Abhigyan Sengupta

Modulation of Photophysics and pKa Shift of the Anti‐cancer Drug Camptothecin in the Nanocavities of Supramolecular Hosts

This article reports the pK(a) shift of an anti-cancer drug, 20(S)-camptothecin (CPT), upon encapsulation into the nanocavity of a cucurbit[7]uril (CB7) macrocycle. Steady-state, time-resolved fluorescence and electrospray ionisation mass spectrometry (ESI-MS) studies provide evidence for the formation of both 1:1 and 2:1 (CB7⋅CPT) stoichiometries. Astonishingly, we have found that protonation of CPT takes place at a higher concentration of macrocycle (≥50 μM) when the 2:1 stoichiometric complex develops. However, we did not find any proof for protonation of CPT when it is encased by a β-cyclodextrin cavity, which has a cavity size almost the same as that of CB7. Hence, we conclude that electron-rich carbonyl portals of CB7 have an important role in protonation of the drug in the 2:1 inclusion complex. Docking and semi-empirical quantum chemical calculations have been employed to gain an insight into the molecular picture of orientation of CPT in the inclusion complexes. It is clearly seen from the optimised structure of the 2:1 (CB7⋅CPT) inclusion complex that the quinoline nitrogen of CPT does not reside within either of the CB7 cavities, rather it is almost sandwiched between two CB7 rings, and therefore, it experiences huge electron density exerted by both carbonyl portals of the macrocycles. As a result, the pK(a) of CPT shifts from 1.2 to 6.2. Finally, controlled release of the drug has been achieved through the introduction of NaCl, which is rich in cells, as an external stimulus. We hope this recognition-mediated binding and release mechanism can be useful for activation of the drug and controlled release of the drug in therapeutic uses.

Comparative Study of Flavins Binding with Human Serum Albumin: A Fluorometric, Thermodynamic, and Molecular Dynamics Approach

Flavin adenine dinucleotide (FAD) and flavin mononucleotide (FMN) are derivatives of riboflavin (RF), a water-soluble vitamin, more commonly known as vitamin B(2). Flavins have attracted special attention in the last few years because of the recent discovery of a large number of flavoproteins. In this work, these flavins are used as extrinsic fluorescence markers for probing the microheterogeneous environment of a well-known transport protein, human serum albumin (HSA). Steady-state and time-resolved fluorescence experiments confirm that both FMN and FAD bind to the Sudlows site-1 (SS1) binding pocket of HSA, where Trp214 resides. In the case of RF, a fraction of RF molecules binds at the SS1, whereas the major fraction of RF molecules remains unbound or surface bound to the protein. Moreover, flavin(s)-HSA interactions are monitored with the help of isothermal titration calorimetry, which provides free energy, enthalpy, and entropy changes of binding along with the binding constants. The molecular picture of binding interaction between flavins and HSA is well explored by docking and molecular dynamics studies.

How Does the Urea Dynamics Differ from Water Dynamics inside the Reverse Micelle

In this study, the urea dynamics inside AOT reverse micelle (RM) has been monitored without intervention of water using time-resolved fluorescence techniques from the picosecond to nanosecond time regime. It has been observed that urea dynamics inside the reverse micelle is severely retarded compared to water RM due to the formation of highly networked urea cluster inside the RM. Time-resolved fluorescence anisotropy study also confirms the existence of a confined environment around the dye at higher concentrations of urea inside the reverse micelle. The dynamics of urea-water mixtures inside AOT reverse micelle has also been monitored with increasing urea concentration to get insight about the effect of urea on the overall solvation dynamics feature. It has been observed that with the increase in urea concentration, the overall dynamics becomes slower, and it infers the presence of few water or urea molecules, those strongly associated with surrounding urea and (or) water by hydrogen bonds.

Cucurbit[7]uril assisted ultraviolet to visible fluorescence switch of a heart medicine

Host-guest interactions between cucurbit[7]uril (CB7) and a cardiotonic drug, milrinone, have been explored using steady state and pico-second time-resolved techniques. A novel fluorescence switch from ultraviolet (UV) to visible (cyan) is observed as a consequence of upward pKa shift of the drug inside the nano-cavity of cucurbit[7]uril.

Supramolecular Host‐Inhibited Excited‐State Proton Transfer and Fluorescence Switching of the Anti‐Cancer Drug, Topotecan

