Interaction of Astramol poly(propyleneimine) dendrimers with linear polyanions

Interaction between Astramol poly(propylene imine) dendrimers of five generations, DAB-dendr-(NH2)(x) (where x is equal to 4, 8, 16, 32, or 64) and linear poly(sodium acrylate), poly(acrylic) acid, poly(sodium styrenesulfonate), or native DNA was studied in salt-free solutions by means of potentiometric, argentometric, and turbidimetric titrations. In addition to x outer primary amine groups the dendrimer molecule contains x - 2 inner tertiary amine groups, It is found that the pH-controlled interpolyelectrolyte coupling reaction resulting in formation of the corresponding interpolyelectrolyte complexes occurs on mixing of the dendrimer and the polyanion solutions. A peculiar finding was that all dendrimer amine groups being protonated can form ion pairs with carboxylate or sulfonate groups of the polyanions. In other words DAB-dendr-(NH2)(x) molecules are fully penetrable for rather flexible oppositely charged polyelectrolyte chains. However, this is not the case for rigid negatively charged DNA double helixes which apparently bind only to the dendrimer shell. The complexes of dendrimers, with polyanions containing equal amounts of cationic and anionic groups mostly ion-paired (stoichiometric complexes), are insoluble in water. At the same time water soluble nonstoichiometric interpolyelectrolyte complexes can be obtained if the dendrimer is a deficient component of the complex species. This behavior is similar to that of complexes formed by oppositely charged linear polyions.