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The interaction of the macrocyclic host, cucurbit[7]uril (CB7), with riboflavin (RF) and its derivative, flavin adenine dinucleotide (FAD), has been investigated using absorption and steady-state and time-resolved fluorescence measurements. Interestingly, in the presence of CB7, the fluorescence intensity of RF is quenched, whereas the fluorescence intensity of FAD is enhanced. It is proposed that the fluorescence quenching of RF results from the tautomerization of its isoalloxazine moiety from the lactam to the lactim forms, upon binding to CB7. Such a tautomerization can be brought about since the two lactim forms have higher dipole moments than the lactam form of RF, and thus experience much stronger dipole-dipole interactions (and hence greater binding affinities) with CB7 in the former cases than in the latter. This tautomerization in the presence of CB7 leads to a significant reduction in the observed radiative decay rate and hence a reduction in the fluorescence intensity of RF. Binding of CB7 with RF is confirmed by an increase in the rotational correlation time of RF in the presence of CB7. Geometry optimization studies indicate the formation of an exclusion complex between CB7 and RF, possibly stabilized by H-bonding interactions, as also suggested by the characteristic red shift in the absorption spectra of the CB7-RF system. In the case of FAD, both the isoalloxazine ring and the adenine moiety can interact with the CB7 host. In aqueous solutions, a good fraction of FAD molecules exists in a "closed" conformation with the adenine and isoalloxazine rings stacked together, thus leading to very efficient fluorescence quenching due to the ultrafast intramolecular electron transfer from adenine to the isoalloxazine moiety. Binding of the adenine and/or the isoalloxazine moiety of FAD with CB7 inhibits the stacking interaction and changes the "closed" conformation to the "open" conformation, wherein the adenine and isoalloxazine moieties are widely separated, thus prohibiting the electron transfer process. This reduces the inherent fluorescence quenching of FAD molecule and results in the observed fluorescence enhancement. As observed for RF, the interaction of CB7 with the isoalloxazine ring of FAD should cause fluorescence quenching due to the lactam to lactim tautomerization process. However, in the interplay between the above two opposing effects, the fluorescence enhancement due to the modulation in the conformational dynamics of FAD by the CB7 host predominates. The conformational change is in fact supported by the observation of a long lifetime component in the fluorescence decay of FAD in the presence of CB7. Moreover, at acidic pH, when FAD is already present mainly in the "open" form, the conformational dynamics no longer plays any major role and the fluorescence of FAD is quenched by CB7, as expected, due to the tautomerization at the isoalloxazine moiety.

Noncovalent interaction of Thioflavin T (ThT) with versatile macrocyclic host molecules, namely, cucurbit[7]uril (CB7) and cucurbit[5]uril (CB5), has been investigated in aqueous solutions by photophysical methods. Steady-state and time-resolved fluorescence studies illustrate significant enhancements/modifications in the ThT fluorescence yield, lifetime, and spectral features on interaction with the CBs and are assigned due to the formation of 1:1 and 2:1 complexes between the CBs and the ThT. The high binding constant values for the 1:1 complex (K(1) approximately 10(5) M(-1)) indicate the strong ion-dipole interaction between the host and guest molecules, whereas the 2:1 complex formation is mainly driven by weaker forces like hydrophobic interaction as evident from the lower binding constants (K(2) approximately 10(3) M(-1)). From the characteristic differences in the photophysical properties of the CB7-ThT and CB5-ThT complexes, it has been adjudged that ThT forms an inclusion complex with CB7 whereas with CB5, the interaction is through an exclusion complex formation. These contentions have been further verified by the rotational relaxation dynamics, NMR, and quantum chemical calculations on CB-ThT systems. The present results have also been compared with those reported for the dye in the presence of cyclodextrin hosts.