The fluorescence enhancement of 1-anilinonaphthalene-8-sulfonate (ANS) by modified β-cyclodextrins

The fluorescence enhancement of the probe molecule 1-anilino-8-naphthalenesulfonate (ANS) by a number of modified beta-cyclodextrins has been studied. Alkyl-and hydroxyalkyl-substituted beta-cyclodextrins show significantly greater enhancement of ANS fluorescence than does the parent unmodified beta-cyclodextrin (beta-CD). In the cases of methyl-beta-cyclodextrin (Me-beta) and hydroxypropyl-beta-cyclodentrin (HP-beta), enhancements by a factor of 120 and 180, respectively, were observed for ANS fluorescence, compared to a factor of only 8.4 in the case of beta-CD The binding constant for formation of the 1:1 ANS:CD complex was determined to be 370 +/- 80 M-1 for Me-beta and 430 +/- 70 M-1 for HP-beta. The large increase in enhancement ability was shown to be a result of the relatively less polar environment experienced by the ANS probe incorporated in the modified as compared to the unmodified cyclodextrin cavities. The dielectric constants of the HP-beta and Me-beta cavities experienced by the included ANS were found to be 22 and 25, respectively. These polarities are similar to that of ethanol, whereas that of beta-CD was found to be 54, a polarity similar to that of a 3.1 methanol:water mixture. (C) 1998 Elsevier Science S.A. All rights reserved.