Prototropical
and Photophysical Properties of Ellipticine
inside the Nanocavities of Molecular Containers
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Abstract
Host–guest interactions between
an anticancer drug, ellipticine
(EPT), and molecular containers (cucurbitruils (CB<i>n</i>) 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, α-CD
does not form an inclusion complex, whereas β-CD forms a 1:1
inclusion complex with the protonated form of the drug, and the binding
affinity of EPT with β-CD is less compared to CB7/CB8. Interestingly,
in the case of γ-CD, drug exists in different forms depending
on the concentration of the host. At lower concentration of γ-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 (γ-CD:EPT)
is observed, in which EPT is completely buried inside the hydrophobic
cavity of the capsule formed by two γ-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