Molecular Dynamics Study of Charged Dendrimers in Salt-Free Solution: Effect of Counterions

TL;DR

This study uses molecular dynamics simulations to explore how counterions influence the size, structure, and dynamics of charged dendrimers in salt-free solutions, revealing a non-monotonic size dependence on electrostatic interaction strength.

Contribution

It provides detailed insights into the structural and dynamic behavior of charged dendrimers considering explicit counterions, highlighting effects often neglected in implicit models.

Findings

Dendrimer size varies non-monotonically with electrostatic interaction strength.

Counterions significantly affect dendrimer structure and dynamics.

Maximum dendrimer swelling occurs when Bjerrum length equals bead diameter.

Abstract

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.