A Discussion of the pH-Dependent Protonation Behaviors of Poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and Poly(ethylenimine-ran-2-ethyl-2-oxazoline) (P(EI-r-EOz))

In this article, we present results of our experimental and atomistic simulation studies of the pH-dependent protonation behaviors of poly(2-(dimethylamino)ethyl methacrylate) (PDMAEMA) and poly(ethylenimine) (PEI). The potentiometric titration profiles of the PDMAEMA polymer and its unpolymerized monomer (i.e., DMAEMA) were measured under identical conditions in order to study the influence of the covalent linkage of the amine groups on their protonation behavior. The titration curves of poly(ethylenimine-ran-2-ethyl-2-oxazoline) (P(EI-r-EOz)) random copolymers with varying monomer composition were measured in order to study the effect of the spacing between the EI monomers on the protonation behavior of the P(EI-r-EOz) copolymer. The results of these two sets of measurements demonstrate that the connectivity and tight spacing between amine groups in a polyamine chain causes the retardation of the protonation of the amine groups relative to the same compounds in their isolated state. The same titration measurements were also performed with added NaCl. The results of these measurements demonstrate that added NaCl weakens the electrostatic repulsion between charged amine groups in a polyamine chain and thus enhances the protonation of the chain, and this effect is quite significant at a physiological NaCl concentration of 150 mM. However, on the quantitative level, the effect of added NaCl was found to be very different between the PDMAEMA and P(EI-r-EOz) cases. In PDMAEMA, since the amine groups are located at the termini of the side chains, the interaction between adjacent charged monomers occurs through the aqueous medium, and therefore at a sufficiently high concentration of added NaCl, the amine groups on the chain behave almost identically to their unpolymerized equivalents. In contrast, the electrostatic interaction between two closely spaced charged EI monomers in a P(EI-r-EOz) chain is significantly less influenced by a change of the ionic strength of the medium, because it is dominated by the local dielectric property of the polymer segment located between the charged monomers. This interpretation is further supported by ab initio electron density functional theory (DFT) calculations on model oligomeric compounds whose structures imitate the repeat unit structures of the polymers. Lastly, in connection with potential applications of the PEI and PDMAEMA polymers in gene delivery technologies, it was also examined how complexation with negatively charged polymers at the physiological NaCl concentration (150 mM) impacts the protonation behaviors of the polyamines. We found that the oppositely charged polyanion greatly stabilizes the protonated form of the amine groups on the polyamine chain. However, the proton buffering capacity of the polyamine in the complexed form under the influence of added 150 mM NaCl for the intracellularly relevant pH change was found to be significantly lower than that of the pure polyamine in the uncomplexed state with no added salt.