Spectroscopy and Dynamics of Cryptolepine in the Nanocavity of Cucurbit[7]uril and DNA

Herein, we explored the photophysical properties of the antimalarial, anticancer drug cryptolepine (CRYP) in the presence of the macrocyclic host cucurbit[7]uril (CB7) and DNA with the help of steady-state and time-resolved fluorescence techniques. Ground-state and excited-state calculations based on density functional theory were also performed to obtain insight into the shape, electron density distribution, and energetics of the molecular orbitals of CRYP. CRYP exists in two forms depending on the pH of the medium, namely, a cationic (charge transfer) form and a neutral form, which emit at λ=540 and 420 nm, respectively. In a buffer solution of pH 7, the drug exists in the cationic form, and upon encapsulation with CB7, it exhibits a huge enhancement in fluorescence intensity due to a decrement in nonradiative decay pathways of the emitting cryptolepine species. Furthermore, docking and quantum chemical calculations were employed to decipher the molecular orientation of the drug in the inclusion complex. Studies with natural DNA indicate that CRYP molecules intercalate into DNA, which leads to a huge quenching of the fluorescence of CRYP. Keeping this in mind, we studied the DNA-assisted release of CRYP molecules from the nanocavity of CB7. Strikingly, DNA alone could not remove the drug from the nanocavity of CB7. However, an external stimulus such as acetylcholine chloride was able to displace CRYP from the nanocavity, and subsequently, the displaced drug could bind to DNA.