Complexation and proton dissociation behavior of 7-hydroxy-4-methylcoumarin and related compounds in the presence of beta-cyclodextrin

The pH effects on the 1:1 complexation behavior of beta -cyclodextrin (beta -CDx) with the title compounds have been investigated by measuring ultravi olet (UV) absorption, circular dichroism (CD), fluorescence and H-1 nuclear magnetic resonance (NMR) spectra and fluorescence lifetimes of the beta -C Dx inclusion complexes formed. Steric effects of substituent attached at th e four position of a guest molecule on the stability of the inclusion compl ex as well as on the induced CD spectral intensity demonstrated that the gu est is included into the beta -CDx cavity so as to locate this substituent around the rim of the cavity at low and even at high pH (where a tautomer a nion is exclusively produced). The H-1 NMR data were consistent with the in tracavity immersion of a given guest molecule from the secondary hydroxy gr oup side and, additionally, substantiated the structure of the beta -CDx in clusion complex generated at low pH. Through an analysis of the pH-dependen t UV absorption and fluorescence spectra of 7-hydroxycoumarine derivatives, it was found that the proton dissociation abilities of these guests underg o negligible effects of the beta -CDx inclusion in both the ground-state an d the excited singlet-state. This finding strongly suggested that both the quinoid-type carbonyl and oxide groups in the tautomer anion guest are expo sed to the bulk aqueous phase to interact with the surrounding water molecu les. (C) 2001 Elsevier Science B.V. All rights reserved.