Bishwajit Ganguly

Ratiometric Detection of Cr3+ and Hg2+ by a Naphthalimide-Rhodamine Based Fluorescent Probe

Newly synthesized rhodamine derivatives, L(1) and L(2), are found to bind specifically to Hg(2+) or Cr(3+) in presence of large excess of other competing ions with associated changes in their optical and fluorescence spectral behavior. These spectral changes are significant enough in the visible region of the spectrum and thus, allow the visual detection. For L(1), the detection limit is even lower than the permissible [Cr(3+)] or [Hg(2+)] in drinking water as per standard U.S. EPA norms; while the receptor, L2 could be used as a ratiometric sensor for detection of Cr(3+) and Hg(2+) based on the resonance energy transfer (RET) process involving the donor naphthalimide and the acceptor Cr(3+)/Hg(2+)-bound xanthene fragment. Studies reveal that these two reagents could be used for recognition and sensing of Hg(2+)/Cr(3+). Further, confocal laser microscopic studies confirmed that the reagent L(2) could also be used as an imaging probe for detection of uptake of these ions in A431 cells.

New Chemodosimetric Reagents as Ratiometric Probes for Cysteine and Homocysteine and Possible Detection in Living Cells and in Blood Plasma

In this work, we have rationally designed and synthesized two new reagents (L(1) and L(2)), each bearing a pendant aldehyde functionality. This aldehyde group can take part in cyclization reactions with β- or γ-amino thiols to yield the corresponding thiazolidine and thiazinane derivatives, respectively. The intramolecular charge-transfer (ICT) bands of these thiazolidine and thiazinane derivatives are distinctly different from those of the molecular probes (L(1) and L(2)). Such changes could serve as a potential platform for using L(1) and L(2) as new colorimetric/fluorogenic as well as ratiometric sensors for cysteine (Cys) and homocysteine (Hcy) under physiological conditions. Both reagents proved to be specific towards Cys and Hcy even in the presence of various amino acids, glucose, and DNA. Importantly, these two chemodosimetric reagents could be used for the quantitative detection of Cys present in blood plasma by using a pre-column HPLC technique. Such examples are not common in contemporary literature. MTT assay studies have revealed that these probes have low cytotoxicity. Confocal laser scanning micrographs of cells demonstrated that these probes could penetrate cell membranes and could be used to detect intracellular Cys/Hcy present within living cells. Thus, the results presented in this article not only demonstrate the efficiency and specificity of two ratiometric chemodosimeter molecules for the quantitative detection of Cys and Hcy, but also provide a strategy for developing reagents for analysis of these vital amino acids in biological samples.

Regioselectivity of Vinyl Sulfone Based 1,3-Dipolar Cycloaddition Reactions with Sugar Azides by Computational and Experimental Studies

DFT (M06-L) calculations on the transition state for the 1,3-dipolar cycloadditions between substituted vinyl sulfones with sugar azide have been reported in conjunction with new experimental results, and the origin of reversal of regioselectivity has been revealed using a distortion/interaction model. This study provides the scientific justification for combining organic azides with two different types of vinyl sulfones for the preparation of 1,5-disubstituted 1,2,3-triazoles and 1,4-disubstituted triazolyl esters under metal-free conditions.

Folding and Unfolding Movements in a [2]Pseudorotaxane

A new dibenzo[24]crown-8 derivative (1) was synthesized and functionalized with aromatic moieties such as naphthalene and coumarin units. These two fluorophores are known to form an effective FRET (Forster resonance energy transfer) pair, and this formed the basis for the design of this host crown ether derivative. Results of the steady-state and time-resolved fluorescence studies confirmed the resonance energy transfer between the donor naphthalene moiety and acceptor coumarin fragment, while NMR spectra and computational studies support a folded conformation for the uncomplexed crown ether 1. This was found to form an inclusion complex, a [2]pseudorotaxane type with imidazolium ion derivatives as the guest molecules with varying alkyl chain lengths ([C(4)mim](+) or [C(10)mim](+)). The host crown ether (1) tends to adopt an open conformation on formation of the interwoven inclusion complex (1·[C(4)mim](+) or 1·[C(10)mim](+)). This change in conformation, from the folded to a open one, was predicted by computational as well as (1)H NMR studies and was confirmed by single crystal X-ray structure for one (1·[C(4)mim](+)) of the two inclusion complexes. The increase in the effective distance between the naphthalene and coumarin moieties in the open conformation of these inclusion complexes was also supported by the decrease in the effective FRET process that was operational between naphthalene and coumarin moieties in the free molecule (1). Importantly, this inclusion complex formation was found to be reversible, and in the presence of a stronger base/polar solvent, such as triethyl amine/DMSO, the deprotonation/effective solvation of the cationic imidizolium ions ([C(4)mim](+) or [C(10)mim](+)) resulted in decomplexation or dethreading with restoration of the original emission spectra for 1, which signifies the subsequent increase in the FRET process. Thus we could demonstrate that a molecular folding-unfolding type of movement in the crown ether derivative could be induced by chemical input as an imidazolium ion.

Strategic design of small and versatile bicyclic organic superbases: a density functional study

It is shown by a reliable DFT method that bicyclic structures 7–10, constructed using 1,3-diaminopropane as an essential building block, can act as powerful neutral organic superbases in the gas phase, aqueous solution and in acetonitrile. This new molecular framework is versatile in terms of anchoring different functional groups to achieve higher basicities. The buttressing effect of substituents on the basicity of these organic bases was also observed. The proton affinities calculated at the B3LYP/6-311+G**//B3LYP/6-31+G* level for 1,2-diaminoethane and 1,3-diaminopropane were found to be in good agreement with the experimental results. Barriers for proton transfer between the N atoms of the diamine and diimine cations are also reported.

