The authors analyzed extensively the dynamics of polymer chains in solutions simulated with dissipative particle dynamics (DPD), with a special focus on the potential influence of a low Schmidt number of a typical DPD fluid on the simulated polymer dynamics. It has been argued that a low Schmidt number in a DPD fluid can lead to underdevelopment of the hydrodynamic interaction in polymer solutions. The authors’ analyses reveal that equilibrium polymer dynamics in dilute solution, under typical DPD simulation conditions, obey the Zimm [J. Chem. Phys. 24, 269 (1956)] model very well. With a further reduction in the Schmidt number, a deviation from the Zimm model to the Rouse model is observed. This implies that the hydrodynamic interaction between monomers is reasonably developed under typical conditions of a DPD simulation. Only when the Schmidt number is further reduced, the hydrodynamic interaction within the chains becomes underdeveloped. The screening of the hydrodynamic interaction and the excluded volume interaction as the polymer volume fraction is increased are well reproduced by the DPD simulations. The use of soft interaction between polymer beads and a low Schmidt number do not produce noticeable problems for the simulated dynamics at high concentrations, except for the entanglement effect which is not captured in the simulations.

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 ∼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.