  DNA interactions with polycations are not only important for our
understanding of chromatin compaction but also for characterizing DNA-binding
proteins involved in transcription, replication and repair. DNA is known to
form several types of liquid-crystalline phases depending, among other factors,
on polycation structure and charge density. Theoretical studies and simulations
have predicted the wrapping of DNA around spherical positively charged
polycations. As a potential mimic of the histone octamer or other DNA wrapping
proteins, poly(amido amine) generation 6 (PAMAM6) dendrimers have been chosen
for our study. The self-assembly of DNA induced by PAMAM6 has been investigated
using small angle X-ray scattering (SAXS) in order to reveal the assemblies'
structure dependence on the pH of the environment and on dendrimers
concentration. We demonstrate that at pH 8.5 dense phases are formed and
characterized by a 2D-columnar hexagonal lattice which is transformed into a 3D
hexagonal lattice with increasing dendrimer concentration (charge ratio N/P).
Moreover, a systematic analysis of the scattering data collected from diluted
samples at pH 8.5 and 5.5 have led us to propose a wrapping scenario of DNA
around PAMAM6 at pH 5.5.
