Solvent Quality Changes the Structure of G8 PAMAM Dendrimer, a Disagreement with Some Experimental Interpretations

We have performed \sim20-40 ns of molecular dynamics (MD) simulations for the generation 8 PAMAM dendrimer in explicit water under varying pH conditions to study the structure of the dendrimer (\sim156 738 atoms at low pH). This is the first report of such a long MD simulation of a larger generation PAMAM dendrimer including the effect of salt and counterions with explicit water molecules. We find that changing the pH from a high value (\sim12) to a low value (\sim3) changes the radius of gyration from $R_g=37.8$ to 43.1\AA(increasing by 13%). We also find significant back-folding of the primary amines and a large amount of water penetration inside the polymer. The increase in size with decrease in pH is consistent with our earlier studies on G3-G6 and agrees with the Monte Carlo theory by Welch and Muthukumar of G8 (Macromolecules, 1998, 31, 5892) and the experiments on G5 and G8 PAMAM dendrimer by Topp et al. (Macromolecules,1999, 32, 7232). However, these results disagree dramatically with the interpretations of SANS experiments of G8 PAMAM dendrimers by Nisato et al. (Macromolecules, 2000, 33, 4172) who observe no change in the size of the dendrimer with variations of solution pH and ionic strength. We assume that the disagreement might arise from neglecting nonspherical shape, penetration of water and ions into the core, and aggregation, all of which might depend on pH