Abstract
Polyanionic DNA can bind electrostatically with cationic PAMAM dendrimer to form the complex exhibiting rich self-assembled structure at various length scales. This class of bioassembly has been considered as a non-viral gene delivery system for gene therapy and as a template or building block for DNA-based nanotechnology. Understanding the self-assembly behavior of DNA-dendrimer complex is crucial for the developments of both non-viral gene vectors and biomolecule-directed nanostructures. This thesis presents a comprehensive study of the supramolecular structure of the complexes of DNA with poly(amidoamine) (PAMAM) dendrimers. The structure in pure water has been investigated as a function of dendrimer generation number, N/P ratio, temperature and dendrimer charge density controlled by the degree of protonation (dp). Here N/P ratio represents the molar ratio of the amine groups of dendrimer to the phosphate groups of DNA. For the complexes with low-generation dendrimers, i.e., G2 and G3, two types of ordered columnar mesophases, in which the DNA chains packed into long-range ordered lattice with in-plane hexagonal or square symmetry, were identified. The type of DNA packing was proposed to be governed by the interplay between DNA-dendrimer attraction and DNA-DNA repulsion. In general, hexagonal packing was the favorable structure for G2 complexes, while G3 system tended to form square phase. Smaller N/P ratio and larger dp could induce the formation of disordered columnar phase. As dp approached zero, a special hexagonal structure termed “H’ phase” formed to accommodate a large number of dendrimer within the hexagonal lattice. For the complexes with G4 dendrimer, we observed the coexistence of two lattice orderings of both DNA and dendrimer components. Over a certain range of N/P ratio and dp, the DNA chains in the complexes were found to pack in the square lattice, and the dendrimer molecules situating in the interstitial tunnels further organized into a three-dimensional tetragonal lattice. The result indicated that interesting hierarchical nanostructure can be constructed by proper combination of building blocks with well-defined geometry, such as cylinder and sphere found in this study. As the dendrimer generation was increased to nine, the complexation resulted in the beads-on-string structure found in DNA-histone complex constituting chromatin. At low dp, DNA chains wrapped around the dendrimer tightly with a distribution of pitches, and the chromatin-like fiber thus formed had a small persistent length due to weaker electrostatic repulsion. At dp = 0.5, the DNA chain wrapped around the dendrimer regularly and tightly with the pitch of ca. 2.6 nm. The resultant chromatin-like fiber was highly stiff with the persistent length probably close to several hundred nm. Finally, a simple method for constructing a 2-D densely packed DNA nanostructure using the electrostatic complex of DNA with PAMAM G2 dendrimer was reported. Ordered DNA arrays were formed by drop-casting an aqueous solution containing positively overcharged complexes onto mica followed by a prolonged incubation. During the incubation, the complexes tended to adsorb onto the negatively charged mica surface through electrostatic attraction. The rodlike complexes organized to form ordered arrays to increase the surface density of the adsorbed complexes and hence the attractive free energy of adsorption.