Ions modulate key interactions between pHLIP and lipid membranes

Abstract The pH-Low Insertion Peptide (pHLIP) is used for targeted delivery of drug cargoes to acidic tissues such as tumors. The extracellular acidosis found in solid tumors triggers pHLIP to transition from a membrane-absorbed state to fold into a transmembrane α-helix. Different factors influence the acidity required for pHLIP to insert into lipid membranes. One of them is the lipid headgroup composition, which defines the electrostatic profile of the membrane. However, the molecular interactions that drive the absorption of pHLIP to the bilayer surface are poorly understood. In this study we combine biophysical experiments and all-atom molecular dynamics (MD) simulations to understand the role played by electrostatics in the interaction between pHLIP and a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) bilayer. We observed that the solution ionic strength affects the structure of pHLIP at the membrane surface, as well as the acidity needed for different steps in the membrane insertion process. In particular, our simulations revealed that an increase in ionic strength affected both pHLIP and the bilayer: coordinaton of sodium ions with the C-terminus of pHLIP led to localized changes in helicitiy, while coordination of sodium ions with the phosphate moiety of the PC headgroups had a condensing effect on our model bilayer. These results are relevant to our understanding of environmental influences on the ability of pHLIP to absorb to the cell membrane and are useful in our fundamental understanding of the absorption of pH-responsive peptides and cell-penetrating peptides.