), 5 (B), and 10 μM (C) C10-OU4. The spectra were recorded in 0.01 M PBS dissolved in D₂O. The red line corresponds to the spectra recorded with s-polarized light, while the black line corresponds to the spectra recorded with p-polarized light. Spectra of intact membrane were used as a reference to assess the effect strictly related to lipooligourea action.