Chemistry

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Shiv Nadar University

Ketol-isomerases catalyze the reversible isomerization between aldoses and ketoses. d-Xylose isomerase carries out the first reaction in the catabolism of d-xylose, but is also able to convert d-glucose to d-fructose. The first step of the reaction is an enzyme-catalyzed ring opening of the cyclic substrate. The active-site amino-acid acid/base pair involved in ring opening has long been investigated and several models have been proposed. Here, the structure of the xylose isomerase E186Q mutant with cyclic glucose bound at the active site, refined against joint X-ray and neutron diffraction data, is reported. Detailed analysis of the hydrogen-bond networks at the active site of the enzyme suggests that His54, which is doubly protonated, is poised to protonate the glucose O5 position, while Lys289, which is neutral, promotes deprotonation of the glucose O1H hydroxyl group via an activated water molecule. The structure also reveals an extended hydrogen-bonding network that connects the conserved residues Lys289 and Lys183 through three structurally conserved water molecules and residue 186, which is a glutamic acid to glutamine mutation. © 2014 International Union of Crystallography.

(SoNS) School of Natural Sciences

World University root node

Acta Crystallographica Section D: Biological Crystallography

Neutron structure of the cyclic glucose-bound xylose isomerase E186Q mutant

Detection of trace amounts of explosive materials is significantly important for security concerns and pollution control. Four multicomponent metal-organic frameworks (MOFs-12, 13, 23 and 123) have been synthesized by employing ligands embedded with fluorescent tags. The multicomponent assembly of the ligands was utilized to acquire a diverse electronic behavior of the MOFs and the fluorescent tags were strategically chosen to enhance the electron density in the MOFs. The phase purity of the MOFs was established by PXRD, NMR spectroscopy and finally by single-crystal XRD. Single-crystal structures of the MOFs-12 and 13 showed the formation of three-dimensional porous networks with the aromatic tags projecting inwardly into the pores. These electron-rich MOFs were utilized for detection of explosive nitroaromatic compounds (NACs) through fluorescence quenching with high selectivity and sensitivity. The rate of fluorescence quenching for all the MOFs follows the order of electron deficiency of the NACs. We also showed the detection of picric acid (PA) by luminescent MOFs is not always reliable and can be misleading. This attracts our attention to explore these MOFs for sensing picryl chloride (PC), which is as explosive as picric acid and used widely to prepare more stable explosives like 2,4,6-trinitroaniline from PA. Moreover, the recyclability and sensitivity studies indicated that these MOFs can be reused several times with parts per billion (ppb) levels of sensitivity towards PC and 2,4,6-trinitrotoluene (TNT).

Chemistry - A European Journal

Multicomponent Assembly of Fluorescent-Tag Functionalized Ligands in Metal-Organic Frameworks for Sensing Explosives

Three new electron-rich metal-organic frameworks (MOF-1-MOF-3) have been synthesized by employing ligands bearing aromatic tags. The key role of the chosen aromatic tags is to enhance the $\pi$-electron density of the luminescent MOFs. Single-crystal X-ray structures have revealed that these MOFs form three-dimensional porous networks with the aromatic tags projecting inwardly into the pores. These highly luminescent electron-rich MOFs have been successfully utilized for the detection of explosive nitroaromatic compounds (NACs) on the basis of fluorescence quenching. Although all of the prepared MOFs can serve as sensors for NACs, MOF-1 and MOF-2 exhibit superior sensitivity towards 4-nitrotoluene (4-NT) and 2,4-dinitrotoluene (DNT) compared to 2,4,6-trinitrotoluene (TNT) and 1,3,5-trinitrobenzene (TNB). MOF-3, on the other hand, shows an order of sensitivity in accordance with the electron deficiencies of the substrates. To understand such anomalous behavior, we have thoroughly analyzed both the steady-state and time-resolved fluorescence quenching associated with these interactions. Determination of static Stern-Volmer constants (KS) as well as collisional constants (K C) has revealed that MOF-1 and MOF-2 have higher KS values with 4-NT than with TNT, whereas for MOF-3 the reverse order is observed. This apparently anomalous phenomenon was well corroborated by theoretical calculations. Moreover, recyclability and sensitivity studies have revealed that these MOFs can be reused several times and that their sensitivities towards TNT solution are at the parts per billion (ppb) level. Copyright {\textcopyright} 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Modification of extended open frameworks with fluorescent tags for sensing explosives: Competition between size selectivity and electron deficiency

