Structure and pH Dependence of Membranolytic Mechanisms by Truncated Oxidized Phospholipids

Membrane lipid oxidation is a universal process that occurs in situations of oxidative stress and is encountered in numerous physiological and pathological situations. Oxidized truncated phospholipids make up a large part of the oxidation products and alter the membrane properties in a way that can lead to cell death. However, the underlying mechanisms are not well understood nor is it clear whether environmental factors, such as pH, can modulate these effects. Using model membranes, we investigate how individual lipid aldehydes and carboxylic acids with truncated acyl chains alter the membrane structure. Our data shows that lipid aldehydes and carboxylic acids have different permeabilization efficiencies towards molecules of varying charge and size and that ΔC9 truncated lipids are usually more efficient in permeabilizing membranes than ΔC5. In terms of physical mechanisms, the ΔC9 truncated lipid carboxylic acid induces permeabilization and membrane curvature in a pH-dependent fashion. This is explained by ionization-dependent exposure of the carboxyl group to the water-bilayer interface, which increases the intrinsic molecular curvature of the oxidized lipid. Conversely, ΔC9 truncated lipid aldehydes and nonionized carboxyls do not induce curved structures but are more efficient in increasing permeability toward larger molecules. We further show that truncated lipids can escape the bilayer and accumulate at interfaces, implying that they might act on neighboring cells. This study indicates that oxidized phospholipids with truncated acyl chains disrupt membrane structure, depending on their specific molecular structure and the pH of the environment, opening a possible route for the design of lipid nanoparticles with pH-dependent drug release.