Molecular Dynamics Simulations of Charged Dendrimers: Low-to-Intermediate Half-Generation PAMAMs

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Abstract
We have performed atomistic molecular dynamics simulations of PAMAM dendrimers of generations 0.5, 1.5, 2.5, 3.5, and 4.5. The simulated systems comprise the charged dendrimer and its counterions embedded in a dielectric continuum (i.e., without explicit solvent). Structural properties of these dendrimers, like the radius of gyration, the principal moments of inertia, and the segment density profiles, were evaluated from the simulations. The average radius of gyration obtained for the intermediate half-generations 2.5, 3.5, and 4.5 follows the same scaling law that was previously inferred from simulations of full-generation PAMAMs, Rg approximately M1/3, and is characteristic of space-filling objects. The low half-generations 0.5 and 1.5 deviate, however, to greater Rg values. The shape of the smaller dendrimers is approximately that of a prolate ellipsoid, which becomes more spherical for higher generations. The segment density profiles show features identical to those obtained in other simulations of flexible-chain dendrimers, like dendron-backfolding. Two slightly different configurations, in terms of size and shape, were identified for generation 2.5. The radial distributions of counterions extracted from the simulations compare well with the solutions of Poisson-Boltzmann cell model, and the dendrimer's effective charge was estimated using the Bjerrum criterion. The influence of electrostatic interactions in the dendrimer's conformation due to repulsion between the charged end-groups and its relation to counterion effects is discussed for the several generations simulated. The form factors calculated from the simulations are compared with the model of a homogeneous ellipsoid of revolution. The overall results are in agreement with the previously established morphological transition of PAMAM dendrimers toward a more spherical and compact conformation above generations 3 or 4.