Divalent metal ion triggered activity of a synthetic antimicrobial in cardiolipin membranes

Abstract

One member of a prototypical class of antimicrobial oligomers was used to study pore formation in cardiolipin-rich membranes. Both vesicle dye-leakage assays and small-angle X-ray scattering were used to study bilayer remodeling. The results indicate that the presence of negative intrinsic curvature lipids is essential for pore formation by this class of molecules: In Gram-positive bacteria, cardiolipin and divalent metal cations like Ca(2+) and Mg(2+) are needed. This is consistent with the role of phosphatidylethanolamine (PE) lipid in Gram-negative bacteria, where antimicrobial activity is dependent on the negative intrinsic curvature of PE rather than a specific interaction with PE.

Figure 1

AMO 2 (2.5 µg/mL for A, B and D, 0.25 µg/mL for C) induced calcein dye leakage from (A, B) 100% CL vesicles, (C) 20/80 PG/PE vesicles, (D) 50/50 PG/CL vesicles with 0 mM, 0.02 mM, 0.2 mM, 1.0 mM, 2.0 mM, 4.0 mM, 8.0 mM, and 10.0 mM (A, D) Mg²⁺, and (B, C) Ca²⁺. Final lipid concentration was 5.0 µM for assay.

Figure 2

(A) Synchrotron SAXS data show that, at sufficiently high [Mg²⁺] (between 1–4 mM), Mg²⁺ ions can induce SUVs composed of CL to form an inverted hexagonal structure. (B) Synchrotron SAXS data show that AMO 2 induces SUVs composed of DOPG/CL = 32/68 into a hexagonal structure at reduced concentrations of divalent Mg²⁺ions.