Synthesis and Characterization of a Pyrene-Labeled Gemini Surfactant Sensitive to the Polarity of Its Environment

Interactions between DNA and a Pyrene-Labeled Surfactant Probed by Pyrene Excimer Formation, Transmission Electron Microscopy, and Dynamic Light Scattering

The interactions between calf thymus DNA and a pyrene-labeled gemini surfactant termed PyO-3-12 were investigated by using dynamic light scattering (DLS), transmission electron microscopy (TEM), and pyrene excimer formation (PEF) between an excited and a ground-state pyrene. PyO-3-12 was prepared with two dimethylammonium bromide headgroups linked with a propyl spacer, with one ammonium bearing a dodecyl tail and the other bearing a 1-pyrenemethoxyhexyl pendent. The size and electrostatic properties of the colloidal species were characterized by DLS, and TEM was used to describe the overall dimensions and morphologies of the aggregates. PEF enabled the study of PyO-3-12 at the molecular level. The association of PyO-3-12 and DNA was studied in detail for two specific PyO-3-12 concentrations of 16 and 56 μM using a (∓) ratio equal to the [DNA]/[PyO-3-12] ratio with [DNA] expressed in terms of DNA base pairs, yielding a (∓) ratio ranging from 0.1 to 10. An increase in PyO-3-12 association upon adding DNA was detected through an increase in PEF. Analysis of the fluorescence decays indicated a progressive binding of PyO-3-12 to DNA until the equicharge point was reached after which all PyO-3-12 surfactants were bound to DNA. The formation of large colloidal aggregates was observed by DLS, which showed a spike in the hydrodynamic diameter (Dh) as the (∓) ratio approached unity. Analysis of the fluorescence spectra suggested that the excited pyrenyl labels experienced a more hydrophobic environment for the lipoplexes generated by the 56 μM PyO-3-12 solutions, which appeared smaller and more globular in the TEM images than the lipoplexes obtained with the lower 16 μM PyO-3-12 concentration. Together, these experiments suggest that the concentrations of the cationic gemini surfactants and DNA are important parameters for the morphology of lipoplexes with possible implications for their transfection efficiency.

Probing Macromolecular Conformation in Restricted Geometry by PEF: Application to Hydrophobically Modified PAMAM Dendrimers Isolated Inside Surfactant Micelles

The conformation of a series of zero-generation polyamidoamine dendrimers end-labeled with four 1-pyrene-butyroyl, -hexanoyl, -octanoyl, -decanoyl, and -dodecanoyl derivatives, referred to as the PyCX-PAMAM-G0 samples with X = 4, 6, 8, 10, and 12, respectively, was characterized in N,N-dimethylformamide (DMF), dimethyl sulfoxide (DMSO), and aqueous solutions of 50 mM sodium dodecyl sulfate (SDS) or 50 mM dodecyltrimethylammonium bromide (DTAB). The conformation of the PyCX-PAMAM-G0 samples was determined from the global model-free analysis (MFA) of the fluorescence decays, which yielded the average rate constant (⟨k⟩) for pyrene excimer formation (PEF) between an excited and a ground-state pyrenyl labels, with ⟨k⟩ being proportional to the local concentration ([Py]loc) of the pyrenyl labels within the macromolecular volume; ⟨k⟩-vs-[Py]loc plots yielded straight lines passing through the origin in DMF and DMSO, demonstrating that the internal segments of the dendrimers obeyed Gaussian statistics in these two solvents. In aqueous surfactant solutions, the hydrophobic pyrenyl labels induced the interactions of the PyCX-PAMAM-G0 dendrimers with the SDS and DTAB micelles. Plots of ⟨k⟩ as a function of [Py]loc yielded straight lines passing through the origin for the PyCX-PAMAM-G0 samples with X equal to 4, 6, and 8, indicating that the internal segments of these three dendrimers obeyed Gaussian statistics within the surfactant micelles. However, ⟨k⟩ departed from the straight lines for the PyCX-PAMAM-G0 samples with X = 10 and 12 associated with the SDS and DTAB micelles. This behavior indicated that [Py]loc was much larger than expected for the dendrimers prepared with longer alkanoyl linkers. This behavior was attributed to strong hydrophobic interactions between the longer linkers and the dodecyl tails of the surfactants in the hydrophobic core of the micelles, which induced a conformational change for the dendrimers inside the micelles, that could be probed at the molecular level by PEF.

