Mesomorphic Complexes of Poly(amidoamine) Dendrimer with DNA

Abstract

The self-assembly behavior of the complexes of DNA with fully surface-protonated poly(amidoamine) (PAMAM) dendrimer of generation four has been studied as a function of the overall complex composition. The complex composition (x) was expressed by the molar ratio of the positively charged ammonium groups in the dendrimer to the DNA base pairs. The complexation was found to result in DNA condensation through which the dendrimer-bound DNA chains aggregated significantly to form ordered structures. A condensed nematic phase in which the locally oriented DNA chains did not exhibit coherent positional order formed at x = 2. Although the numbers of positive and negative charges were identical at this composition, the charge matching was frustrated by the DNA−DNA repulsion which limited the number of DNA chains surrounding each dendrimer molecule. Therefore, the nematic mesophase was built up by the irregularly packed square columnar cells (with each dendrimer molecule surrounded by four DNA chains in each cell), yielding defective DNA networks with the average interhelical distance of 4.2 nm. A significant fraction of the phosphate groups on the DNA chains in the network remained unbound to the dendrimer due to limited supply of dendrimer molecules. The condensed DNA structure transformed into a long-range ordered square columnar phase with the interhelical distance of 4.25 nm at x = 4.0. Here the number of dendrimers became abundant enough to maximize the charge matching for the DNA chains, and the interconnection of the square columnar unit cells led to a long-range ordered lattice.