  Polyamidoamine (PAMAM) dendrimers, being protonated under physiological
conditions, represent a promising class of nonviral, nano-sized vectors for
drug and gene delivery. We performed extensive molecular dynamics simulations
of a generic model dendrimer in a salt-free solution with dendrimer's terminal
beads positively charged. Solvent molecules as well as counterions were
explicitly included as interacting beads. We find that the size of the charged
dendrimer depends non-monotonically on the strength of electrostatic
interactions demonstrating a maximum when the Bjerrum length equals the
diameter of a bead. Many other structural and dynamic characteristics of
charged dendrimers are also found to follow this pattern. We address such a
behavior to the interplay between repulsive interactions of the charged
terminal beads and their attractive interactions with oppositely charged
counterions. The former favors swelling at small Bjerrum lengths and the latter
promotes counterion condensation. Thus, counterions can have a dramatic effect
on the structure and dynamics of charged dendrimers and, under certain
conditions, cannot be treated implicitly.
