Role of pH on the compaction energies and phase behavior of low generation PAMAM-DNA complexes

Cationic polymers are capable of spontaneously condensing DNA into complexes suitable for nonviral gene therapy. The precisely defined molecular weights and highly symmetric surface chemistries of dendrimers, such as poly(amidoamine) (PAMAM), have made them attractive alternatives to conventional linear polymers for DNA delivery applications. In this paper, we use small-angle X-ray scattering (SAXS) coupled with osmotic stress to directly measure the effect of pH on the packaging and forces in low generation (G0 and G1) PAMAM-DNA complexes or dendriplexes. Because of the presence of both primary and tertiary amines with differing pK_as, PAMAM changes its net charge with pH. We show that changing the pH at condensation results in large differences in the packaging and intermolecular forces in PAMAM dendriplexes. Both dendrimer/DNA systems show a large increase in attractions with decreasing pH scaling linearly with the inverse of the dendrimer charge, while repulsions in the system are nearly unaffected. pH also greatly influences the resulting phase behavior and salt sensitivities of the PAMAM-DNA complexes. With increasing salt concentration, a discontinuous phase transition is observed from a hexagonal packaging of DNA helices to a more loosely ordered cholesteric phase for both PAMAM dendriplex systems studied. The phase transition occurs at a critical salt concentration, c, that is dependent on both the PAMAM generation number and the pH. When normalized by c, this salt-induced melting transition is found to be universal for a given dendrimer/DNA system.