Understanding the Protonation Behavior of Linear Polyethylenimine in Solutions through Monte Carlo Simulations

Abstract

The success of polyethyleneimine (PEI) as a non-viral based gene delivery vector has been attributed to its proton buffering capacity. Despite the great interest in PEI for its use in non-viral based gene delivery, the protonation behavior of PEI in solution is not well understood. Earlier experimental studies have reported inconsistent values of the protonation state of PEI. In this work, we report our investigation of the protonation behavior of a realistic linear PEI (lPEI) with computational approaches. Reported experimental pKa values of several diamine compounds are first examined. A screened coulombic interaction with a distance dependence dielectric is shown to reproduce the shifted pKa values of the model diamine compounds. Then atomistic molecular dynamics simulations of lPEI chain with 20 repeating units are performed and the results are used to provide parameters for a coarse-grained polyamine model. The screened coulombic interaction is then incorporated in the coarse-grained lPEI chain and computational titrations are performed. The obtained computational titration curves of lPEI in solutions were found to be in best agreement with experimental results by Smits et al, but the computational titration curves have too strong of a dependence on salt concentration compared to the experimental results by Smits et al. Disregarding the discrepancy in the salt dependence, our computational titrations reveal that approximately 55% of the lPEI amine groups are protonated under physiological conditions in solution with a nearly alternating arrangement of protonated and non-protonated amines. Titrations of lPEI in the presence of a polyanion are also performed to determine how the charge state of lPEI could be affected by complexation with DNA in gene therapy preparations. While the presence of the polyanion increases the degree of protonation of the PEI, many of PEI amines remain unprotonated under physiological conditions, providing evidence that PEI complexed with DNA could still have proton buffering capacity. Potential sources of error that have resulted in the inconsistency of previously reported protonation states of PEI were also discussed.

Binding and biomimetic cleavage of the RNA poly(U) by synthetic polyimidazoles

“…The midpoint of the titration of polymer 2 was ca. 7.0, consistent with previous measurements on such polyamines (24). The broad titration range reflects the effects of protonated amino groups on the ease of protonation of additional amino groups.…”

Impact of molecular weight and degree of conjugation on the thermodynamics of DNA complexation and stability of polyethylenimine-graft-poly(ethylene glycol) copolymers

“…As DNA electrostatically binds, this charge repulsion is relieved and the pK a s of nearby amines should rise. A similar 25% increase in the protonation state of linear PEI amines after DNA binding was found by Ziebarth and Wang through Monte Carlo simulations [63]. Our results provide a nice experimental compliment in support of the fact that PEI retains buffering capacity, even in its bound form, which has implications for endosomal escape.…”

Chemistry specificity of DNA–polycation complex salt response: a simulation study of DNA, polylysine and polyethyleneimine

“…35,36 Furthermore, the structure of the polymer, 37 polyanion binding, and the ionic strength of the solution also affect the pKa of the PEI protonation sites. 24 Therefore, we choose to employ linear polyethyleneimine molecules with every second (50%) or every fourth (25%) backbone nitrogen (N) protonated. These are referred PEI50 and PEI25, respectively.…”