Susobhan Choudhury

Facile synthesis of Pd nanostructures in hexagonal mesophases as a promising electrocatalyst for ethanol oxidation

One of the significant challenges for the commercialization of direct ethanol fuel cells (DEFCs) is the preparation of active, robust, and low-cost catalysts. In this work, a facile and reproducible method is demonstrated for the synthesis of Pd assembled nanostructures in a hexagonal mesophase formed by a quaternary system (Pd-doped water, surfactant, oil, and cosurfactant) via photoirradiation. The formation of Pd nanostructures in the confined region of hexagonal mesophases was further supported by water relaxation dynamics study using a solvation probe. The mesophases can be doped with high concentrations of a palladium salt (0.1 M) without any disturbance to the structure of the mesophases which results in a high yield and facilitates the clean synthesis of Pd nanostructures without using any toxic chemicals. Electrochemical measurement confirms that the as-prepared catalysts exhibit significant electrocatalytic activity for ethanol oxidation in alkaline solution. Additionally, we present an alternative strategy using reduced graphene oxide nanosheets in combination with Nafion (a proton conducting phase) as a support, revealing the pronounced impact on dramatically enhanced electrocatalytic activity and stability of Pd nanostructures compared to Nafion alone. This unique combination allowed the effective dispersion of the Pd nanostructures that is responsible for the enhancement of the catalytic activity. Our approach paves the way towards the rational design of practically relevant catalysts with both enhanced activity and durability for fuel cell applications.

Ultrafast dynamics of solvation and charge transfer in a DNA-based biomaterial.

Charge migration along DNA molecules is a key factor for DNA-based devices in optoelectronics and biotechnology. The association of a significant amount of water molecules in DNA-based materials for the intactness of the DNA structure and their dynamic role in the charge-transfer (CT) dynamics is less documented in contemporary literature. In the present study, we have used a genomic DNA-cetyltrimethyl ammonium chloride (CTMA) complex, a technological important biomaterial, and Hoechest 33258 (H258), a well-known DNA minor groove binder, as fluorogenic probe for the dynamic solvation studies. The CT dynamics of CdSe/ZnS quantum dots (QDs; 5.2 nm) embedded in the as-prepared and swollen biomaterial have also been studied and correlated with that of the timescale of solvation. We have extended our studies on the temperature-dependent CT dynamics of QDs in a nanoenvironment of an anionic, sodium bis(2-ethylhexyl)sulfosuccinate reverse micelle (AOT RMs), whereby the number of water molecules and their dynamics can be tuned in a controlled manner. A direct correlation of the dynamics of solvation and that of the CT in the nanoenvironments clearly suggests that the hydration barrier within the Arrhenius framework essentially dictates the charge-transfer dynamics.

Modulation of stability and functionality of a phyto-antioxidant by weakly interacting metal ions: curcumin in aqueous solution

The natural polyphenol curcumin and its metal coordinated complexes show obvious benefits in the medical therapies of cancer and several neurodegenerative diseases. On the other side their stability and bioavailability are critical issues. The present study is an attempt to address the stability and functionality of curcumin upon complexation with transition metal ions. We have synthesized and optically characterized metallo–curcumin complexes with Cu(II) and Zn(II). From femtosecond resolved upconversion studies an interaction at the molecular level is revealed based on an observed photoinduced electron transfer from curcumin to the metal ions. In order to investigate the antioxidant activity of the complexes, we have performed a 2,2-diphenyl-1-picrylhydrazyl (DPPH) assay in dark. The Cu(II)–curcumin complex exhibits an enhanced and recyclable activity, more pronounced compared to that of the Zn(II)–curcumin complex, which can be attributed to the weaker O–H bond present in the former case. In contrast, the Zn(II) complex has a higher solubility and stability in aqueous media than the Cu(II) complex. To address stability vs. functionality issues, we have suggested a facile method that enhances the solubility and stability of curcumin in aqueous media by metalation with Zn(II) and a successional replacement of Zn(II) in the complex by Cu(II) through a simple route to enhance the activity prior to its use. We have also used the complex in a model anti-bacteriological assay experiment where it shows significantly higher activity compared to pure curcumin. The dichlorofluorescin (DCFH) oxidation indicates an enhancement in ROS generation, which in turn is responsible for the enhanced antioxidative property of the Cu(II)–curcumin complex. Our results provide a promising method to use metallo–curcumin complexes in diverse biological applications.

