Mechanistic features of Salmonella typhimurium propionate kinase (TdcD): Insights from kinetic and crystallographic studies

Short-chain fatty acids (SCFAs) play a major role in carbon cycle and can be utilized as a source of carbon and energy by bacteria. Salmonella typhimurium propionate kinase (StTdcD) catalyzes reversible transfer of the gamma-phosphate of ATP to propionate during L-threonine degradation to propionate. Kinetic analysis revealed that StTdcD possesses broad ligand specificity and could be activated by various SCFAs (propionate > acetate approximate to butyrate), nucleotides (ATP approximate to GTP > CTP approximate to TTP; dATP > dGTP > dCTP) and metal ions (Mg2+ approximate to Mn2+ > Co2+). Inhibition of StTdcD by tricarboxylic acid (TCA) cycle intermediates such as citrate, succinate, alpha-ketoglutarate and malate suggests that the enzyme could be under plausible feedback regulation. Crystal structures of StTdcD bound to PO4 (phosphate), AMP, ATP, Ap4 (adenosine tetraphosphate), GMP, GDP, GTP, CMP and CTP revealed that binding of nucleotide mainly involves hydrophobic interactions with the base moiety and could account for the broad biochemical specificity observed between the enzyme and nucleotides. Modeling and site-directed mutagenesis studies suggest Ala88 to be an important residue involved in determining the rate of catalysis with SCFA substrates. Molecular dynamics simulations on monomeric and dimeric forms of StTdcD revealed plausible open and closed states, and also suggested role for dimerization in stabilizing segment 235-290 involved in interfacial interactions and ligand binding. Observation of an ethylene glycol molecule bound sufficiently close to the gamma-phosphate in StTdcD complexes with triphosphate nucleotides supports direct in-line phosphoryl transfer. (C) 2013 Elsevier B.V. All rights reserved.

Synthesis, structural and optical properties of nanocrystalline Ba2NaNb5O15

Fine powders comprising nanocrystallites of barium sodium niobate, Ba2NaNb5O15 (BNN) were obtained via a citrate assisted sol-gel route at a much lower temperature than that of the conventional solid-state reaction route. The phase evolution of BNN as a function of temperature was investigated by thermogravimetric analysis (TGA), differential thermal analysis (DTA), Fourier transform infrared spectroscopy (FTIR) and X-ray powder diffraction (XRD). DTA data followed by XRD studies confirmed the BNN formation temperature to be around 923 K. The as-synthesized powders heat-treated at 923 K/10 h attained an orthorhombic structure akin to that of the parent BNN phase. Transmission electron microscopy revealed that the nanocrystallites are associated with dislocations. The optical band gap was calculated using the Kubelka-Munk function. These nanocrystallites exhibited strong visible photoluminescence (PL) at room temperature. The PL mechanism was explained by invoking the dielectric confinement effect, defect states and generation of self-trapped excitons.

Salt and cocrystals of sildenafil with dicarboxylic acids: solubility and pharmacokinetic advantage of the glutarate salt

Sildenafil is a drug used to treat erectile dysfunction and pulmonary arterial hypertension. Because of poor aqueous solubility of the drug, the citrate salt, with improved solubility and pharmacokinetics, has been marketed. However, the citrate salt requires an hour to reach its peak plasma concentration. Thus, to improve solubility and bioavailability characteristics, cocrystals and salts of the drug have been prepared by treating aliphatic dicarboxylic acids with sildenafil; the N-methylated piperazine of the drug molecule interacts with the carboxyl group of the acid to form a heterosynthon. Salts are formed with oxalic and fumaric acid; salt monoanions are formed with succinic and glutaric acid. Sildenafil forms cocrystals with longer chain dicarboxylic acids such as adipic, pimelic, suberic, and sebacic acids. Auxiliary stabilization via C-H center dot center dot center dot O interactions is also present in these cocrystals and salts. Solubility experiments of sildenafil cocrystal/salts were carried out in 0.1N HCl aqueous medium and compared with the solubility of the citrate salt. The glutarate salt and pimelic acid cocrystal dissolve faster than the citrate salt in a two hour dissolution experiment. The glutarate salt exhibits improved solubility (3.2-fold) compared to the citrate salt in water. Solubilities of the binary salts follow an inverse correlation with their melting points, while the solubilities of the cocrystals follow solubilities of the coformer. Pharmacokinetic studies on rats showed that the glutarate salt exhibits doubled plasma AUC values in a single dose within an hour compared to the citrate salt. The high solubility of glutaric acid, in part originating from the strained conformation of the molecule and its high permeability, may be the reason for higher plasma levels of the drug.

