Intramolecular structural change of PAMAM dendrimers in aqueous solutions revealed by small-angle neutron scattering

Small-angle neutron scattering (SANS) experiments were carried out to investigate the structure of aqueous (D₂O) G4 PAMAM dendrimer solutions as a function of molecular protonation and dendrimer concentration. Our results indicate unambiguously that, although the radius of gyration R _G remains nearly invariant, the dendrimer radial density profile ρ(r) decreases in the dendrimer core with a continuous increase in protonation. This discovery also suggests that R_G, which is commonly adopted by numerous simulation and experimental works in describing the global dendrimer size, is not suitable as the index parameter to characterize the dendrimer conformation change. We also found that R_G and ρ(r), for dendrimers dissolved in both neutral and acidified solutions, remain nearly constant over the studied concentration range. We further demonstrate that the outcome of the widely used Guinier method is questionable for extracting R _G in the concentration range studied. Our results reveal the polymer colloid structural duality as benchmarks for future experimental and theoretical studies and provide a critical step toward understanding drug encapsulation by ionic bonds.