Enhanced Characterization of Pyrene Binding in Mixed Cyclodextrin Systems via Fluorescence Spectroscopy
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ORIGINAL ARTICLE
Enhanced Characterization of Pyrene Binding in Mixed Cyclodextrin Systems via Fluorescence Spectroscopy Mindy Levine 1
&
Benjamin R. Smith 2
Received: 4 February 2020 / Accepted: 23 June 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Although significant effort has been expended to analyze the binding of pyrene in β-cyclodextrin and γ-cyclodextrin, little has been published on the binding of this guest in β-cyclodextrin derivatives (methyl-β-cyclodextrin and 2-hydroxypropyl-βcyclodextrin) or in mixtures of such derivatives, despite the fact that these derivatives are known to have different supramolecular properties that facilitate unique modes of complexation. Reported herein is a detailed spectroscopic investigation of the binding of pyrene in β-cyclodextrin derivatives and in binary mixtures of cyclodextrins. Py values, defined as the ratio of representative vibronic bands in the fluorescence emission of pyrene, were used to measure changes in the pyrene microenvironment in the presence of the cyclodextrin hosts, and indicated that unmodified β-cyclodextrin is unique in providing a fully hydrophobic environment for pyrene through the use of two cyclodextrins to bind a single pyrene guest. By comparison, both γ-cyclodextrin and modified β-cyclodextrin analogues bind pyrene in a less hydrophobic environment through 1:1 binding stoichiometries that allow for continued interactions between the incompletely encapsulated pyrene guest and the aqueous solvent system. Binary mixtures of cyclodextrins were also explored and reinforce the unique properties of the unmodified β-cyclodextrin host. Keywords Cyclodextrin . Pyrene . Py values . Fluorescence spectroscopy
Introduction Cyclodextrin hosts have been reported to bind a variety of small molecule guests [1], with applications of such binding including enhanced drug delivery [2], pollutant removal [3], chemical sensing [4], and supramolecular catalysis [5]. Guest binding to cyclodextrin can be driven by several different factors, including the displacement of “high-energy” water from the cyclodextrin cavity [6], favorable hydrophobic association between a hydrophobic guest and the hydrophobic cyclodextrin cavity [7], and intermolecular hydrogen bonding between small molecule guests and the hydroxyl groups of the
Electronic supplementary material The online version of this article (https://doi.org/10.1007/s10895-020-02572-5) contains supplementary material, which is available to authorized users. * Mindy Levine [email protected] 1
Department of Chemical Sciences, Ariel University, 65 Ramat HaGolan Street, Ariel, Israel
2
Atlanta, USA
cyclodextrin host [8]. A variety of cyclodextrin hosts have been reported in the literature, with the most commonly used host, β-cyclodextrin, also one of the least soluble [9]. Derivatives of β-cyclodextrin have been developed to provide improved solubility while maintaining the high complexation efficiency of β-cyclodextrin, and include randomly methylated β-cyclodextrin [10],
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