Effect of the Protonation Level and Ionic Strength on the Structure of Linear Polyethyleneimine

Polyethylenimine (PEI) is a highly studied vector for nonviral gene delivery, with a high transfection efficiency that has been linked with its pH responsiveness. Atomistic molecular dynamics simulations of a linear 40 mer PEI chain were performed for nine protonation states and various NaCl concentrations to examine how the structure of PEI depends on pH and salt concentration. PEI continuously expands as it transitions from being unprotonated to fully protonated; however, we observe that two different regimes underlie this expansion. Sparsely protonated chains behave as weakly charged polyelectrolytes whose expansion is associated with the reduction of intrachain hydrophobic interactions. In contrast, the expansion of densely protonated chains with increased protonation involves increasing chain stiffness and breaking intrachain hydrogen bonds. The weakly to highly charged transition occurred at similar to 40% protonation, suggesting it may occur in endosomal conditions. These results provide a microscopic picture of changes in PEI structure during the gene delivery process.