Abstract
The quenching of 2-acetylnaphthalene (2-AN) fluorescence by hydroxypropyl cyclodextrins (HP-CD) has been analyzed using modified Stern–Volmer plots to obtain binding constants as a function of temperature for 2-AN:HP-CD complexes. The HP-CDs were commercially available and contained 4–7 HP groups per CD molecule for α-CD, β-CD, and γ-CD. HP substitution causes a 12 to over 40% increase in binding constant (<i>K</i><sub>ave</sub>) for 2-AN compared to that for unsubstituted CDs, although the <i>K</i><sub>ave</sub> value is not strongly dependent on the extent of HP substitution for β-CD. No evidence of formation of a 2:2 complex, such as that observed with 2-AN and γ-CD, is observed with 2-AN and HP-γ-CD. Thermodynamic parameters (ΔH° and ΔS°) suggest that the increase in <i>K</i><sub>ave</sub> with HP substitution is due to an enlarged binding site for the HP-CDs that allows greater motional freedom for 2-AN. Comparison is made to the binding of 2-methylnaphthoate (2-MN) to CDs and HP-CDs, and the larger <i>K</i><sub>ave</sub> values for 2-MN over 2-AN are attributed to greater dispersion forces for 2-MN complex formation.
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