Tanmoy Saha

Metal-organic framework based highly selective fluorescence turn-on probe for hydrogen sulphide

Hydrogen sulphide (H2S) is known to play a vital role in human physiology and pathology which stimulated interest in understanding complex behaviour of H2S. Discerning the pathways of H2S production and its mode of action is still a challenge owing to its volatile and reactive nature. Herein we report azide functionalized metal-organic framework (MOF) as a selective turn-on fluorescent probe for H2S detection. The MOF shows highly selective and fast response towards H2S even in presence of other relevant biomolecules. Low cytotoxicity and H2S detection in live cells, demonstrate the potential of MOF towards monitoring H2S chemistry in biological system. To the best of our knowledge this is the first example of MOF that exhibit fast and highly selective fluorescence turn-on response towards H2S under physiological conditions.

Hopping-mediated anion transport through a mannitol-based rosette ion channel.

Artificial anion selective ion channels with single-file multiple anion-recognition sites are rare. Here, we have designed, by hypothesis, a small molecule that self-organizes to form a barrel rosette ion channel in the lipid membrane environment. Being amphiphilic in nature, this molecule forms nanotubes through intermolecular hydrogen bond formation, while its hydrophobic counterpart is stabilized by hydrophobic interactions in the membrane. The anion selectivity of the channel was investigated by fluorescence-based vesicle assay and planar bilayer conductance measurements. The ion transport by a modified hopping mechanism was demonstrated by molecular dynamics simulation studies.

A colorimetric and fluorometric BODIPY probe for rapid, selective detection of H2S and its application in live cell imaging

A BODIPY-azide based colorimetric and fluorescence turn-ON probe for rapid, selective and sensitive detection of H2S is reported. The probe displayed a fast response time (10 min in HEPES and 30 s in serum albumin), 28-fold fluorescence enhancement and low detection limit up to 259 nM. The application of the probe to the estimation of H2S in live cells was demonstrated.

Structural imposition on the off–on response of naphthalimide based probes for selective thiophenol sensing

N-Phenyl-1,8-naphthalimide based fluorescent probes are reported with various positions of dinitrobenzenesulfonyl (DNs) quencher on the phenyl ring. The probe with –DNs at the ortho-position was predicted to show better response compared to those with the quencher at the meta and para positions. The probe showed selective and very fast response (response time, tR = 1 min) toward thiophenol with 20-fold off–on fluorescence sensitivity. Thiophenol sensing applications by the probe were demonstrated via water sample analysis and live cell imaging studies.

Chloride-Mediated Apoptosis-Inducing Activity of Bis(sulfonamide) Anionophores

Transmembrane anion transport modality is enjoying a renewed interest because of recent advances toward anticancer therapy. Here we show bis(sulfonamides) as efficient receptors for selective Cl(-) ion binding and transport across lipid bilayer membranes. Anion-binding studies by (1)H NMR indicate a logical correlation between the acidity of sulfonamide N-H proton and binding strength. Such recognition is influenced further by the lipophilicity of a receptor during the ion-transport process. The anion-binding and transport activity of a bis(sulfonamide) system are far superior compared to those of the corresponding bis(carboxylic amide) derivative. Fluorescent-based assays confirm the Cl(-)/anion antiport as the operational mechanism of the ion transport by bis(sulfonamides). Disruption of ionic homeostasis by the transported Cl(-) ion, via bis(sulfonamide), is found to impose cell death. Induction of a caspase-dependent intrinsic pathway of apoptosis is confirmed by monitoring the changes in mitrochondrial membrane potential, cytochrome c leakage, activation of family of caspases, and nuclear fragmentation studies.

