Subhendu Sekhar Bag

Installation/modulation of the emission response via click reaction.

We have demonstrated the installation of a fluorescence property into a nonfluorescent precursor and modulation of an emission response of a pyrene fluorophore via click reaction. The synthesized fluorophores show different solvatochromicity and/or intramolecular charge transfer (ICT) feature as is revealed from the UV-visible, fluorescence photophysical properties of these fluorophores, and DFT/TDDFT calculation. We observed that some of the synthesized fluorophores showed purely ICT character while emission from some of them arose from the LE state. A structureless and solvent polarity-sensitive dual emission behavior was observed for one of the triazolylpyrene fluorophores that contains an electron-donating -NMe(2) substituent (fluorophore, 7a). Conversely, triazolylpyrene with an electron-withdrawing -CN group (fluorophore, 7b) showed a solvent polarity-independent vibronic emission. The effect of ICT on the photophysical properties of these fluorophores was studied by fluorescence emission spectra and DFT/TDDFT calculations. Fluorescence lifetimes were also measured in different solvents. All of our findings revealed the delicate interplay of structure and emission properties and thus having broader general utility. As the CT to LE intensity ratio can be employed as a sensing index, the dual emissive fluorophore can be utilized in designing the molecular recognition system too. We envisage that our investigation is of importance for the development of new fluorophores with predetermined photophysical properties that may find a wide range of applications in chemistry, biology, and material sciences.

Singly and doubly labeled base-discriminating fluorescent oligonucleotide probes containing oxo-pyrene chromophore.

We have developed new oxo-pyrene labeled fluorescent nucleoside, (Oxo-Py)U which showed a strong fluorescence dependency on solvent polarity at long wavelength. The designed singly and doubly (Oxo-Py)U labeled fluorescent oligonucleotide probes were found highly efficient for the discrimination of A and consecutive AA bases of target DNA opposite to the labeled base via generation of enhanced fluorescence signal.

Selectivity in Garratt−Braverman Cyclization: An Experimental and Computational Study

Bispropargyl sulfones equipped with aromatic rings of dissimilar nature were synthesized. Under basic conditions, these sulfones isomerized to the bisallenic sulfones, creating a competitive scenario between two alternate Garratt-Braverman (GB) cyclization pathways. The observed product distribution ruled out the involvement of any ionic intermediate and supported the diradical mechanism with greater involvement of the electron-rich aromatic ring via the more nucleophilic radical. DFT-based calculations supported the diradical mechanism along with the observed selectivity.

A Novel Enediynyl Peptide Inhibitor of Furin That Blocks Processing of proPDGF-A, B and proVEGF-C

Background Furin represents a crucial member of secretory mammalian subtilase, the Proprotein Convertase (PC) or Proprotein Convertase Subtilisin/Kexin (PCSK) superfamily. It has been linked to cancer, tumorgenesis, viral and bacterial pathogenesis. As a result it is considered a major target for intervention of these diseases. Methodology/Principal Findings Herein, we report, for the first time, the synthesis and biological evaluation of a newly designed potent furin inhibitor that contains a highly reactive beta-turn inducing and radical generating “enediynyl amino acid” (Eda) moiety. “Eda” was inserted between P1 and P1′ residues of hfurin98–112 peptide, derived from the primary cleavage site of furins own prodomain. The resulting hexadecapeptide derivative inhibited furin in vitro with IC50 ∼40 nM when measured against the fluorogenic substrate Boc-RVRR-MCA. It also inhibited furin-mediated cleavage of a fluorogenic peptide derived from hSARS-CoV spike protein with IC50 ∼193 nM. Additionally it also blocked furin-processing of growth factors proPDGF-A, B and VEGF-C that are linked to tumor genesis and cancer. Circular dichroism study showed that this inhibitor displayed a predominantly beta-turn structure while western blots confirmed its ability to protect furin protein from self degradation. Conclusion/Significance These findings imply its potential as a therapeutic agent for intervention of cancer and other furin-associated diseases.