Pyrene monomer-excimer dynamics to reveal molecular organization in mesoporous hybrid silica films

Self-assembly and characteristics of hybrid mesoporous silica film templates remain a subject of inquiry. The short time scale of the inorganic condensation and formation of micelles makes our understanding of this process insufficient. To provide an insight into the evaporation-induced self-assembly of such films, we synthesized an efficient molecular probe of the triethoxysilane precursor bearing a pyrene derivative. The probe was introduced into the porous film at the synthesis stage through the sol-gel co-condensation method. At different synthesis stages, the emission of pyrene moieties was measured by fluorescence spectroscopy, revealing the placement of probes within the film. We also report dynamic excimer formation upon template removal. Moreover, we evaluate the influence of several parameters on the pyrene excimer formation phenomenon. The pore geometry, probe concentration, and the presence of another organosilane precursor are investigated in this work.

Location of a Hydrophobic Load in Poly(oligo(ethylene glycol) methyl ether methacrylate)s (PEGMAs) Dissolved in Water and Probed by Fluorescence

Two series of pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s referred to as PyEG5-PEGnMA and PyC4-PEGnMA were prepared to probe the region surrounding the polymethacrylate backbone by using the fluorescence of the dye pyrene. PyEG5-PEGnMA and PyC4-PEGnMA were prepared by copolymerizing the EGnMA methacrylate monomers with penta(ethylene glycol) 1-pyrenemethyl ether methacrylate or 1-pyrenebutyl methacrylate, respectively. In organic solvents, the much longer 18 non-hydrogen atom linker connecting the pyrene moieties to the polymethacrylate backbone in the PyEG5-PEGnMA samples enabled the deployment of the pyrenyl labels into the solution. In water, however, an excited pyrene for PyEG5-PEGnMA was found to probe a same volume as for the PyC4-PEGnMA samples where a much shorter 6 non-hydrogen atom spacer connected pyrene to the backbone. Another surprising observation, considering that the hydrophobicity of pyrene induces strong pyrene aggregation for many pyrene-labeled water-soluble polymers (Py-WSPs) in water, was the little pyrene aggregation found for the PyEG5-PEGnMA and PyC4-PEGnMA samples in water. These effects could be related to the organic-like domain (OLD) generated by the oligo(ethylene glycol) side chains densely arranged around the polymethacrylate backbone of the polymeric bottlebrush (PBB). Additional fluorescence experiments conducted with the penta(ethylene glycol) 1-pyrenemethyl ether derivative indicated that the cylindrical OLD surrounding the polymethacrylate backbone had a chemical composition similar to that of ethylene glycol. Binding of hydrophobic pyrene molecules to unlabeled PEGnMA bottlebrushes in water further supported the existence of the OLD. The demonstration, that PEGnMA samples form an OLD in water, which can host and protect hydrophobic cargoes like pyrene, should lead to the development of improved PEGnMA-based drug delivery systems.

Characterization of the Interactions between an Unassociated Cationic Pyrene-Labeled Gemini Surfactant and Anionic Sodium Dodecyl Sulfate

The gemini surfactant PyO-3-12, made of two dimethylammonium bromides joined by a propyl linker and bearing a dodecyl pendant on one side and a 1-pyrenemethoxyhexyl group on the other side, was employed to probe the interactions between positively charged PyO-3-12 and negatively charged sodium dodecyl sulfate (SDS). PyO-3-12 was selected for its ability to respond to the polarity of its local environment through the fluorescence intensity ratio I₁/I₃ of the first-to-third fluorescence peaks of the pyrene monomer and the local pyrene concentration [Py]_loc through the I_E/I_M ratio of the pyrene excimer-to-pyrene monomer fluorescence intensity. Furthermore, analysis of the fluorescence decays of aqueous solutions of PyO-3-12 and SDS yielded a measure of the internal dynamics, local concentration, and state (associated vs unassociated) of PyO-3-12 in solution. By following these parameters for aqueous solutions prepared with a constant PyO-3-12 concentration of either 1, 4, or 16 μM and SDS concentrations ranging from 0 to 200 mM, six SDS concentration regimes were identified to describe the interactions between PyO-3-12 and SDS in pure water. Sharp transitions of the parameters describing the fluorescence of pyrene marked the boundaries between the different regimes. Perhaps the most important transition was the one defining the formation of the PyO-3-12/SDS aggregates, which was completed at the equicharge point, implying that they were constituted of 1 meq of PyO-3-12 and 2 meq of SDS. The low I₁/I₃ ratio obtained for the PyO-3-12/SDS aggregates suggested that they were multilamellar aggregates, which would shield the pyrenyl labels from polar water. The formation of these multilamellar aggregates was confirmed by transmission electron microscopy (TEM), which demonstrated the existence of multilamellar vesicles, whose presence increased with decreasing PyO-3-12 concentration. This study suggests that the combination of pyrene excimer formation and TEM provides an interesting experimental means to probe the assemblies generated from oppositely charged surfactants at surfactant concentrations, which are much lower than their critical micelle concentration.