Complex formation of DNA with oppositely charged polyelectrolytes of different chain topology:: Cylindrical brushes and, dendrimers

The complex formation between DNA (pUC19-supercoiled DNA, 2686 base pairs) and some polycations of different chain topologies in aqueous solution was studied by light scattering, gel electrophoresis, and AFM. The investigated polycations comprised cylindrical brush polymers with quaternized poly(vinylpyridine) and polyethylene imine side chains as well as a fifth generation dendrimer thus covering a broad molar mass regime of 3 x 10(4) g mol(-1) < M-w < 1 X 10(7) g mol(-1) and very different chemical charges/molecule, Z(+), of 127 < Z+ < 5500. Irrespective of the polycation, the complexes formed in dilute solution exhibited a similar size in terms of the mean square radius of gyration, (R-g(2)), i.e., 30 nm < R, < 40 nm (excess of DNA) and 40 nm < R-g < 55 nm (excess of polycation). At a large excess of either DNA or polycation, the complexes were shown to coexist with the uncomplexed molecules of the excess component and did not vary in size with increasing weight fraction of the minority component. Only if the number of complexes became comparable to the number of uncomplexed molecules was inter complex bridging observed to occur, which eventually led to phase separation. The extremely large charge density mismatch between the DNA and the polycations caused strongly overcharged cationic complexes to be formed at excess polycation whereas at excess DNA a small anionic charge of the complexes was found. The results are explained qualitatively in terms of kinetically controlled complexation.