Ultrafast interfacial solvation dynamics in specific protein DNA recognition

An overwhelming number of structural and functional studies on specific protein-DNA complexes reveal the existence of water molecules at the interaction interface. What role does the interfacial water molecules play in determining the specificity of association is thus a critical question. Herein, we have explored the dynamical role of minor groove water molecules and DNA side chain flexibility in lambda repressor-operator DNA interaction using well-characterized DNA minor groove binder dye, Hoechst 33258. The most striking finding of our studies reveals that the solvation time scale corresponding to the minor groove water molecules (∼50 ps) and DNA side chain flexibility (∼10 ns) remain unaltered even in protein-DNA complex in comparison to unbound operator DNA. The temperature dependent study further reveals the slower exchange of minor grove water molecules with bulk water in DNA-protein complex in comparison to the unbound DNA. Detailed structural studies including circular dichroism (CD) and Förster resonance energy transfer (FRET) have also been performed to elucidate the interaction between protein and DNA.

Ultrafast spectroscopy on DNA-cleavage by endonuclease in molecular crowding

The jam-packed intracellular environments differ the activity of a biological macromolecule from that in laboratory environments (in vitro) through a number of mechanisms called molecular crowding related to structure, function and dynamics of the macromolecule. Here, we have explored the structure, function and dynamics of a model enzyme protein DNase I in molecular crowing of polyethylene glycol (PEG; MW 3350). We have used steady state and picosecond resolved dynamics of a well-known intercalator ethidium bromide (EB) in a 20-mer double-stranded DNA (dsDNA) to monitor the DNA-cleavage by the enzyme in absence and presence PEG. We have also labelled the enzyme by a well-known fluorescent probe 8-anilino-1-naphthalenesulfonic acid ammonium salt (ANS) to study the molecular mechanism of the protein-DNA association through exited state relaxation of the probe in absence (dictated by polarity) and presence of EB in the DNA (dictated by Förster resonance energy transfer (FRET)). The overall and local structures of the protein in presence of PEG have been followed by circular dichroism and time resolved polarization gated spectroscopy respectively. The enhanced dynamical flexibility of protein in presence of PEG as revealed from excited state lifetime and polarization gated anisotropy of ANS has been correlated with the stronger DNA-binding for the higher nuclease activity. We have also used conventional experimental strategy of agarose gel electrophoresis to monitor DNA-cleavage and found consistent results of enhanced nuclease activities both on synthetic 20-mer oligonucleotide and long genomic DNA from calf thymus.

Modulation of Ultrafast Conformational Dynamics in Allosteric Interaction of Gal Repressor Protein with Different Operator DNA Sequences

Although all forms of dynamical behaviour of a protein under allosteric interaction with effectors are predicted, little evidence of ultrafast dynamics in the interaction has been reported. Here, we demonstrate the efficacy of a combined approach involving picosecond‐resolved FRET and polarisation‐gated fluorescence for the exploration of ultrafast dynamics in the allosteric interaction of the Gal repressor (GalR) protein dimer with DNA operator sequences OE and OI. FRET from the single tryptophan residue to a covalently attached probe IAEDANS at a cysteine residue in the C‐terminal domain of GalR shows structural perturbation and conformational dynamics during allosteric interaction. Polarisation‐gated fluorescence spectroscopy of IAEDANS and another probe (FITC) covalently attached to the operator directly revealed the essential dynamics for cooperativity in the protein–protein interaction. The ultrafast resonance energy transfer from IAEDANS in the protein to FITC also revealed different dynamic flexibility in the allosteric interaction. An attempt was made to correlate the dynamic changes in the protein dimers with OE and OI with the consequent protein–protein interaction (tetramerisation) to form a DNA loop encompassing the promoter segment.

Direct Observation of Kinetic Pathways of Biomolecular Recognition.

The pathways of molecular recognition, which is a central event in all biological processes, belong to the most important subjects of contemporary research in biomolecular science. By using fluorescence spectroscopy in a microfluidics channel, it can be determined that molecular recognition of α-chymotrypsin in hydrous surroundings at two different pH values (3.6 and 6.3) follows two distinctly different pathways. Whereas one corroborates an induced-fit model (pH 3.6), the other one (pH 6.3) is consistent with the selected-fit model of biomolecular recognition. The role of massive structural perturbations of differential recognition pathways could be ruled out by earlier XRD studies, rather was consistent with the femtosecond-resolved observation of dynamic flexibility of the protein at different pH values. At low concentrations of ligands, the selected-fit model dominates, whereas increasing the ligand concentration leads to the induced-fit model. From molecular modelling and experimental results, the timescale associated with the conformational flexibility of the protein plays a key role in the selection of a pathway in biomolecular recognition.

