Fluorescent Probes for the Supramolecular Interactions responsible for Binding of Polycyclic Aromatic Hydrocarbons to Hyperbranched Polyelectrolytes in Aqueous Media

Abstract

Cationic poly(amidoamine) (PAMAM) dendrimers are known as good supramolecular hosts for a variety of smaller water‐soluble guests. We expanded their binding scope to the uptake of very hydrophobic guests such as polycyclic aromatic hydrocarbons (PAH) in neutral aqueous solution. We used anthracene and pyrene as representatives of this family of hydrocarbons, and as capable fluorescent emitters to probe mode and location of interaction for these hydrocarbons with polycationic amine‐terminated PAMAM dendrimers. We used steady‐state and time‐resolved fluorescence emission spectroscopy, as well as fluorescence anisotropy measurements and selective quenching experiments, to establish the mode and location of these binding interactions, demonstrating that, although these probes may look relatively similar, their interaction with PAMAM dendrimers is nevertheless significantly different. The results presented here provide insight into the attractive forces at play in the uptake of featureless hydrophobic PAH guests by hydrophilic PAMAM dendrimer hosts, whose applications span practical challenges including chemical separations, analytical discrimination, solubility and bioavailability enhancement of hydrophobic compounds.

Discrimination and Quantitation of Biologically Relevant Carboxylate Anions Using A [Dye•PAMAM] Complex

“…The amount of G5 required to reach saturation was higher for calcein than for pyranine, suggesting that pyranine binds to PAMAM G5 more strongly than calcein. This cannot stem from the electrostatic inter-actions (they are both trianions); instead, it is likely that the more hydrophilic calcein establishes fewer favorable hydrophobic interaction with the dendrimer [ 31 ]. This already unfavorable situation is exacerbated by calcein’s higher energetic cost of de-solvation upon binding: carboxylate groups in calcein are better H bonding acceptors than the sulfonate groups in pyranine, therefore calcein is likely to form stronger hydrogen bonding interactions with water molecules compared to pyranine and to be better solvated in water.…”

pH- and Functionalization-Dependent Host–Guest Interactions Between Fluorescein and Various Poly(amidoamine) Dendrimers

“…These interactions have been established for both uncharged and anionic aromatic molecules. 11,13,41 Although these CH−π effects cannot be observed directly in the present work, their presence in comparable dendrimer-guest systems suggest that they might support PAMAM-fluorescein binding as well.…”

“…Furthermore, dendrimers can bind selectively to water pollutants such as heavy metals and dyes. , In addition, dendrimers can be used to detect dissolved anions . Consequently, dendrimers are being studied for drug delivery, wastewater pollutant removal, and sensing of anionic molecules. − Although these applications of dendrimers are promising, their implementations are complex because a large variety of dendrimers can interact with an equally large variety of external molecules: each dendrimer-guest combination can be driven by interactions including electrostatic effects, hydrogen bonding, π-bonding, and hydrophobic interactions. − In turn, each of these interactions depends on a range of environmental factors, such as temperature and pH. , Due to this complexity, it can be challenging to predict in advance to which extent a dendrimer will interact with a specific target molecule under particular conditions . It is therefore critical to study the factors that dictate the interactions between dendrimers and external “guest” molecules.…”

“…When this happens throughout a dendrimer, its conformation changes: increased protonation causes the dendrimer to swell and open up for potential guest molecules. , Additionally, protonated amines can interact electrostatically with dissolved anionic species. This property has been explored for dendrimer-drug and dendrimer-dye interactions ,,− and can even be used to create multidendrimer assemblies in the presence of anionic dye molecules that are capable of π–π stacking. , These dendrimer-guest interactions require protonated dendrimer amines, which present a prime opportunity for pH-controlled binding. Although this strategy has received attention, ,− there are relatively few studies that probe dendrimer-guest interactions across a wide pH range and as a function of dendrimer functionalization.…”

“…Notably, each of these protoliths could interact with PAMAM in different manners. For example, fluorescein’s anionic forms might electrostatically interact with amine groups on the dendrimer, , whereas the neutral form might be driven through aromatic and hydrophobic effects, and the cation could electrostatically bind with the carboxylate groups on G5.5COOH . In addition, each protolith can hydrogen-bond to its dendrimer host .…”