Fluorescence of the Complexes of 2-Methylnaphthoate and 2-Hydroxypropyl-α-, -β-, and -γ-Cyclodextrins in Aqueous Solution

Abstract

Fluorescence techniques were employed to study the inclusion complexes of 2-methylnaphthoate (MN) with 2-hydroxypropyl-α-cyclodextrin (αHPCD), 2-hydroxypropyl-β-cyclodextrin (βHPCD), and 2-hydroxypropyl-γ-cyclodextrin (γHPCD). Emission spectra of MN show two vibronic bands whose intensity ratio R is very sensitive to the polarity of the medium. The stoichiometry and formation constants of these complexes were investigated by obtaining R as a function of the cyclodextrin (CD) concentration. Results showed identical stoichiometry (1/1) for the three MN/αHPCD, MN/β-HPCD, and MN/γHPCD complexes. Formation constants at 25 °C were 780 ± 15, 2700 ± 130, and 165 ± 10 M−1, respectively. ΔH0 and ΔS0 were obtained from linear van't Hoff plots. Results reveal that the complexation of MN with αHPCD is enthalpy driven. With βHPCD, both the entropy and enthalpy terms favor the process, whereas the formation of the complex with γHPCD is entropically governed. The extrapolation of R at infinite CD concentration allows us to estimate the effective dielectric constants of the inner CD cavities, which are around 50, but which differ from their counterparts, the naturally occurring α-, β-, and γ-CDs. Fluorescence anisotropy, quencher lifetimes, and average lifetimes can also give additional information about the structure and driving forces accompanying the formation of such complexes.

Xylylene Clips for the Topology-Guided Control of the Inclusion and Self-Assembling Properties of Cyclodextrins

“…Further experimental observations were in agreement with this hypothesis. Thus, the R value extrapolated at infinite concentration of the guest ( R ∞ ), known to be independent of the instrumental conditions and broadly used to probe the polarity of the surrounding medium ( R ∞ decreases from 1.1 to 1.2 in bulk water to <1 and progressively lower in media with increasing hydrophobicity), unequivocally demonstrates that MN is located in a more polar environment when complex with 5 ( R ∞ = 0.63–0.54 in the 5–45 °C range) relative to 4 ( R ∞ = 0.37–0.15). Moreover, the 1 H NMR spectrum of the 5 :MN complex, but not that of the 4 :MN complex (see the SI, Figures S31 and S32), exhibited broad signals from the guest, which is characteristic of the existence of a chemical exchange process relatively slow in the NMR time scale.…”

Study of the Conformational and Self-Aggregation Properties of 2I,3I-O-(o-Xylylene)-per-O-Me-α- and -β-cyclodextrins by Fluorescence and Molecular Modeling

“…Quenching measurements were carried out in o Xy and X m CD water solutions at different temperatures by using 2,3-butanedione (diacetyl) as a quencher. This quencher was employed before in the complexation studies of some naphthalene derivatives with CDs, showing that the host−guest (1:1) equilibrium apparently does not change upon quencher addition. , Measurements for X m αCD and X m βCD were performed at 3.96 and 3.72 mM concentrations, respectively, for which the dimer (X m CD) 2 molar fractions were similar (∼22%). Stern−Volmer ⟨τ⟩ q =0 /⟨τ⟩ plots, which are reasonably linear in the 0 to 7 × 10 −2 M quencher concentration range, exhibit a downhill deviation above this concentration.…”

“…These changes influence the fluorescence characteristics of aromatic guest molecules, allowing the thermodynamics of complexation to be studied. [5][6][7][8][9][10] A chromophore group can also be covalently attached to a given CD or CD derivative to obtain fluorescent hosts. Depending on the size and shape of this group and the flexibility of the junction to the CD core, it is capable of hindering, inducing, or even modulating the complexation of guest molecules, as well as modifying the self-association properties of the modified CD.…”

“…They are able to form inclusion complexes with a variety of low molecular weight and polymer guests in water solution. − As the inner CD cavity is relatively hydrophobic, both polarity and microviscosity of the medium surrounding the guest molecule are substantially modified upon complexation in water. These changes influence the fluorescence characteristics of aromatic guest molecules, allowing the thermodynamics of complexation to be studied. − A chromophore group can also be covalently attached to a given CD or CD derivative to obtain fluorescent hosts. Depending on the size and shape of this group and the flexibility of the junction to the CD core, it is capable of hindering, inducing, or even modulating the complexation of guest molecules, as well as modifying the self-association properties of the modified CD. − Both the complexation of a guest and the host self-association processes are competitive with each other and, if self-inclusion phenomena take place, eventually with the inside-to-outside conformational equilibrium of the CD substituent.…”

Predicting self-assembly and structure in diluted aqueous solutions of modified mono- and bis-β-cyclodextrins that contain naphthoxy chromophore groups

“…These changes influence guest chromophore spectroscopic properties, allowing the thermodynamics and the structure of the complex to be studied. [3][4][5][6][7][8][9][10][11] A chromophore can be covalently attached to a bCD macroring in order to obtain fluorescent mono-CD derivative hosts. Appended groups are capable of hindering or favouring the complexation of guests, or even modifying their capabilities of aggregation into dimers, according to their shape, size and flexibility of the link to the core of the molecule.…”