Conformation of Pyrene End-Labeled Polyamidoamine Dendrimers in Catanionic Surfactant Micelles Probed by Pyrene
Donghan Liu1, Sanjay Patel1, Jean Duhamel1
1Institute for Polymer Research, Waterloo Institute for Nanotechnology, Department of Chemistry, University of Waterloo, Waterloo, ON N2L 3G1, Canada.
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A series of generation-0 polyamidoamine (PAMAM-G0) dendrimers, referred to as the PyCX-PAMAM-G0 samples, were end-labeled with four identical 1-pyrenealkanoyl derivatives with either (X = 4) butyroyl, (X = 6) hexanoyl, (X = 8) octanoyl, (X = 10) decanoyl, or (X = 12) dodecanoyl linkers. Their conformation was investigated in mixed micelles composed of sodium dodecyl sulfate (SDS) and dodecyltrimethylammonium bromide (DTAB). As the molar fraction (fDTAB) of DTAB in the SDS:DTAB mixtures was increased from 0.0 to 0.3, the micellar aggregates grew in size and changed their shape from spheres to ellipsoids. Global model free analysis of the fluorescence decays yielded the average rate constant (⟨k⟩) for pyrene excimer formation (PEF) between excited and ground-state pyrenyl labels bound to the PyCX-PAMAM-G0 dendrimers. Plots of ⟨k⟩ as a function of the local concentration ([Py]loc) of ground-state pyrenyl labels indicated that the conformation of the PyCX-PAMAM-G0 dendrimers bearing shorter X = 4, 6, and 8 alkanoyl linkers was that expected if the internal segments of these dendrimers obeyed Gaussian statistics. The ⟨k⟩-vs-[Py]loc plots also indicated that the PyCX-PAMAM-G0 samples with the longer linkers underwent a conformational inversion. However, the conformational inversion experienced by the PyCX-PAMAM-G0 samples with longer linkers appeared to become less pronounced with increasing fDTAB. Analysis of the ⟨k⟩-vs-[Py]loc trends obtained with increasing fDTAB led to the conclusion that the PyCX-PAMAM-G0 dendrimers with longer alkanoyl linkers underwent conformational inversion in the SDS-rich capping ends of the ellipsoidal micelles but not in their more hydrophobic central section and that partitioning of the dendrimers between the two regions of the surfactant aggregates resulted in the decrease in conformational inversion identified by the PEF study. Consequently, these PEF experiments represent an interesting experimental means for probing the conformation of complex hydrophobically modified water-soluble macromolecules as they interact with the microdomains generated by small amphiphilic molecules.