Triazolyl Donor/Acceptor Chromophore Decorated Unnatural Nucleosides and Oligonucleotides with Duplex Stability Comparable to That of a Natural Adenine/Thymine Pair

We report the design and synthesis of triazolyl donor/acceptor unnatural nucleosides via click chemistry and studies on the duplex stabilization of DNA containing two such new nucleosides. The observed duplex stabilization among the self-pair/heteropair has been found to be comparable to that of a natural A/T pair. Our observations on the comparable duplex stabilization has been explained on the basis of possible π-π stacking and/or charge transfer interactions between the pairing partners. The evidence of ground-state charge transfer complexation came from the UV-vis spectra and the static quenching of fluorescence in a heteropair. We have also exploited one of our unnatural DNAs in stabilizing abasic DNA.

Click-Reagent Version of Sonogashira Coupling Protocol to Conjugated Fluorescent Alkynes with No or Reduced Homocoupling

A click-reagent version of the Sonogashira-coupling protocol has been developed. Diarylalkynes with donor and/or acceptor substituents have been synthesized via this protocol at moderate to excellent yields and with no or drastically reduced quantities of undesired homocoupled side products. This protocol is green-solvent compatible, air-insensitive, and effective under microwave conditions.

Synthesis of Functionalized Pyrazoles via Vanadium-Catalyzed C–N Dehydrogenative Cross-Coupling and Fluorescence Switch-On Sensing of BSA Protein

Vanadium-catalyzed C-N dehydrogenative cross-coupling of alkenyl hydrazones leading to functionalized pyrazoles is described in a 1:1 mixture of toluene/H2O using air as the terminal oxidant. Significant practical features include use of the commercial nontoxic VOSO4 as a recyclable catalyst, mild reaction conditions, scalability, and the broad substrate scope. Some of the product pyrazoles exhibit interesting photophysical properties. Fluorescence light-up sensing of BSA protein by one of the pyrazoles is also highlighted.

Highly solvatochromic fluorescent naphthalimides: design, synthesis, photophysical properties and fluorescence switch-on sensing of ct-DNA.

We report the design, synthesis and photophysical properties of highly solvatochromic donor/acceptor substituted naphthalimide based fluorophores. The synthesized naphthalimides containing propargyl ends showed highly solvatochromic intramolecular charge transfer (ICT) feature as was revealed from the UV-visible, fluorescence photophysical properties of these fluorophores and DFT/TDDFT calculation. Fluorescence life times for the imide fluorophores were also measured in different solvents. The solid state photophysical property of donor substituted naphthalimide 1 showed promising for future application in material sciences. Furthermore, both the donor/acceptor substituted naphthalimide fluorophores 1-2 were exploited in sensing calf-thymus DNA via switch-on fluorescence response. The propargyl linker containing naphthalimides can further be exploited for the synthesis of labeled biomolecular building blocks.

Synthesis, reactivity and conformational preferences of novel enediynyl peptides: a possible scaffold for β-sheet capping turns

Novel enediynyl tripeptides 2(a–c) in fully protected forms have been prepared via a sequence of palladium(0)-based Sonogashira coupling. The thermal reactivity of these peptides was shown to be dependent upon the nature of the side chain in the amino acids. Analysis of the CD-spectra of these peptides as well as the variation of chemical shifts with temperature revealed the presence of a β-sheet nucleating conformation in equilibrium with a conformation induced by H-bond formation between the CO and NH belonging to the enediynyl amino acid.

Mechanistic Studies on Garratt–Braverman Cyclization: The Diradical–Cycloaddition Puzzle

In this work, we present the results of extensive multiprong studies involving the fate of deuterium-labeled substrates, EPR, trapping experiments, and LA-LDI mass spectrometry to sort out the controversies relating to the mechanism of Garratt-Braverman cyclization in two systems, namely bis-propargyl sulfones and ethers. The results are in conformity with a diradical mechamism for the sulfone, while for the ether, the anionic [4 + 2] appears to be the preferred pathway. This shows that the mechanistic pathway toward GB cyclization is dependent upon the nature of heteroatom (O or S in sulfone) bridging the propargyl arms.