Diffusion NMR study of generation-five PAMAM dendrimer materials

Commercial generation-five poly(amidoamine) dendrimer material (G5c) was fractionated into its major structural components. Monomeric G5 (G5m; 21-30 kDa) was isolated to compare its functional properties to the G5c material. Diffusion-ordered nuclear magnetic resonance spectroscopy was employed to measure the self-diffusion coefficients and corresponding hydrodynamic radii of G5m and other G5c components as a function of dendrimer size (i.e., molecular weight) and tertiary structure (i.e., generational or oligomeric nature). It was found that the hydrodynamic radius (R_H) scales with approximate numbers of atoms in the trailing generations, G5m, and oligomeric material at a rate of R_H ∝ N^0.35, in good agreement with previous reports of R_H scaling for PAMAM dendrimer with generation. G5c materials can be thought of as a heterogeneous mixture of dendrimers ranging in size from trailing generation two to tetramers of G5, approximately the same in size as a G7 dendrimer, with G5m comprising ∼65% of the material. The radius of hydration for G5m was measured to be 3.1 ± 0.1 nm at pH 7.4. The 10% swelling in response to a drop in pH observed for the G5c material was not observed for isolated G5m; however, the isolated G5-G5 dimers had an increase of 44% in R_H, indicating that the G5c pH response results from the increase in R_H of the oligomeric fraction upon protonation. Finally, the data allow for an experimental test of the "slip" and "stick" boundary models of the Stokes-Einstein equation for PAMAM dendrimer in water.