Prototropical and Photophysical Properties of Ellipticine inside the Nanocavities of Molecular Containers

Host-guest interactions between an anticancer drug, ellipticine (EPT), and molecular containers (cucurbitruils (CBn) and cyclodextrins (CD)) are investigated with the help of steady state and time-resolved fluorescence measurements. Our experimental results confirm the formation of 1:1 inclusion complexes with CB7 and CB8. The protonated form of EPT predominantly prevails in the inclusion complexes due to the stabilization achieved through ion-dipole interaction between host and positively charged drug. Drug does not form an inclusion complex with CB6, which is smaller in cavity size compared to either CB7 or CB8. In the case of cyclodextrins, alpha-CD does not form an inclusion complex, whereas beta-CD forms a 1:1 inclusion complex with the protonated form of the drug, and the binding affinity of EPT with beta-CD is less compared to CB7/CB8. Interestingly, in the case of gamma-CD, drug exists in different forms depending on the concentration of the host. At lower concentration of gamma-CD, 1:1 inclusion complex formation takes place and EPT exists in protonated form due to accessibility of water by the drug in the inclusion complex, whereas, at higher concentration, a 2:1 inclusion complex (gamma-CD:EPT) is observed, in which EPT is completely buried inside the hydrophobic cavity of the capsule formed by two gamma-CD molecules, and we believe the hydrophobic environment inside the capsule stabilizes the neutral form of the drug in the 2:1 inclusion complex. Deep insight into the molecular picture of these host-guest interactions has been provided by the docking studies followed by quantum chemical calculations.