The understanding of interactions between double stranded
(ds) DNA and charged nanoparticles will have a broad bearing
on many important applications from drug delivery [ 1–4 ] to DNAtemplated
metallization. [ 5 , 6 ] Cationic nanoparticles (NPs) can
bind to DNA, a negatively charged molecule, through a combination
of electrostatic attraction, groove binding, and intercalation.
Such binding events induce changes in the conformation
of a DNA strand. In nature, DNA wraps around a cylindrical
protein assembly (diameter and height of 6 nm) [ 7 ] with an ≈ 220
positive charge, [ 8 ] creating the complex known as chromatin.
Wrapping and bending of DNA has also been achieved in the
laboratory through the binding of highly charged species such
as molecular assemblies, [ 9 , 10 ] cationic dendrimers, [ 11 , 12 ] and
nanoparticles. [ 13–15 ] The charge of a nanoparticle plays a crucial
role in its ability to induce DNA structural changes. If a
nanoparticle has a highly positive surface charge density, the
DNA is likely to wrap and bend upon binding to the nanoparticle
[ 13 ] (as in the case of chromatin). On the other hand, if a
nanoparticle is weakly charged it will not induce dsDNA compaction.
[ 9 , 10 , 15 ] Consequently, there is a transition zone from
extended to compact DNA conformations which depends on
the chemical nature of the nanoparticle and occurs for polycations
with charges between 5 and 10. [ 9 ] While the interactions
between highly charged NPs and DNA have been extensively
studied, the processes that occur within the transition zone are
less explored.