Variations of fluorescent molecular sensing for organic guests by regioselective anthranilate modified β- and γ-cyclodextrins

Flexible hosts, regioselectively modified, namely disodium 6(A),6(B)-, 6(A),6(C)- and 6(A),6(D)-dianthranilato-beta-cyclodextrins (beta-1, beta-2 and beta-3, respectively) and gamma-cyclodextrin analogues, disodium 6(A),6(B)-, 6(A),6(C)-, 6(A),6(D)- and 6(A),6(E)-dianthranilato-gamma-cyclodextrins (gamma-1, gamma-2, gamma-3 and gamma-4, respectively) have been synthesized as a sensor for organic guests including terpenoids and bile acids. These host compounds show pure monomer fluorescence, in which beta-1 shows an increase in fluorescence intensity on accommodation of guest species. On the other hand, beta-3 exhibits an increase in intensity on complexation of bile acids and a decrease in intensity for smaller guests such as terpenoids. Host beta-2 exhibits a mixed type of beta-1 and beta-3. The extent of fluorescence variation with a guest is employed to display the sensing abilities of those hosts. The sensing parameter (Delta I/I-0) to describe the sensing ability of the hosts was used. Host beta-1 can detect both small and large guests with high sensitivity. Hosts beta-2 and beta-3 show a similar sensing pattern for guests, while the monoderivative (beta-4) can detect small guests with higher sensitivity, but cannot detect larger guests such as bile acids. In the case of larger hosts such as gamma-1, gamma-2 and gamma-3, they show positive parameter values for small guests such as the terpenoids examined, which means the fluorescence intensity increases on accommodation of a guest, whereas gamma-4 shows negative parameter values. Host gamma-3 exhibits the highest sensitivity for bile acids. The sequence of the binding ability of these hosts is gamma-3 > gamma-4 > gamma-2 > gamma-1. The behavior of the appended moieties of those hosts during a host-guest complexation are studied by induced circular dichroism (ICD) spectra and fluorescence spectra. The ICD spectral patterns of beta-1, beta-2 and beta-3 are quite different. On the other hand, the ICD patterns of gamma-cyclodextrin analogs are similar. For example, the spectrum of gamma-2, alone or in the presence of a guest is very similar to that of gamma-3, indicating that the movements of the appended moieties are very similar. The guest-induced variations in the fluorescence or ICD intensity suggest that the appended moieties act as a spacer or hydrophobic cap which enables the cyclodextrin to form a 1 : 1 host-guest complex.