In continuation of our interest in pyrazole based multifunctional metal-organic frameworks (MOFs), we report herein the construction of a series of Co(II) MOFs using a bis-pyrazole ligand and various benzene polycarboxylic acids. Employment of different acids has resulted in different architectures ranging from a two-dimensional grid network, porous nanochannels with interesting double helical features such as supramolecular chicken wire, to three-dimensional diamondoid networks. One of the distinguishing features of the network is their larger dimensions which can be directly linked to a relatively larger size of the ligand molecule. Conformational flexibility of the ligand also plays a decisive role in determining both the dimensionality and topology of the final structure. Furthermore, chirality associated with helical networks and magnetic properties of two MOFs have also been investigated. {\textcopyright} 2014 American Chemical Society.

Crystal Growth and Design

Construction of bis-pyrazole based Co(II) metal-organic frameworks and exploration of their chirality and magnetic properties

We demonstrate here that supramolecular interactions enhance the sensitivity towards detection of electron-deficient nitro-aromatic compounds (NACs) over discrete analogues. NACs are the most commonly used explosive ingredients and are common constituents of many unexploded landmines used during World War II. In this study, we have synthesised a series of pyrene-based polycarboxylic acids along with their corresponding discrete esters. Due to the electron richness and the fluorescent behaviour of the pyrene moiety, all the compounds act as sensors for electron-deficient NACs through a fluorescence quenching mechanism. A Stern-Volmer quenching constant determination revealed that the carboxylic acids are more sensitive than the corresponding esters towards NACs in solution. The high sensitivity of the acids was attributed to supramolecular polymer formation through hydrogen bonding in the case of the acids and the enhancement mechanism is based on an exciton energy migration upon excitation along the hydrogen-bond backbone. The presence of intermolecular hydrogen bonding in the acids in solution was established by solvent-dependent fluorescence studies and dynamic light scattering (DLS) experiments. In addition, the importance of intermolecular hydrogen bonds in solid-state sensing was further explored by scanning tunnelling microscopy (STM) experiments at the liquid-solid interface, in which structures of self-assembled monolayer of the acids and the corresponding esters were compared. The sensitivity tests revealed that these supramolecular sensors can even detect picric acid and trinitrotoluene in solution at levels as low as parts per trillion (ppt), which is much below the recommended permissible level of these constituents in drinking water.

Explosives sensing by using electron-rich supramolecular polymers: Role of intermolecular hydrogen bonding in significant enhancement of sensitivity

Glucose-appended photocytotoxic iron(III) complexes of a tridentate Schiff base phenolate ligand [Fe(bpyag)(L)](NO3) (1-3), where bpyag is N,N-bis(2-pyridylmethyl)-2-aminoethyl-$\beta$-d-glucopyranoside and H 2L is 3-(2-hydroxyphenylimino)-1-phenylbutan-1-one (H 2phap) in 1, 3-(2-hydroxyphenylimino)-9-anthrylbutan-1-one (H 2anap) in 2 and 3-(2-hydroxyphenylimino)-1-pyrenylbutan-1-one (H2pyap) in 3, were synthesized and characterized. The complex [Fe(dpma)(anap)](NO3) (4), having bis-(2-pyridylmethyl)benzylamine (dpma), in which the glucose moiety of bpyag is substituted by a phenyl group, was used as a control and the complex [Fe(dpma)(anap)](PF6) (4a) was structurally characterized by X-ray crystallography. The structure shows a FeN4O2 core in a distorted octahedral geometry. The high-spin iron(III) complexes with magnetic moment value of ∼5.9 $\mu$B showed a low-energy phenolate-to-Fe(III) charge-transfer (CT) absorption band as a shoulder near 500 nm with a tail extending to 700 nm and an irreversible Fe(III)-Fe(II) redox couple near -0.6 V versus saturated calomel electrode. The complexes are avid binders to calf thymus DNA and showed photocleavage of supercoiled pUC19 DNA in red (647 nm) and green (532 nm) light. Complexes 2 and 3 displayed significant photocytotoxicity in red light, with an IC50 value of ∼20 $\mu$M in HeLa and HaCaT cells and no significant toxicity in dark. The cell death is via an apoptotic pathway, by generation of reactive oxygen species. Preferential internalization of the carbohydrate-appended complexes 2 and 3 was evidenced in HeLa cells as compared to the control complex 4. A 5-fold increase in the cellular uptake was observed for the active complexes in HeLa cells. The photophysical properties of the complexes are rationalized from the density functional theory calculations. {\textcopyright} 2014 American Chemical Society.

