Non-disruptive uptake of anionic and cationic gold nanoparticles in neutral zwitterionic membranes

The potential toxicity of ligand-protected nanoparticles (NPs) on biological targets is crucial for their clinical translation. A number of studies are aimed at investigating the molecular mechanisms shaping the interactions between synthetic NPs and neutral plasma membranes. The role played by the NP surface charge is still widely debated. We compare, via liposome leakage assays, the perturbation induced by the penetration of sub-6 nm anionic and cationic Au NPs into model neutral lipid membranes composed of the zwitterionic 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Our charged Au NPs are functionalized by a mixture of the apolar 1-octanethiol and a ω-charged thiol which is either the anionic 11-mercapto-1-undecanesulfonate or the cationic (11-mercaptoundecyl)-N,N,N-trimethylammonium. In both cases, the NP uptake in the bilayer is confirmed by quartz crystal microbalance investigations. Our leakage assays show that both negatively and positively charged Au NPs do not induce significant membrane damage on POPC liposomes when penetrating into the bilayer. By means of molecular dynamics simulations, we show that the energy barrier for membrane penetration is the same for both NPs. These results suggest that the sign of the NP surface charge, per se, does not imply different physicochemical mechanisms of interaction with zwitterionic lipid membranes.