Effect of Ionic and Nonionic Compounds Structure on the Fluidity of Model Lipid Membranes: Computer Simulation and EPR Experiment

This article investigates the influence of dopant molecules on the structural and dynamic properties of lipid bilayers in liposomes, with a focus on the effects of dopant concentration, size, and introduced electric charge. Experimental studies were performed using electron paramagnetic resonance (EPR) spectroscopy with spin probes, complemented by Monte Carlo simulations. Liposomes, formed via lecithin sonication, were doped with compounds of varying concentrations and analyzed using EPR spectroscopy to assess changes in membrane rigidity. Parallel simulations modeled the membrane's surface layer as a system of electric dipoles on a 20 × 20 rectangular matrix. As in the EPR experiments, the simulation explored the effects of dopant molecules differing in size and charge, while gradually increasing their concentrations in the system. Minimum binding energy configurations were determined from the simulations. The results revealed a strong correlation between the EPR data and simulation outcomes, indicating a clear dependence of membrane stiffening on the concentration, size, and charge of dopant molecules. This effect was most pronounced at low dopant concentrations (~1-1.5% for q = 2 and 1.5-2% for q ≥ 3). No significant stiffening was observed for neutral molecules lacking charge. These findings offer valuable insights into the mechanisms of membrane modulation by dopants and provide a quantitative framework for understanding their impact on lipid bilayer properties.