Sulfur K-edge XAS and DFT calculations on [Fe4S4]2+ clusters: effects of H-bonding and structural distortion on covalency and spin topology

Sulfur K-edge X-ray absorption spectroscopy of a hydrogen-bonded elongated [Fe4S4]2+ cube is reported. The data show that this synthetic cube is less covalent than a normal compressed cube with no hydrogen bonding. DFT calculations reveal that the observed difference in electronic structure has significant contributions from both the cluster distortion and from hydrogen bonding. The elongated and compressed Fe4S4 structures are found to have different spin topologies (i.e., orientation of the delocalized Fe2S2 subclusters which are antiferromagnetically coupled to each other). It is suggested that the H-bonding interaction with the counterion does not contribute to the cluster elongation. A magneto-structural correlation is developed for the Fe4S4 cube that is used to identify the redox-active Fe2S2 subclusters in active sites of HiPIP and ferredoxin proteins involving these clusters.     

Sulfur K-edge XAS and DFT studies on NiII complexes with oxidized thiolate ligands: implications for the roles of oxidized thiolates in the active sites of Fe and Co nitrile hydratase

S K-edge X-ray absorption spectroscopy data on a series of NiII complexes with thiolate (RS-) and oxidized thiolate (RSO2-) ligands are used to quantify Ni-S bond covalency and its change upon ligand oxidation. Analyses of these results using geometry-optimized density functional theory (DFT) calculations suggest that the Ni-S sigma bonds do not weaken on ligand oxidation. Molecular orbital analysis indicates that these oxidized thiolate ligands use filled high-lying S-O pi* orbitals for strong sigma donation. However, the RSO2- ligands are poor pi donors, as the orbital required for pi interaction is used in the S-O sigma-bond formation. The oxidation of the thiolate reduces the repulsion between electrons in the filled Ni t2 orbital and the thiolate out-of-plane pi-donor orbital leading to shorter Ni-S bond length relative to that of the thiolate donor. The insights obtained from these results are relevant to the active sites of Fe- and Co-type nitrile hydratases (Nhase) that also have oxidized thiolate ligands. DFT calculations on models of the active site indicate that whereas the oxidation of these thiolates has a major effect in the axial ligand-binding affinity of the Fe-type Nhase (where there is both sigma and pi donation from the S ligands), it has only a limited effect on the sixth-ligand-binding affinity of the Co-type Nhases (where there is only sigma donation). These oxidized residues may also play a role in substrate binding and proton shuttling at the active site.     

Electrochemical approach for synthesis of 3-substituted indole derivatives

An efficient and economical method was developed for synthesis of 3-substituted indole by using electrochemically induced condensation of various aldehyde, indole and malononitrile.     

Sensing of Phosphates by Using Luminescent EuIII and TbIII Complexes: Application to the Microalgal Cell Chlorella vulgaris

Phenanthroline‐based chiral ligands L¹ and L² as well as the corresponding Eu^III and Tb^III complexes were synthesized and characterized. The coordination compounds show red and green emission, which was explored for the sensing of a series of anions such as F^?, Cl^?, Br^?, I^?, NO₃^?, NO₂^?, HPO₄^2?, HSO₄^?, CH₃COO^?, and HCO₃^?. Among the anions, HPO₄^2? exhibited a strong response in the emission property of both europium(III) and terbium(III) complexes. The complexes showed interactions with the nucleoside phosphates adenosine triphosphate (ATP), adenosine diphosphate (ADP), and adenosine monophosphate (AMP). Owing to this recognition, these complexes have been applied as staining agents in the microalgal cell Chlorella vulgaris. The stained microalgal cells were monitored through fluorescence microscopy and scanning electron microscopy. Initially, the complexes bind to the outer cell wall and then enter the cell wall through holes in which they probably bind to phospholipids. This leads to a quenching of the luminescence properties.     

Anion Storage Chemistry of Organic Cathodes for High-Energy and High-Power Density Divalent Metal Batteries

Multivalent batteries show promising prospects for next-generation sustainable energy storage applications. Herein, we report a polytriphenylamine (PTPAn) composite cathode capable of highly reversible storage of tetrakis(hexafluoroisopropyloxy) borate [B(hfip)₄] anions in both Magnesium (Mg) and calcium (Ca) battery systems. Spectroscopic and computational studies reveal the redox reaction mechanism of the PTPAn cathode material. The Mg and Ca cells exhibit a cell voltage >3 V, a high-power density of ∼∼3000 W kg⁻¹ and a high-energy density of ∼∼300 Wh kg⁻¹, respectively. Moreover, the combination of the PTPAn cathode with a calcium-tin (Ca−Sn) alloy anode could enable a long battery-life of 3000 cycles with a capacity retention of 60 %. The anion storage chemistry associated with dual-ion electrochemical concept demonstrates a new feasible pathway towards high-performance divalent ion batteries.     

