Molecular Dynamics Simulation on the Conformational Change of a pH-Switchable Lipid

pH-sensitive lipids are important components of lipid nanoparticles, which enable the targeted delivery and controlled release of drugs. Understanding the mechanism of pH-triggered drug release at the molecular level is important for the rational design of ionizable lipids. Based on a recently reported pH-switchable lipid, named SL2, molecular dynamics (MD) simulations were employed to explore the microscopic mechanism behind the membrane destabilization induced by the conformational change of pH-switchable lipids. The simulated results showed that, at neutral pH, the neutral SL2 lipids assembled with other components (helper lipids and cholesterol) to form a structurally ordered bilayer structure. At this moment, the two hydrocarbon chains of SL2 were closely aligned and inserted in an orderly manner inside of the membrane. With a decrease in pH, the protonation of the pyridinium ring caused a large degree of molecular structural change. The pyridinium ring preferred to form intramolecular H-bonds with the methoxy groups and intermolecular H-bonds with water, resulting in the flip of the pyridinium ring. Meanwhile, due to the structural flip, the two alkane chains showed a more open state, which perturbed the arrangement of molecules within the membrane. The perturbations caused local collapse of the membrane and the formation of water molecule channels, which contributed to the pH-induced drug release. Our results verified the experimentally proposed mechanism at the molecular level and provided more complementary information, which are expected to have deeper insights into the pH-triggered drug release.