Chloride Transport through Supramolecular Barrel-Rosette Ion Channels: Lipophilic Control and Apoptosis-Inducing Activity

Despite the great interest in artificial ion channel design, only a small number of channel-forming molecules are currently available for addressing challenging problems, particularly in the biological systems. Recent advances in chloride-mediated cell death, aided by synthetic ion carriers, encouraged us to develop chloride selective supramolecular ion channels. The present work describes vicinal diols, tethered to a rigid 1,3-diethynylbenzene core, as pivotal moieties for the barrel-rosette ion channel formation, and the activity of such channels was tuned by controlling the lipophilicity of designed monomers. Selective transport of chloride ions via an antiport mechanism and channel formation in the lipid bilayer membranes were confirmed for the most active molecule. A theoretical model of the supramolecular barrel-rosette, favored by a network of intermolecular hydrogen bonding, has been proposed. The artificial ion-channel-mediated transport of chloride into cells and subsequent disruption of cellular ionic homeostasis were evident. Perturbation of chloride homeostasis in cells instigates cell death by inducing the caspase-mediated intrinsic pathway of apoptosis.

A fluorescent off–on NBD-probe for F− sensing: theoretical validation and experimental studies

The design, synthesis and fluoride sensing ability of a 7-nitro-2,1,3-benzoxadiazole (NBD) based chemodosimeter is reported. Theoretical calculations were used to design a more applicable off-on response, by choosing NBD as the accurate fluorophore. Reaction of the NBD-probe with 300 equivalents of tetrabutyl ammonium fluoride (TBAF) exhibited a response time of 80 minutes and the reaction was selective to F(-) and sensing of the ion was marked by a 110-fold enhancement of green fluorescence. The off-on fluorescence characteristics of the probe enabled its application in live-cell imaging of intracellular F(-) ions.

Trimodal Control of Ion-Transport Activity on Cyclo-oligo-(1→6)-β-D-glucosamine-Based Artificial Ion-Transport Systems.

Cyclo-oligo-(1→6)-β-D-glucosamines functionalized with hydrophobic tails are reported as a new class of transmembrane ion-transport system. These macrocycles with hydrophilic cavities were introduced as an alternative to cyclodextrins, which are supramolecular systems with hydrophobic cavities. The transport activities of these glycoconjugates were manipulated by altering the oligomericity of the macrocycles, as well as the length and number of attached tails. Hydrophobic tails of 3 different sizes were synthesized and coupled with each glucosamine scaffold through the amide linkage to obtain 18 derivatives. The ion-transport activity increased from di- to tetrameric glucosamine macrocycles, but decreased further when flexible pentameric glucosamine was introduced. The ion-transport activity also increased with increasing length of attached linkers. For a fixed length of linkers, the transport activity decreased when the number of such tails was reduced. All glycoconjugates displayed a uniform anion-selectivity sequence: Cl(-) >Br(-) >I(-) . From theoretical studies, hydrogen bonding between the macrocycle backbone and the anion bridged through water molecules was observed.

Pink fluorescence emitting fluoride ion sensor: investigation of the cascade sensing mechanism and bioimaging applications

A resorufin based colorimetric and fluorescence off–on probe for selective fluoride ion detection is reported. 1H-NMR and HPLC experiments confirm the formation of phthalide and resorufin in the deprotection–cyclization based sensing mechanism. The cascade reaction strategy ensures the rapid release (t1/2 = 1.96 min) of pink fluorescent resorufin dye. Selective sensing of fluoride ions results in the 1820-fold fluorescence enhancement. The probe also displays a detection limit of 60 nM (i.e. 1.15 ppb) during the sensing of the ion in water. The permeability of the probe and sensing of intracellular fluoride ions is demonstrated by live cell imaging.

Performance comparison of two cascade reaction models in fluorescence off–on detection of hydrogen sulfide

Comparative studies on the performances of two cascade reaction based fluorescent H2S probes are reported. These probes were also designed to address the solubility issues of existing probes. The Reso-N3 probe favors the H2S mediated azide-to-amine reduction followed by a cyclization to release the resorufin fluorophore. Reso-Br undergoes a bromide-to-thiol nucleophilic substitution followed by a similar cyclization releasing the same fluorophore. Reso-N3 exhibited lower background fluorescence and better H2S sensing behavior in water compared to Reso-Br. Reso-Br underwent hydrolysis in aqueous buffer conditions (pH = 7.4) while, Reso-N3 was quite stable. Reso-N3 displayed high selectivity and sensitivity towards H2S. Live cell imaging of the species by the probe was also established.