Dynamic Aspects in Host-Guest Interactions. 5. Kinetics of the Metal Complexation Reactions of 2-(5-Bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol Associated with Cyclodextrin Inclusion Reactions

摘要

The kinetics of the 1 : 1 complex formation reactions of 2-(5-bromo-2-pyridylazo)-5-(N-propyl-N-sulfopropylamino)phenol (5-Br-PAPS) with some metal ions in the presence of α-, β-, and γ-cyclodextrin (α-, β-, and γ-CDx) have been investigated by the stopped-flow spectrophotometric method. The inclusion of 5-Br-PAPS by CDx causes a substantial decrease in the second-order formation rate constants owing to the steric hindrance to effective approach of metal ion to the specific orientation of 5-Br-PAPS in the CDx cavity and/or to the slower release of metal-coordinated water molecules in the apolar cavity. Binding of 5-Br-PAPS with cyclodextrins is indicated by change in the UV-vis absorption spectra and the ~1H NMR spectra. It is suggested that the reaction rates are governed by the presence of the encapsulated 5-Br-PAPS, which reacts more slowly with some metal ions in aqueous solution.