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 pK(a) value of 7.5, which has been shifted to 8.1 in the presence of SBE(7)βCD. The changes in the pK(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 pK(a) shift (pK(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 pK(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.