The effect of cucurbit[7]uril (CB[7]) nano-caging on the photophysical properties, particularly excited-state proton transfer (ESPT) reaction, of an eminent anti-cancer drug, topotecan (TPT), is demonstrated through steady-state and time-resolved fluorescence measurements. TPT in water (pH 6) exists exclusively as the cationic form (C) in the ground state. However, the drug emission mainly comes from the excited-state zwitterionic form (Z*) of TPT, and is attributed to water-assisted ESPT between the 10-hydroxyl group and water, which leads to the transformation of C* to Z* of TPT. In the presence of CB[7], it is found that selective encapsulation of the C form of TPT results in the formation of a 1:1 inclusion complex (CB[7]:TPT), and the ESPT process is inhibited by this encapsulation process. As a result, C* becomes the dominant emitting species in the presence of CB[7] rather than Z*, and fluorescence switching takes place from green to blue. Time-resolved studies also support the existence of CB[7]-encapsulated cationic species as the major emitting species in the presence of the macrocyclic host. Semi-empirical quantum chemical calculations are employed to gain insight into the molecular picture of orientation of TPT in the inclusion complex. It is clearly seen from the optimised structure of 1:1 CB[7]:TPT inclusion complex that both 10-hydroxyl and 9-dimethylaminomethylene groups of TPT lie partly inside the cavity, and thereby inhibit the excited-state transformation of C* to Z* by the ESPT process. Finally, controlled release of the drug is achieved by means of fluorescence switching by introducing NaCl, which is rich in cells, as an external stimulus.

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, 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.

A Green Solvent Induced DNA Package

Mechanistic details of DNA compaction is essential blue print for gene regulation in living organisms. Many in vitro studies have been implemented using several compaction agents. However, these compacting agents may have some kinds of cytotoxic effects to the cells. To minimize this aspect, several research works had been performed, but people have never focused green solvent, i.e. room temperature ionic liquid as DNA compaction agent. To the best of our knowledge, this is the first ever report where we have shown that guanidinium tris(pentafluoroethyl)trifluorophosphate (Gua-IL) acts as a DNA compacting agent. The compaction ability of Gua-IL has been verified by different spectroscopic techniques, like steady state emission, circular dichroism, dynamic light scattering and UV melting. Notably, we have extensively probed this compaction by Gua-IL through field emission scanning electron microscopy (FE-SEM) and fluorescence microscopy images. We also have discussed the plausible compaction mechanism process of DNA by Gua-IL. Our results suggest that Gua-IL forms a micellar kind of self aggregation above a certain concentration (≥1 mM), which instigates this compaction process. This study divulges the specific details of DNA compaction mechanism by a new class of compaction agent, which is highly biodegradable and eco friendly in nature.

Spectroscopic and Thermodynamic Insights into the Interaction between Proflavine and Human Telomeric G‑Quadruplex DNA

The G-quadruplex (GQ-DNA), an alternative structure motif of DNA, has emerged as a novel and exciting target for anticancer drug discovery. GQ-DNA formed in the presence of monovalent cations (Na(+)/K(+)) by human telomeric DNA is a point of interest due to their direct relevance for cellular aging and abnormal cell growths. Small molecules that selectively target and stabilize G-quadruplex structures are considered to be potential therapeutic anticancer agents. Herein, we probe G-quadruplex and proflavine (a well-known DNA intercalator, hence acting as an anticarcinogen) association through steady state and time-resolved fluorescence spectroscopy to explore the effect of stabilization of GQ-DNA by this well-known DNA intercalator. The structural modifications of G-quadruplex upon binding are highlighted through circular dichroism (CD) spectra. Moreover, a detailed insight into the thermodynamics of this interaction has been provided though isothermal titration calorimetry (ITC) studies. The thermodynamic parameters obtained from ITC help to gain knowledge about the nature as well as the driving forces of binding. This present study shows that proflavine (PF) can act as a stabilizer of telomeric GQ-DNA through an entropically as well as enthalpically feasible process with high binding affinity and thereby can be considered as a potential telomerase inhibitor.

Unraveling the mode of binding of the anticancer drug topotecan with dsDNA

The binding interactions between antitumor drugs and DNA are of burgeoning interest due to increasing demand in medicinal science. In the present work, we have tried to examine the mode of binding of topotecan (TPT) with DNA. TPT, an eminent anti-cancer drug from the Camptothecin family, is found to interact with DNA Topoisomerase-I and inhibits the DNA replication process. Steady state, time resolved fluorescence, circular dichroism and thermal melting studies have been utilized to explore the mode of binding of TPT with synthetic polynucleotides ((dA-dT)15, (dG-dC)15) and natural DNA (CT-DNA). The mode of binding of TPT with the DNA double helix has been substantiated to be principally groove binding. It is found that even though the ground state cationic form (C) of the drug binds to dsDNA irrespective of DNA sequences, the emission mainly appears from Z*, and it is attributed to the intermolecular excited state proton transfer (ESPT) reaction between the drug and surrounding water molecules. However, in the case of (dA-dT)15, the emission profile indicates the existence of a small population of excited state cationic form (C*) of the drug in the minor groove of DNA. The different photophysical behavior of TPT in the case of (dA-dT)15 compared to others is attributed to the narrower and deeper minor groove of (dA-dT)15 than that of the others. The exact molecular picture of the binding interaction between the drug and DNAs has been explored from molecular modeling studies.

An anticancer drug to probe non-specific protein–DNA interactions

A visible fluorescence switch of an eminent anti-carcinogen, ellipticine has been used to probe non-specific protein-DNA interaction. The unique pattern of protein-DNA complexation is depicted for the first time through field emission scanning electron microscopy (FE-SEM) images and spectroscopic techniques.