Nilotpal Barooah

Thioflavin T as an Efficient Inducer and Selective Fluorescent Sensor for the Human Telomeric G-Quadruplex DNA

The quest for a G-quadruplex specific fluorescent sensor among other DNA forms under physiological salt conditions has been addressed in this article. We demonstrate for the first time the application of a water-soluble fluorogenic dye, Thioflavin T (ThT), in a dual role of exclusively inducing quadruplex folding in the 22AG human telomeric DNA, both in the presence and absence of Tris buffer/salt, and sensing the same through its fluorescence light-up having emission enhancement of the order of 2100-fold in the visible region. Appropriate conditions allow an apparent switch over of the parallel quadruplex structure in 22AG-ThT (50 mM Tris, pH 7.2) solution to the antiparallel form just by the addition of K(+) ions in the range 10-50 mM. Moreover, addition of ThT cooperatively stabilizes the K(+) induced antiparallel quadruplexes by a ΔT(m) ∼11 °C. The distinction of ThT as a quadruplex inducer has been contrasted with the erstwhile used structurally related dye, Thiazole Orange (TO), which did not induce any quadruplex folding in the 22AG strand in the absence of salt. The striking fluorescence light-up in ThT on binding to the human telomeric G-quadruplex is shown to be highly specific compared to the less than 250-fold enhancement observed with other single/double strand DNA forms. This work has implication in designing new generation dyes based on the ThT scaffold, which are highly selective for telomeric DNA, for potential diagnostic, therapeutic, and ion-sensing applications.

Stimulus-responsive supramolecular pKa tuning of cucurbit[7]uril encapsulated coumarin 6 dye.

This article reports an efficient host-assisted guest protonation mechanism in coumarin 6 (C6) dye, upon its interaction with cucurbit[7]uril (CB7) macrocycle. C6 uptakes the CB7 macrocycle both in 1:1 and 2:1 (CB7/C6) stoichiometries, which brings out a large upward pK(a) shift (from ~0.8 to 6), and facilitates the protonation of C6 at normal pH conditions, having significant photochemical implications. Controlled dissociation of the assemblies has been achieved through their response to stimuli like temperature change or metal ions. By this approach, a specific form of the guest can be activated and could provide a simple stimulus for the controlled drug/dye delivery. Furthermore, the host-assisted guest protonation improves the stability and aqueous solubility of C6 and is a promising candidate for aqueous-based supramolecular dye laser system. Such simple protocol leading to photoswitchable systems having aqueous solubility and biocompatibility can in principle be evolved into a general strategy to deliver and operate potential functional molecular components under various trigger control.

Recognition-Mediated Light-Up of Thiazole Orange with Cucurbit[8]uril: Exchange and Release by Chemical Stimuli

This article reports a convenient supramolecular strategy to construct fluorescent photoswitchable molecular assemblies between a macrocyclic host, cucurbit[8]uril (CB8), and a fluorogenic dye, thiazole orange (TO). The interaction mechanism and the stable stoichiometric host-guest arrangements have been claimed on the basis of the optical absorption, steady-state and time-resolved fluorescence lifetime and anisotropy measurements, and also the geometry optimization studies. The CB8 recognized TO in its 2:2 stoichiometry exhibited spectacular fluorescence enhancement of the order of 1700 fold, which is the largest directly determined value so far reported for a dye in an organic macrocyclic system. This prospective 2CB8:2TO assembly responded to selected chemical stimuli such as metal ions, adamantylamine, and tryptophan, providing different dissociation mechanisms and demonstrating a controlled exchange and release action desired with such noncovalently linked assemblies. Positively, considering the aqueous solubility and biocompatibility of the host-guest constituents, this methodology can evolve into a general approach to deliver and operate intracellularly functional molecular components under chemical/thermal/optical trigger control, especially for therapeutic applications.

