Macromolecular Conformation of Low-Generation PAMAM Dendrimers Probed by Pyrene Excimer Formation
Sanjay Patel, Jean Duhamel
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 k diff, which is the bimolecular rate constant for PEF generated by n -hexyl-1-pyrenebutyramide used as a model compound. The ⟨ k ⟩/ k diff ratios obtained for all the PyCX-PAMAM-GY samples in DMF and DMSO collapsed onto a single master line where ⟨ k ⟩/ k diff 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 ( R g ) of PAMAM dendrimers and compare them to the R g values obtained by molecular dynamics simulations. The excellent agreement obtained between the two sets of R g 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.