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Binding Properties of 2-Acetylnaphthalene with Hydroxypropyl Cyclodextrins from Fluorescence Quenching Experiments

Volume 59, Number 5 (May 2005) Page 691-695

Seel, Maximilian; Werner, T.C.

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 (Kave) for 2-AN compared to that for unsubstituted CDs, although the Kave 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 Kave 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 Kave values for 2-MN over 2-AN are attributed to greater dispersion forces for 2-MN complex formation.