We investigated the thermodynamic, structural, and dynamics changes in dendrimer-membrane systems during dendrimer adsorption to biological membrane systems by combining atomistic molecular dynamics simulations with umbrella sampling techniques to understand the atomistic interactions between the dendrimer and biological membranes. An ethylenediamine core polyamidoamine dendrimer (generation 3) with amine terminal groups and both zwitterionic dipalmitoyl-phosphatidyl-choline (DPPC) and anionic palmitoyl-oleoyl-phosphatidyl glycerol (POPG) lipid bilayer membranes were used as the model dendrimer and biological membranes, respectively, in this study. The free energy of the dendrimer adsorption onto two model membranes with different charge states was quantitatively determined. For the zwitterionic DPPC membrane, the dendrimer has a minimum free energy of approximately 50 kcal/mol, which is 15 kcal/mol higher than that observed in previous studies. The dominant contribution to the adsorption potential energy is the van der Waals attraction between the dendrimer and the DPPC membrane. However, the anionic POPG membrane pulls the positively charged dendrimer with an attractive mean force of about 200 pN, finally positioning the dendrimer in the membrane headgroup region. As a result of these strong attractive dendrimer and membrane interactions, the dendrimer structurally undergoes the transition from spherical to a pancake conformation, which slows its lateral mobility, especially in the presence of the POPG membrane. The bilayer lipid membranes are also perturbed by the dendrimer adsorption.
The thermodynamic properties of polyelectrolyte
solutions are of
long-standing interest. Theoretical complexity arises not only from
the long-ranged electrostatic interaction but also from the multicomponent
nature of the solution, In this work, we report grand canonical Monte
Carlo simulations for the effect of added salt on the osmotic pressure
of a primitive model of polyelectrolyte solutions. The polymer chains
are freely jointed charged hard spheres, and counterions and co-ions
are charged hard spheres. We use an ensemble that allows us to calculate
directly the osmotic pressure for a solution in equilibrium with a
bulk salt solution. As the bulk salt concentration is increased, the
concentration of salt in the polyelectrolyte solution decreases and
for semidilute solutions the salt concentration is very low. In dilute
solution, the salt contribution to the osmotic pressure arises from
electrostatic screening and excluded volume interactions. Semidilute
solutions behave like salt-free solutions. The simulations show that
both polymer molecules and small ions make a significant contribution
to the osmotic pressure, thus questioning theories that ignore the
polymer contribution. The latter effect results in the decrease in
magnitude and a strong concentration dependence of the osmotic pressure.
The simulation results are in qualitative accord with experiments
on DNA. Scaling theories for the osmotic pressure, however, are not
in agreement with the simulations or experiments.
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