Effect of pH on Size and Internal Structure of Poly(propylene imine) Dendrimers: A Molecular Dynamics Simulation Study.

The behavior of poly(propylene imine) dendrimers at three different solution pH is investigated through molecular dynamics (MD) simulations in explicit solvent. MD simulations provide an insight into the conformational properties of dendrimers via the evaluation of their size, shape, radial density distribution, static structure factor, and scattering intensity. The size of the dendrimer increases from high solution pH to low pH. The internal structure of the dendrimer is quantified in terms of the radial atomic density profile and the terminal amine group density distribution. While the radial atomic density distribution shifts away from the core of the dendrimer with decreasing pH, a significant degree of back-folding of the outer generations is observed at high pH for higher generations of growth. Results from the structure factor and scattering intensity indicate two types of conformational transitions: (i) as a function of the solution pH, where the dendrimer evolves from an expanded structure at low pH to a highly compact one at high pH (except for higher generations), and (ii) with increasing generations, where the open structure of the dendrimer at lower generations transforms to a compact structure at higher generations at both high and low pH, characterized by a change in the fractal dimension.