Abstract
In this study, we investigate the nanostructures of the complexes of DNA with poly(amdoamine) (PAMAM) dendrimer of generation four (G4). The role of electrostatic interaction in the structure formation is examined by studying the effects of dendrimer charge density (prescribed by its degree of protonation, dp) and addition of monovalent salt NaCl. For the complex prepared in pure water, we show that increasing the charge density of dendrimer effectively compensates the DNA bending energy, thereby allowing DNA conformation to be templated by the surface curvature of the charged sphere. In this case, the DNA packaging state transforms from the square columnar phase to tetragonally-packed undulated DNA phase to beads-on-string structure with increasing dp. The addition of NaCl has a strong impact on the nanostructure of the complex. The structure transformation with increasing salt concentration tends to follow the trend of: square columnar phase□ undulated phase □ hexagonal columnar phase or beads-on-string structure□ undulated phase □ hexagonal columnar phase. The observed phase transition sequence implies that the screening of the electrostatic attraction between DNA and dendrimer is the most significant effect, while the screening of the intra-chain electrostatic repulsion of DNA is also operative to induce the transformation from square columnar phase to undulated phase.