2,7-Diaminofluorene-based organic dyes for dye-sensitized solar cells: effect of auxiliary donor on optical and electrochemical properties

New organic dyes containing a diarylaminofluorene unit as an electron donor and cyanoacrylic acid as acceptor and anchoring group in a donor-π-donor-π-acceptor architecture have been synthesized and characterized as sensitizers for nanocrystalline TiO(2)-based dye-sensitized solar cells. They have shown three major electronic absorptions originating from the π-π* and charge-transfer transitions covering the broad visible range (250-550 nm) in solution. The charge-transfer transition of the dyes exhibited negative solvatochromism, suggesting a polarized ground state. They have also displayed acidochromism in solution owing to the presence of a protonation-deprotonation equilibrium. On comparison with the triphenylamine and carbazole-based parent dyes (E)-2-cyano-3-(4-(diphenylamino)phenyl)acrylic acid and (E)-2-cyano-3-(9-ethyl-9H-carbazol-3-yl)acrylic acid they exhibited longer wavelength absorptions and facile oxidation, indicating the stronger electron-donating ability of the auxiliary chromophores. In addition, they exhibited nearly two times larger light-to-electron conversion efficiency under simulated AM 1.5 G irradiation (100 mW cm(-2)) with an aperture mask when compared to the parent dyes. Among the new dyes, the one containing the naphthylphenylamine segment showed better device characteristics attributable to the higher HOMO energy level which probably facilitates the regeneration of the dye and effective suppression of the back reaction of the injected electrons with the I(3)(-) in the electrolyte. The optical properties of the dyes were modeled using TDDFT simulations employing different theoretical models (B3LYP, CAM-B3LYP, and MPW1K), and the best correlations with the observed parameters have been found for CAM-B3LYP and MPW1K calculations. The electron lifetimes extracted from the electrochemical impedance measurements of the dye-sensitized solar cells were used to interpret the solar cell efficiency alternations.     

Kinetics of Enzymatic Synthesis of Cinnamyl Butyrate by Immobilized Lipase

This work illustrates the enzymatic synthesis of cinnamyl butyrate by esterification of butyric acid and cinnamyl alcohol. Experiments were performed to study the various operating parameters such as molar ratio, enzyme concentration, temperature, and speed of agitation. Also, the suitable kinetic model for esterification reaction was predicted and the various kinetic parameters were determined. It has been observed that the experimental results agree well with the simulated results obtained by following the ping-pong bi-bi mechanism with dead-end inhibition by both the substrate acid and alcohol. The highest 90% conversion of butyric acid was observed after 12 h at the following reaction conditions: substrate molar ratio 1:2 (butyric acid/cinnamyl alcohol), temperature 50 °C, enzyme loading 2% (with respect to the weight of the substrates), and agitation speed 250 rpm. Diffusional mass transfer limitations between substrate and enzyme surface do not show significant effect on reaction kinetics. Enzyme reusability study reveals that it retains 85% of its catalytic activity after five consecutive cycles.     

Validated Chiral LC Method for the Enantiomeric Separation of β-Amino-β-(3-Methoxyphenyl) Propionic Acid

A chiral liquid chromatographic method for enantiomeric resolution of β-amino-β-(3-methoxyphenyl) propionic acid was developed and validated. The “hybrid” π-electron donor–acceptor based stationary phase (R,R) Whelk-01 was found to be enantiomerically selective for (R) and (S) enantiomers of β-amino-β-(3-methoxyphenyl) propionic acid with a resolution greater than 2.0. The effects of isopropyl alcohol and ethanol on enantioselectivity and resolution of enantiomers were evaluated. Calibration curves were linear over the range of 0.10–1.00, with a regression coefficient (r) of 0.999. The limit of detection (LOD) and limit of quantification (LOQ) were 300 and 1,000 ng mL⁻¹ respectively for 10 μL injection volume. The percentage RSD of the peak area of six replicate injections of (S) enantiomer at LOQ concentration was 2.8. The percentage recovery of (S) enantiomer from (R) enantiomer samples ranged from 92 to 102. The test solution was observed to be stable up to 24 h after the preparation. The developed normal phase chiral LC method can be used for the enantiomeric purity evaluation of R-β-amino-β-(3-methoxyphenyl) propionic acid.     

Intermediates involved in serotonin oxidation catalyzed by Cu bound Aβ peptides

The degradation of neurotransmitters is a hallmark feature of Alzheimer''s disease (AD). Copper bound Aβ peptides, invoked to be involved in the pathology of AD, are found to catalyze the oxidation of serotonin (5-HT) by H₂O₂. A combination of EPR and resonance Raman spectroscopy reveals the formation of a Cu(ii)–OOH species and a dimeric, EPR silent, Cu₂O₂ bis-μ-oxo species under the reaction conditions. The Cu(ii)–OOH species, which can be selectively formed in the presence of excess H₂O₂, is the reactive intermediate responsible for 5-HT oxidation. H₂O₂ produced by the reaction of O₂ with reduced Cu(i)–Aβ species can also oxidize 5-HT. Both these pathways are physiologically relevant and may be involved in the observed decay of neurotransmitters as observed in AD patients.

The mononuclear copper hydroperoxo species (Cu(ii)–OOH) of Cu–Aβ is the active oxidant responsible for serotonin oxidation by Cu–Aβ in the presence of physiologically relevant oxidants like O₂ and H₂O₂, which can potentially cause oxidative degradation of neurotransmitters, a marker of Alzheimer''s disease.     

Influence of chemical composition and sequence length on the transport properties of proton exchange membranes

One of the integral parts of the fuel cell is the proton exchange membrane. Our research group has been engaged in the past few years in the synthesis of several sulfonated poly(arylene ether) random copolymers. The copolymers were varied in both the bisphenol structure as well as in the functional groups in the backbone such as sulfone and ketones. To compare the effect of sequence length, multiblock copolymers based on poly(arylene ether sulfone)s were synthesized. This paper aims to describe our investigation of the effect of chemical composition, morphology, and ion exchange capacity (IEC) on the transport properties of proton conducting membranes. The key properties examined were proton conductivity, methanol permeability, and water self diffusion coefficient in the membranes. It was observed that under fully hydrated conditions, proton conductivity for both random and block copolymers was a function of IEC and water uptake. However, under partially hydrated conditions, the block copolymers showed improved proton conductivity over the random copolymers. The proton conductivity for the block copolymer series was found to increase with increasing block lengths under partially hydrated conditions. © 2006 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 44: 2226–2239, 2006