Supramolecular assembly of hoechst-33258 with cucurbit[7]uril macrocycle

Molecular assemblies of potential guest molecules through non-covalent host-guest interactions have found immense use in many applied areas. In this study supramolecular interaction of a biologically important dye Hoechst-33258 (H33258) has been investigated in aqueous solutions at different pHs, in the presence of a macrocyclic host, namely, cucurbit[7]uril (CB7). The pH dependent emission behaviour of H33258 is inherently connected with its protolytic equilibria which allow the dye in different geometrical conformations. This pH dependent structural orientation is greatly affected by the complexation with CB7. The significant structural changes in the monocationic H33258 brought out by CB7 at pH 7 have been documented in the fluorescence emission and lifetime data, which are comparatively less affected in case of the dicationic form, which is prominent in dye solutions at pH 4.5. The strong ion-dipole interactions provided by the carbonyl portals of the CB7 host adequately stabilize the CB7-H33258 complex, both in 1:1 and 2:1 stoichiometries at both the pH conditions. The Jobs plot method, fluorescence anisotropy, NMR measurements and geometry optimization calculations confirm the stoichiometric arrangement and are found to be tunable with the addition of metal ions. The non-covalently stabilized assembly brings out large enhancement in the fluorescence emission due to the unique structural orientation attained by H33258, which reduces the non-radiative relaxation pathways. Comparison of the spectral data of the dye at different pH conditions in the absence and presence of CB7 proposes a large upward pK(a) shift due to CB7 encapsulation, thus providing a handy tool to modulate the photophysical characteristics of the guest molecules.

Synergistic Effect of Intramolecular Charge Transfer toward Supramolecular pKa Shift in Cucurbit[7]uril Encapsulated Coumarin Dyes

This article presents the process and mechanism of supramolecular pKa shift in two bichromophoric coumarin laser dyes, namely, coumarin 7 (C7), (ΔpK(a) = 4.6) and coumarin 30 (C30), (ΔpK(a) = 3.0), achieved by introducing a synthetic macrocyclic receptor, cucurbit[7]uril (CB7), in aqueous media. The intramolecular charge transfer, from the diethylamino coumarin moiety toward the benzimidazolyl moiety and its protonation, even at pH ∼8, is facilitated by the interaction of the cucurbituril host in a 2:1 (CB7/dye) stoichiometric ratio. The CB7 macrocycle interacts with C7/C30 dyes in a stepwise manner with binding constants of the order of K(1) ≅10(5) M(-1), K2 ≅10(4) M(-1) for both C7 and C30 dyes. This study underlines a structure-property relationship to explain the host induced changes in the stereoelectronic distributions in the guest dyes supporting the supramolecular pK(a) shifts and is appropriately established by both experimental and theoretical considerations. On the other hand, the increased solubility (>250 times) and enhancement in fluorescence intensity (>13-fold) of the coumarin dyes in the presence of CB7 also find applications for developing aqueous dye laser systems where this supramolecular strategy will largely suppress the disadvantages of low solubility, aggregation, lower emission, or low stability of the dye in aqueous medium.

Stimuli-responsive Cucurbit[7]uril-mediated BSA Nanoassembly for Uptake and Release of Doxorubicin

We report the construction of a non-toxic nanoassembly of bovine serum albumin (BSA) protein and the cucurbit[7]uril macrocycle as well as its stimuli-responsive breakage with adamantylamine or pH, which restores the protein structure and recognition properties. The assembly showed efficient loading and controlled release of a standard drug, doxorubicin (DOX), and the same was validated in live cells. The cell viability studies documented that the DOX-loaded assembly mask the cytotoxicity of DOX and the toxicity can be revived at the target on demand, triggering its therapeutic activation. This is found to be more effective in the cancer cells. In addition, such host-assisted protein assemblies are also highly promising for stabilizing/protecting the native protein structure, a viable approach to prevent/inhibit protein misfolding and aggregation.

Metal-ion-mediated assemblies of thiazole orange with cucurbit[7]uril: a photophysical study.

The formation of molecular superstructures by metal-ion-mediated noncovalent self-assembly has been demonstrated using the macrocycle, cucurbit[7]uril (CB7), and the dye, thiazole orange (TO), as building blocks. Interestingly, the association of these molecular building blocks can be tuned by the chemical environment, leading to self-assembled structures of different stoichiometries, which is supported by absorption, fluorescence, (1)H NMR, and AFM measurements. Most importantly, the self-assembly process of the CB7/TO/metal ion system is observed to be remarkably different for alkali (Na(+)) and alkaline earth (Ca(2+)) metal ions. Fluorescence enhancement is observed in the presence of Ca(2+) ions, which is attributed to the formation of short dimeric structures composed of two 1:1 CB7-TO complexes. Solution turbidity is detected in the presence of Na(+) ions, which is proposed to be due to the formation of extended structures by the assembly of many 1:1 CB7-TO complexes.

