Small angle neutron scattering studies of the counterion effects on the molecular conformation and structure of charged G4 PAMAM dendrimers in aqueous solutions

The structural properties of generation 4 (G4) poly(amidoamine) starburst dendrimers (PAMAM) with an ethylenediamine (EDA) central core in D₂O solutions have been studied by small-angle neutron scattering (SANS). Upon the addition of DCl, SANS patterns show pronounced inter-particle correlation peaks due to the strong repulsion introduced by the protonation of the amino groups of the dendrimers. By solving the Ornstein-Zernike integral equation (OZ) with hypemetted chain closure (HNC), the dendrimer-dendrimer structure factor S(Q) is determined and used to fit the experimental data, where Q is the magnitude of the scattering wave vector. Quantitative information such as the effective charge per dendrimer and the radius of gyration, R_G, at different pD values is obtained. The results show that R_G only changes by about 4% when the pD value varies from 10.25 to 4.97, and significant counterion association/condensation occurs, strongly mediating the inter-dendrimer interaction. The influence of interplay between counterions and molecular protonation of dendrimers imposes a strong effect on the dendrimer conformation and effective interaction. Although the change of R_G is very small, careful analyses of the high Q data and fitting parameters indicate a possible internal structure change of a dendrimer when the amino groups are progressively charged.