Inorganic Chemistry

Carbohydrate-appended tumor targeting iron(III) complexes showing photocytotoxicity in red light

Due to the prevalent use of polycarbonate plastics and epoxy resins in packaging materials and paints for ships, there has been a widespread global contamination of environmental water sources with bisphenol A (BPA). BPA, an endocrine disruptor, has been found to cause tremendous health problems. Therefore, there is an urgent need for detecting BPA in a convenient and sensitive manner to ensure water safety. Herein, we develop a fluorescent turn-on BPA probe, named Bisphenol Orange (BPO), which could conveniently detect BPA in a wide variety of real water samples including sea water, drain water and drinking water. BPO shows superior selectivity toward BPA and up to 70-fold increase in fluorescence emission at 580. nm when mixed with BPA in water. Mechanistic studies suggest a plausible water-dependent formation of hydrophobic BPA clusters which favorably trap and restrict the rotation of BPO and recover its inherent fluorescence. © 2014 Elsevier B.V.

Analytica Chimica Acta

"Orange alert": A fluorescent detector for bisphenol A in water environments

The Raman signal enhancement depends mainly on four critical factors. These factors are localized surface plasmon resonance (LSPR) of gold substrates along with shape and electronic absorbance of Raman reporters along with laser sources. Hence, the effects of all these four parameters were systematically studied by changing either the different geometries/shapes of the gold substrates having a wide range of SPR or electronic absorbance of Raman reporters under a fixed laser excitation of 633 nm or 785 nm. Enhancement factor (EF) values were evaluated for each individual substrate with respect to five different Raman reporters which cover the maximum absorbance from 530 nm to 800 nm. The EF values suggest that the signal intensity has been increased by at least ten times compared to previous reports. Among the five different shapes of the gold nanoparticles, the substrate of gold nanostars (GNSt) was found to be the most suitable substrate for the highest signal enhancement under both excitation laser sources. The substrate of gold nanosphere (GNSp) is the second best at 633 nm laser excitation; however it is the modest substrate for SERS enhancement at 785 nm laser excitation. After finding the key tools for the development of ultrasensitive SERS nanoprobes, we selected the best Raman reporter (Cy7LA) for the development of biocompatible SERS nanotags in three different SERS substrates for the effective detection of cancer cells under 785 nm laser source. © 2014 The Royal Society of Chemistry.

Fulltext

RSC Advances

Wavelength and shape dependent SERS study to develop ultrasensitive nanotags for imaging of cancer cells

Chemistry - An Asian Journal

Erratum: Synthesis of privileged scaffolds by using diversity-oriented synthesis (Chemistry - An Asian Journal (2013) 8 DOI: 10.1002/asia.201201203)

Exploration of sustainable alternatives to chemicals derived from petro-based industries is the current challenge for maintaining the balance between the needs of a changing world while preserving nature. The major source for sustainable chemicals is either the natural existing plant sources or waste generated from agro-based industries. The utility of such resources will supplement new processed materials with different sets of properties and environmental friendliness due to their biodegradability and low toxicity during preparation, usage and disposal. Amongst other polymers used on a day-to-day basis, phenolic resins account for vast usage. Replacement of petro-based monomers such as phenol and its derivatives either partly or completely utilized for the synthesis of such resins is ongoing. Extraction of natural phenolic components from cashew nut shell liquid, lignin, tannin, palm oil, coconut shell tar or from agricultural and industrial waste, and their utilization as synthons for the preparation of bio-based polymers and properties obtained are reviewed in this paper. This review article is designed to acknowledge efforts of researchers towards the "3C" motto-not only trying to create but also adapting the principles to conserve and care for a sustainable environment. This review paper describes how extraction, separation and recovery of desired phenolic compounds have occurred recently; how substituted phenol compounds, unmodified and modified, act as monomers for polymerization; and how the presence of sustainable phenolic material affects the properties of polymers. There are about 600 references cited and still there is a lot to uncover in this research area. This journal is © the Partner Organisations 2014.