Ionization Behavior of Chitosan and Chitosan–DNA Polyplexes Indicate That Chitosan Has a Similar Capability to Induce a Proton-Sponge Effect as PEI

Polycations having a high buffering capacity in the endosomal pH range, such as polyethylenimine (PEI), are known to be efficient at delivering nucleic acids by overcoming lysosomal sequestration possibly through the proton sponge effect, although other mechanisms such as membrane disruption arising from an interaction between the polycation and the endosome/lysosome membrane, have been proposed. Chitosan is an efficient delivery vehicle for nucleic acids, yet its buffering capacity has been thought to be significantly lower than that of PEI, suggesting that the molecular mechanism responsible for endolysosomal escape was not proton sponge based. However, previous comparisons of PEI and chitosan buffering capacity were performed on a mass concentration basis instead of a charge concentration basis, the latter being the most relevant comparison basis because polycation–DNA complexes form at ratios of charge groups (amine to phosphate), rather than according to mass. We hypothesized that chitosan has a high buffering capacity when compared to PEI on a molar basis and could therefore possibly mediate endolysosomal release through the proton sponge effect. In this study, we examined the ionization behavior of chitosan and chitosan–DNA complexes and compared to that of PEI and polylysine on a charge concentration basis. A mean field theory based on the use of the Poisson–Boltzmann equation and an Ising model were also applied to model ionization behavior of chitosan and PEI, respectively. We found that chitosan has a higher buffering capacity than PEI in the endolysosomal pH range, while the formation of chitosan–DNA complexes reduces chitosan buffering capacity because of the negative electrostatic environment of nucleic acids that facilitates chitosan ionization. These data suggest that chitosans have a similar capacity as PEI to mediate endosomal escape through the proton sponge effect, possibly in a manner which depends on the presence of excess chitosan.