Dynamical perspective of protein-DNA interaction.

Abstract The interactions between protein-DNA are essential for various biological activities. In this review, we provide an overview of protein-DNA interactions that emphasizes the importance of dynamical aspects. We divide protein-DNA interactions into two categories: nonspecific and specific and both the categories would be discussed highlighting some of our relevant work. In the case of nonspecific protein-DNA interaction, solvation studies (picosecond and femtosecond-resolved) explore the role environmental dynamics and change in the micropolarity around DNA molecules upon complexation with histone protein (H1). While exploring the specific protein-DNA interaction at λ-repressor-operator sites interaction, particularly OR1 and OR2, it was observed that the interfacial water dynamics is minimally perturbed upon interaction with DNA, suggesting the labile interface in the protein-DNA complex. Förster resonance energy transfer (FRET) study revealed that the structure of the protein is more compact in repressor-OR2 complex than in the repressor-OR1 complex. Fluorescence anisotropy studies indicated enhanced flexibility of the C-terminal domain of the repressor at fast timescales after complex formation with OR1. The enhanced flexibility and different conformation of the C-terminal domain of the repressor upon complexation with OR1 DNA compared to OR2 DNA were found to have pronounced effect on the rate of photoinduced electron transfer.

Photo-triggered destabilization of nanoscopic vehicles by dihydroindolizine for enhanced anticancer drug delivery in cervical carcinoma

The efficacy and toxicity of drugs depend not only on their potency but also on their ability to reach the target sites in preference to non-target sites. In this regards destabilization of delivery vehicles induced by light can be an effective strategy for enhancing drug delivery with spatial and temporal control. Herein we demonstrate that the photoinduced isomerization from closed (hydrophobic) to open isomeric form (hydrophilic) of a novel DHI encapsulated in liposome leads to potential light-controlled drug delivery vehicles. We have used steady state and picosecond resolved dynamics of a drug 8-anilino-1-naphthalenesulfonic acid ammonium salt (ANS) incorporated in liposome to monitor the efficacy of destabilization of liposome in absence and presence UVA irradiation. Steady state and picosecond resolved polarization gated spectroscopy including the well-known strategy of solvation dynamics and Förster resonance energy transfer; reveal the possible mechanism out of various phenomena involved in destabilization of liposome. We have also investigated the therapeutic efficacy of doxorubicin (DOX) delivery from liposome to cervical cancer cell line HeLa. The FACS, confocal fluorescence microscopic and MTT assay studies reveal an enhanced cellular uptake of DOX leading to significant reduction in cell viability (∼40%) of HeLa followed by photoresponsive destabilization of liposome. Our studies successfully demonstrate that these DHI encapsulated liposomes have potential application as a smart photosensitive drug delivery system.

Resveratrol–ZnO nanohybrid enhanced anti-cancerous effect in ovarian cancer cells through ROS

The use of nanotechnology in medicine and more specifically in drug delivery is expected to spread rapidly. Currently many substances are under investigation for drug delivery and more specifically for cancer therapy. Nano-conjugation of the drug is likely to provide protection against degradation, increasing bioavailability, and improvement in intracellular penetration, enhanced efficacy and control delivery of the drug. In this study, ZnO nanoparticles (NPs) conjugated with a well-known anti-proliferative and chemopreventive trans-resveratrol (RSV) has been designed, characterized and found to be a potential drug for ovarian cancer treatment. Nano-conjugate (RSV–ZnO) has been characterized by FTIR, Raman scattering and Transmission electron microscopy (TEM). Picosecond-resolved fluorescence studies of RSV–ZnO nano-conjugate reveal efficient electron migration from ZnO NPs to RSV, eventually enhancing the ROS activity compared to free RSV. Various in vivo and in vitro studies including MTT assay and apoptosis studies on ovarian cancer (PA1) cell lines reveals the RSV–ZnO nano-conjugate to be more effective in cancer cell death in comparison to free RSV. DCFH assay (in vitro) and DCFDA method (in vivo in PA1 cell lines) demonstrate the huge enhancement of antioxidant property (through ROS) in case of nano-conjugate. JC-1 staining method unravels the increase in depolarization of the mitochondrial membrane in the PA1 cell upon nano-conjugate, consistent with mitochondrial dysfunction. Finally we have performed a Western blot study by expression of some proteins like actin, Bax, Bcl-2 and Caspase-9 to confirm apoptosis.