Fig 1.
Interactions in a hydrophobic polyelectrolyte that is a weak base.
The polymer charge increases with decreasing pH, whereas the charge repulsion is screened by increasing the salt concentration. The backbone extension depends on the balance between inter-chain and intra-chain charge repulsion. Aggregation occurs as intra-chain repulsion is lowered or as inter-chain repulsion is increased. In the figure, the polyelectrolyte chain is depicted as a string of beads, with charged monomers/bead shown in black.
Fig 2.
Variation of the osmotic pressure of PEI solutions as a function of the polymer concentration at constant NaCl concentrations (A), and as a function of the NaCl concentration at constant polymer concentrations (B).
Fig 3.
Distribution of the aggregated and free polymer species at PEI dissolution and with addition of NaCl.
(A) Scaled DLS correlation data showing the predominance of the aggregate form in the absence of NaCl, and the gradual release of free polymer with the addition of salt. The arrows point to the hydrodynamic diameters corresponding to the decay region of the DLS curve. (B) Unscaled intensity contribution showing that the aggregate form releases free polymers as salt is added to the solution. (C) DLS correlation curve showing that filtration removes the contribution from the aggregate form, indicating that the latter is a separable entity. (D) DLS correlation curve of the solution from C over time with the addition of HCL in this case, showing the return of the aggregate contribution. The data in C and D are shown for 2.72 mM PEI and 50 mM NaCl.
Fig 4.
Hydrophobic polyelectrolyte dynamics of PEI in the neutral pH range (pH 7–8).
(A) The hydrodynamic radius of PEI shown as a function of NaCl concentration for different PEI concentrations (legend). The hydrodynamic radius increases in the inter-chain repulsion regime and decreases in the intra-chain repulsion regime. (B) Distribution of free and aggregated PEI forms at neutral pH range with varying PEI and NaCl concentration. The free polymer content increases with salt concentration and decreases again. The salt concentration required for maximum free polymer (black) increases with PEI concentration. (C) In DLS, the level of aggregation initially decreases with PEI concentration and then increases. Correspondingly in osmotic experiments, the number of diffusing species increases with PEI concentration then decreases.
Fig 5.
pH titration curves of 2.5kDa PEI at 4.08 mM PEI at different salt concentrations.
The polymer and salt concentration was maintained constant for all titration samples. The polyelectrolyte-protonation underlying the titration curve is studied in the context of four pH regions.
Fig 6.
pH dependent protonation-polyelectrolyte interplay in 4.08mM PEI solution with 150mM NaCl.
(A) Schematic depicting the protonation and polyelectrolyte state of the polymer in each pH region. The grey shading in each region represents the different charge repulsion regimes described in Fig 1. Charged PEI monomers are shown as black beads and uncharged monomers as white beads. (B) Simultaneous tracking of PEI protonation and backbone extension state in different pH regions. (C) Contribution of the free and aggregated PEI forms to the scattering intensity. (D) Changes in the pH and absorbance of PEI solution at 440 nm following acid and ninhydrin additions.
Fig 7.
Schematic depiction of the stalled protonation states of PEI between which protonation/buffering occurs continuously and discontinuously.
Fig 8.
DNA nanoparticle size as a function of the salt content and pH of the PEI solution.
PEI concentration was maintained at 4.08 mM in all cases. The pH was ~7.5 at the different salt concentrations, while the salt concentration was 150 mM at the different pH.