Supramolecular interaction of sanguinarine dye with sulfobutylether-β-cyclodextrin: modulation of the photophysical properties and antibacterial activity.

The noncovalent host-guest interaction of sanguinarine (SGR), a benzophenanthridine alkaloid, with a nontoxic, water soluble sulfobutylether-β-cyclodextrin (SBE 7 βCD, commercially available as Captisol) macrocyclic host has been investigated using ground-state optical absorption, and steady-state and time-resolved fluorescence measurements. The pH-dependent changes in the absorbance of the dye at 327 nm showed a p K a value of 7.5, which has been shifted to 8.1 in the presence of SBE 7 βCD. The changes in the p K a values, absorption and fluorescence spectra, and fluorescence lifetime values of these two forms of SG with SBE 7 βCD indicate complex formation between them. The cationic form shows 3 times higher interaction towards SEB 7 βCD ( K = 1.2 × 10 4 M -1 ) as compared to the neutral form ( K = 3.9 × 10 3 M -1 ) which leads to a moderate upward p K a shift (p K a values of SGR shifted by more than 0.6 units). The subsequent fluorescence "turn off" was demonstrated to be responsive to chemical stimuli, such as metal ions (Ca 2+ ions). Upon addition of Ca 2+ ions, nearly quantitative dissociation of the complex was established to regenerate the free dye and result in fluorescence "turn on". Apart from improving the stability under ambient light conditions, the upward p K a shift of SGR in the presence of SBE 7 βCD results in increasing the antibacterial activity of the SBE 7 βCD:SGR complex compared to that of the free dye towards four pathogenic micro-organisms at the physiological pH range. This work further compares SGR interaction with parent β-cyclodextrin.