Thermodynamics of charged nanoparticle adsorption on charge-neutral membranes: a simulation study

The interactions between charged nanoparticles (NPs) and charge-neutral phospholipid membranes are investigated by coarse-grained molecular dynamics simulations. Three kinds of nanoparticles are modeled with different surface charge densities: the uncharged one, the positively charged one, and the negatively charged one. We find that the electrostatic attraction improves the adhesion of a charged nanoparticle to the membrane. With the increase of electrostatic energy, a charged NP can be almost fully wrapped by the membrane. In addition, analyses of structural variations suggest that the adhesion of a charged NP induces a local transition in fluid bilayers. Some thermodynamic quantities such as free energy, entropy, and enthalpy are also obtained to explain the process of NPs binding. Furthermore, the bending energy of wrapping of NPs against the electrostatic potential energy is also discussed based on the Helfrich theory, indicating that the driving force of the wrap originates from the gain in electrostatic energy at the cost of the elastic energy of biomembranes. Our observations shed light on the origin of experiments of the wrap as well as the mechanism of structural transitions of membranes due to the electrostatic binding.