Structures and Protonation States of Hydrophilic-Cationic Diblock Copolymers and Their Binding with Plasmid DNA

Complexation between plasmid DNA (pDNA) and a set of diblock copolymers, each with one cationic block and one hydrophilic, charge-neutral block, is examined. A range of hydrophilic block structures are explored, whereas the cationic block is fixed as poly( N-(2-aminoethyl) methacrylamide) (PAEMA) with a degree of polymerization of 60 ± 3. The hydrophilic blocks include poly(ethylene glycol) (PEG45), poly(oligo(ethylene glycol) methyl ether methacrylate) (P(OEGMA)51), and poly(2-deoxy-2-methacrylamido glucopyranose) (PMAG52). The numbers represent the degrees of polymerization and are chosen such that the diblock contour lengths are similar (37 ± 2 nm). The three diblock copolymers and a homopolycation control, PAEMA59, are compared with respect to their state of dissolution in aqueous environments, as well as their complexation with pDNA. The diblock copolymers are found to partially aggregate as pH increases above 6, whereas each separate block generally dissolves well over a wide pH range. The hydrophilic block proves to be a critical parameter in determining the structures of pDNA-diblock complexes. When the molar ratio of polycation amines to pDNA phosphates (i.e., N/P) is less than 1, a bulkier hydrophilic block leads to larger resulting complexes. As more polycations are added to the system (N/P > 1.5), colloidal stability becomes an important factor, making more water-soluble systems stabilize at smaller sizes. Further, the charge density effect on the binding thermodynamics is elucidated via calorimetric measurements. P(OEGMA)51- b-PAEMA60 exhibits a greater amount of endothermic pDNA binding per charged amine at higher pH, implying that lower cationic charge density promotes more phosphate pairing per amine on average. Also, the colloidal stability and the circular dichroism spectral evolution of the pDNA-PAEMA59 complexes are dependent on pH, showing noticeable differences between pH = 6.0 vs 7.4. To summarize, controlling the solution pH may be crucial in pDNA-polycation complexation, as it impacts polycation solubility, binding characteristics, and the final complex properties. The findings reported herein should aid researchers in drawing more rigorous structure-function correlations in the field of polymeric gene delivery.