A density functional study towards substituent effects on anion sensing with urea receptors

AbstractEffects of substituents on anion binding in different urea based receptors have been examined using density functional (B3LYP/6-311+G**) level of theory. The complexes formed by a variety of substituted urea with a halide anion (fluoride) and an oxy-anion (acetate) have been calculated. The stronger complexes were predicted for receptors with fluoride ion than that of acetate ion, however, in water the preference was found to be reversed. The pKa calculations showed the preferred sites of deprotonation for positional isomers, while interacting with anions. The position of the substituent in the receptor, however, could change the preferred sites of deprotonation compared to the site predicted with pKa values. FigureThe substituent effects on anion binding towards different urea receptors have been examined by DFT with B3LYP/6-311+G** level of theory.

Efficacy of carbenes for CO2 chemical fixation and activation by their superbasicity/alcohol: a DFT study

DFT calculations predicted a highly efficient binary system consisting of acyclic or cyclic carbenes and alkylcarbonic acids for an efficient CO2 capture process. The driving force for such processes seems to be governed by the pKa of the base used. The calculated results suggest that the carbenes possess a much higher pKa compared to the superbases DBU and TBD used in earlier studies. The activation barriers computed for the formation of alkylcarbonate salt with the carbene superbases are also favourable compared to the nitrogen bases reported in the literature. The propylcarbonate salt formation is energetically more preferred with these carbenes than their direct CO2 zwitterionic adduct formation. The steric and electronic effects of such carbenes also play an important role towards the determination of the mode of CO2 capture.

Cation-induced fluorescent excimer emission in calix[4]arene-chemosensors bearing quinoline as a fluorogenic unit: experimental, molecular modeling and crystallographic studies

A number of calix[4]arene-based fluorescent chemosensors containing amide as a binding site and quinoline as a fluorogenic unit have been synthesised and characterized by a single crystal X-ray diffraction study. These compounds have been designed with variations in conformation and steric crowding in the calix moiety to investigate its effect on ion selectivity and thereby on excimer emission. The ion-binding property of these fluoroionophores has been investigated with a large number of cations and anions and the ion-recognition event was monitored by luminescence, UV-Vis and 1H NMR (for anions) spectral changes. Out of a large number of cations, Hg2+, Pb2+, Fe3+ and Cu2+ exhibited strong complexation with all of the ionophores, as evident from luminescence and UV-vis spectroscopy. In the case of anions, F− and HSO4− exhibited strong complexation with two of the ionophores, as shown by fluorescence and NMR spectroscopy. Interestingly, complexation with metal ions resulted in a new band at lower energy due to excimer emission, which was not observed for anions. The binding constants for all of the three fluoroionophores with strongly interacting ions have been determined from fluorescence, UV-vis and NMR titration data. The DFT calculations for all of the three compounds were performed and the results show that the modulation of frontier molecular orbital energies upon complexation of metal ions causes the fluorescent excimer emission.

DFT study of urea interaction with potassium chloride surfaces

In this work, interaction of urea on the important surfaces of potassium chloride has been investigated using density functional theory (DFT) approach. We have performed the DFT calculations using slab models for the interactions of urea with {100}, {110} and {111} surfaces of KCl. Furthermore, the influence of solvent effect was computed with the COSMO continuum model. While sodium chloride crystals are known to change their habit in presence of urea impurity, the higher homologue KCl has not been extensively studied. In one of our earlier articles, we have shown that the preferential interaction of urea with the specific surface of sodium chloride can lead to change the habit from cubes to octahedrons [A. Singh, S. Chakraborty and B. Ganguly, Computational study of urea and its homologue glycinamide: conformations, rotational barriers, and relative interactions with sodium chloride, Langmuir 23 (2007), pp. 5406–5411]. The similar surface docking approach for the interaction of urea with the KCl surfaces suggests that urea would not influence the morphology of KCl. The computed interaction energies were found to be comparable for all the important surfaces of KCl with urea. We have performed experimental studies to examine our computational analysis. The experimental results suggest that the growing KCl crystals in urea solution are cubic in nature. The combined computational and experimental studies corroborate the model of effective interaction of additives with the certain face of crystals and responsible for the change in the morphology [R. Speidel, Neues Jahrbuch fur Mineralogie, Part 4, 81m (1961); N. Cabrera, D.A. Vermilyea. Growth and Perfection of Crystals; John Wiley & Sons Inc. 393. (1958); C.W. Bunn, Adsorption oriented overgrowth and the mixed crystal formation, Proc. R. Soc. A 141 (1933), pp. 567–593].

Synthesis of a novel cage-functionalized chiral binaphthol host: a potential new agent for enantioselective recognition of chiral ammonium salts

Abstract Optically active, cage-functionalized crown ether ( R )- 3 which contains a 1,1′-bi-2-naphthol moiety has been prepared. Subsequently, the ability of ( R )- 3 to selectively recognize the enantiomers of guest ammonium salts, i.e., 4 and 5 in transport experiments was studied. Host ( R )- 3 displays significantly enhanced enantiomeric selectivity toward complex formation with 4 vis-a-vis complex formation with 5 . The relative energetics of various relevant host–guest complexes have been investigated computationally.