Macromolecular Conformation of Low-Generation PAMAM Dendrimers Probed by Pyrene Excimer Formation

Abstract

A series of pyrene-labeled PAMAM dendrimers (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) were studied by acquiring their time-resolved fluorescence decays in N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO). The fluorescence decays were fitted globally according to the model-free analysis (MFA), which yielded the average rate constant ⟨k⟩ for pyrene excimer formation (PEF). ⟨k⟩ was compared with the local pyrene concentration [Py]loc inside the dendrimers, which was calculated by assuming that the segments constituting the dendrimer interior obey Gaussian statistics. ⟨k⟩ was found to increase linearly with [Py]loc in both solvents, but the straight lines had different slopes due to differences in solvent viscosity and polarity. These differences were accounted for by dividing ⟨k⟩ by kdiff, which is the bimolecular rate constant for PEF generated by n-hexyl-1-pyrenebutyramide used as a model compound. The ⟨k⟩/kdiff ratios obtained for all the PyCX-PAMAM-GY samples in DMF and DMSO collapsed onto a single master line where ⟨k⟩/kdiff increased linearly with [Py]loc. The strong correlation found between ⟨k⟩ and [Py]loc suggested that the oligomeric segments constituting the interior of the low-generation (Y = 0, 1, and 2) PAMAM dendrimers obeyed Gaussian statistics. This fact was further supported by using the Gaussian approximation to calculate the radius of gyration (Rg) of PAMAM dendrimers and compare them to the Rg values obtained by molecular dynamics simulations. The excellent agreement obtained between the two sets of Rg values validated the assumption. Furthermore, the direct relationship between ⟨k⟩ and [Py]loc established in this study demonstrates that ⟨k⟩ provides a quantitative measure of the internal density of the dendrimers, an observation with considerable implications for the quantitative conformational characterization of macromolecules with a complex architecture in solution.