Molecular-recognition-assisted pKa shifts and metal-ion-induced fluorescence regeneration in p-sulfonatocalix[6]arene-encapsulated acridine.

The host-guest interactions of cationic (AcH(+) ) and neutral (Ac) forms of the dye acridine with the macrocyclic host p-sulfonatocalix[6]arene (SCX6) were investigated by using ground-state absorption, steady-state and time-resolved fluorescence, and NMR measurements. The cationic form undergoes significant complexation with SCX6 (Keq =2.5×10(4)  M(-1) ), causing a sharp decrease in the fluorescence intensity and severe quenching in the excited-state lifetime of the dye. The strong binding of the AcH(+) form of the dye with SCX6 is attributed to ion-ion interactions involving the sulfonato groups (SO3 (-) ) of SCX6 and the positively charged AcH(+) at pH of approximately 4.3. Whereas, the neutral Ac form of the dye undergoes weak complexation with SCX6 (Keq =0.9×10(3)  M(-1) ) and the binding constant is lowered by one order of magnitude compared with that of the SCX6-AcH(+) system. The strong affinity of SCX6 to the protonated form leads to a large upward pKa shift (≈2 units) in the dye. In contrast, strong emission quenching upon SCX6 interaction and the regeneration of fluorescence intensity of the dye in the presence of Gd(3+) through competitive binding have also been demonstrated.

Mechanistic Aspects of Host–Guest Binding in Cucurbiturils: Physicochemical Properties

This article provides a brief account of the supramolecular chemistry of host–guest interactions and their binding mechanisms. The characterization of host–guest complexes, especially those based on cucurbituril (CB) macrocycles, by different methods such as absorption/fluorescence spectroscopy, 1 H NMR, mass spectrometry, and isothermal titration calorimetry has been described, especially with regard to determination of the stoichiometry and binding constants. Emphasis has been laid on the photophysics of CB-based supramolecular assemblies of fluorescent dyes that have technological and biological importance. The spectacular changes in their physicochemical properties such as solubility, stability, aggregation behavior, acidity constants, and cytotoxicity are also described. Effort has been made to give suitable examples of CB-encapsulated guest assemblies and to discuss their dynamic response toward external triggers such as light and metal ions. The potential applications of these host–guest systems to controlled uptake and release of drugs, construction of photo-functional devices, aqueous-based dye lasers, molecular architectures, etc., have been discussed.

Ultra-Bright Rhodamines with Sulfobutylether-β-Cyclodextrin: A Viable Supramolecular Dye Laser in Aqueous Medium

Although aqueous dye lasers are much sought after, they have been of no practical use, as laser dyes show a strong tendency for aggregation in water, thus diminishing their optical output. Contributing towards this shortcoming, we studied the noncovalent interactions of two prominent laser dyes, namely, rhodamine 6G and rhodamine B, with a water soluble macrocyclic host, sulfobutylether-β-cyclodextrin (SBE7 βCD). Spectral changes in the absorption and fluorescence behavior of dyes in presence of the SBE7 βCD host indicated adequate complex formation between dye and host (K∼104  M-1 ). A combination of various photophysical parameters evaluated from measurements such as Job plot, changes in the fluorescence lifetime/anisotropy values, and favorable thermodynamic parameters from isothermal titration calorimetric measurements adjudicated a 1 : 1 stoichiometric complex formation between dye and SBE7 βCD host. Consequently, SBE7 βCD prevents dye aggregation/adsorption and present rhodamine dyes in their monomeric forms with enhanced fluorescence yield and brightness. These vital parameters were utilized to optimize and demonstrate cost-effective supramolecular broad-band and narrow-band aqueous dye laser systems with improved lasing efficiencies (∼25 % higher for the SBE7 βCD : RhB system and ∼10 % higher for SBE7 βCD : Rh6G system), better beam profile, and enhanced durability compared to the respective dyes in optically matched ethanol solutions.