Intramacromolecular Conformational Changes in Low Generation PAMAM Dendrimers Probed by Pyrene Excimer Formation

Pyrene excimer formation (PEF) was used to probe the intramacromolecular conformational change experienced by low generation pyrene-labeled PAMAM dendrimers referred to as PyCX-PAMAM-GY, where X (=4, 8, or 12) and Y (=0, 1, or 2) represent the number of atoms in the pyrenyl linker and the dendrimer generation, respectively. Each sample was studied in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) with and without 5 mM HCl. Global analysis of the monomer and excimer time-resolved fluorescence decays using the model free analysis (MFA) yielded the average rate constant of excimer formation, ⟨k⟩, which was compared with the local pyrene concentration ([Py]_loc) of the PyCX-PAMAM-GY samples calculated by assuming that the oligomeric segments constituting the dendrimer's interior obeyed Gaussian statistics. A notable decrease in ⟨k⟩ was observed upon the addition of 5 mM HCl to the PyCX-PAMAM-GY solutions and was attributed to swelling of the dendrimers resulting from the protonation of the internal tertiary amines. The reversibility of this conformational change could also be monitored via PEF. Solvent differences between DMF and DMSO were accounted for by dividing ⟨k⟩ by k_diff, the bimolecular rate constant for diffusive PEF of a n-hexyl-1-pyrenebutyramide model compound, to yield the ⟨k⟩/k_diff ratio. Comparison between the ⟨k⟩/k_diff ratios obtained for all the PyCX-PAMAM-GY samples with and without 5 mM HCl revealed a 13% increase in the radius of the PAMAM-GY dendrimers upon protonation of their internal tertiary amines in agreement with earlier reports. These experiments illustrate that PEF represents a powerful experimental means to quantitatively probe the intramacromolecular conformational changes of complex macromolecules in solution, in a manner that complements scattering techniques.