Unimodal Lorentzian lifetime distributions for the 2-anilinonaphthalene-6-sulfonate-β-cyclodextrin inclusion complex recovered by multifrequency phase-modulation fluorometry

The inclusion complex of 2-anilinonaphthalene-6-sulfonic acid (2,6-ANS) and β-cyclodextrin (β-CD) was studied by frequency-domain fluorometry. Fluorescence lifetimes were measured in the presence of various concentrations of dynamic quenchers (Cu²⁺, acrylamide, and I^-). Frequency-domain data were analyzed by global analysis linked by the Stern-Volmer equation using both discrete and distributed models. A unimodal Lorentzian distribution model was determined to be the best fit to the experimental data because the quenching constants recovered matched those from steady-state quenching measurements and χ² values were relatively small. This conclusion was also confirmed by measuring the lifetime of the inclusion complex (2,6-ANS-β-CD) at different temperatures. The quenching constants are discussed for these three diverse quenchers in solutions of 2,6-ANS in water, ethanol, and β-CD measured at temperatures ranging from 20 to 50°C. The effects of different solutes on the inclusion complex formation constant are also reported.