Tautomerization of lumichrome promoted by supramolecular complex formation with cucurbit[7]uril

Addition of cucurbit[7]uril to an aqueous solution of lumichrome led to the formation of a new red-shifted band in both the absorption and fluorescence spectra indicating that binding to this rigid macrocyclic host facilitated the partial transformation into isoalloxazine-type structure. Global analysis of the results of spectrophotometric and spectrofluorometric titrations provided log K = 3.92 ± 0.06 for the logarithm of the equilibrium constant of 1:1 association. The dual fluorescence of the complex was quenched by protons in a diffusion-controlled process. Cucurbit[7]uril was found to exert negligible effect on the deprotonation of lumichrome in basic solution.