Investigation on the Sr-doped ceria Ce1-xSrO2-delta (x=0.05-0.2) as an electrolyte for intermediate temperature SOFC

The effect of Sr doping in CeO2 for its use as solid electrolytes for intermediate temperature solid oxide fuel cells (IT-SOFCs) has been explored here. Ce1-xSrxO2-delta (x = 0.05-0.2) are successfully synthesized by citrate-complexation method. XRD, Raman, FT-IR, FE-SEM/EDX and electrochemical impedance spectra are used for structural and electrical characterizations. The formation of well crystalline cubic fluorite structured solid solution is observed for x = 0.05 based on XRD and Raman spectra. For compositions i.e., x > 0.05, however, a secondary phase of SrCeO3 is confirmed by the peak at 342 cm(-1) in Raman spectra. Although the oxygen ion conductivity was found to decrease with increase in x, based on ac-impedance studies, conductivity of Ce0.95Sr0.05O2-delta was found to be higher than of Ce0.95Gd0.1O2-delta and Ce0.8Gd0.2O2-delta. The decrease in conductivity of Ce1-xSrxO2-delta with increasing dopant concentration is ascribed to formation of impurity phase SrCeO3 as well as the formation of neutral associated pairs, Se `' Ce V-o. The activation energies are found to be 0.77, 0.83, 0.85 and 0.90 eV for x = 0.05, 0.1, 0.15 and 0.20, respectively. (C) 2014 Elsevier B.V. All rights reserved.

Effect of Zn substitution on the structural and magnetic properties of Ni-Co ferrites

A series of ferrite samples with the compositional formula, Ni0.5Co0.5-xZnxFe2O4 (0 <= x <= 0.5), was prepared using the citrate based sol gel method for the better understanding of zinc doping on the structural and magnetic properties. The Rietveld-refined X-ray diffraction data revealed that the samples are having cubic structure with the Fd-3m space group. The lattice parameter increased linearly with increasing Zn content. The surface morphology and stoichiometric ratio of the compositional elements were analyzed by scanning electron microscopy equipped with energy dispersive spectroscopy (EDS). EDS showed that the elemental ratios were stoichiometric. An examination of the magnetic properties revealed an increase in saturation magnetization with increasing Zn concentration up to x=0.3 and a decrease thereafter. These results could be explained using Neel's collinear two-sub-lattice model and three-sub-lattice non-collinear model suggested by Yafet and Kittel. The magnetic cubic anisotropy constant determined by the law of approach to saturation decreased with increasing Zn content. The underlying mechanism behind observed behavior was discussed qualitatively. (C) 2014 Elsevier Ltd and Techna Group S.r.l. All rights reserved.

Photoluminescence properties of Eu3+-activated CaMoO4 phosphors for WLEDs applications and its Judd-Ofelt analysis

A series of scheelite-type Eu3+-activated CaMoO4 phosphors were synthesized by the nitrate-citrate gel combustion method. All the compounds crystallized in the tetragonal structure with space group I4(1)/a (No. 88). FESEM results reveal the spherical-like morphology. The CaMoO4 phosphor exhibited broad emission centered at 500 nm under the excitation of 298 nm wavelength, while Eu3+-activated CaMoO4 shows an intense characteristic red emission peak at 615 nm at different excitation wavelengths, due to D-5(0) -> F-7(2) transition of Eu3+ ions. The intensities of transitions between different J levels depend on the symmetry of the local environment of Eu3+ ions and were estimated using the Judd-Ofelt analysis. The high asymmetric ratio revealed that Eu3+ occupies sites with a low symmetry and without an inversion center. The CIE chromaticity co-ordinates (x, y) were calculated from emission spectra, and the values were close to the NTSC standard. Therefore, the present phosphor is highly useful for LEDs applications.

Colorimetric detection of melamine based on the size effect of AuNPs

A simple colorimetric detection of melamine was studied using 15 nm (AuNPs-I), 30 nm (AuNPs-II), and 40 nm (AuNPs-III) citrate-capped gold nanoparticles (AuNPs). The AuNPs aggregated in aqueous solution in the presence of melamine, showing a visual color change from red to blue. This color change led to a shift in the absorption peak from 527 nm, 526 nm, and 525 nm to 638 nm, 626 nm, and 680 nm for AuNPs-I, AuNPs-II, and AuNPs-III, respectively. For all the three AuNPs, linearity was observed between the melamine concentration in aqueous solution and the absorbance ratios, A(638/527), A(626/525), and A(680/526), respectively. The limit of detection (LOD) for melamine for the AuNPs-II was found to be 2.37 x 10(-8) M (correlation coefficient R-2 = 0.9745), which showed better sensitivity as compared to the LOD of the AuNPs-I and AuNPs-III, which were 3.3 x 10(-8) M and 8.9 x 10(-8) M, respectively. The synthesis of AuNPs-II also involved a lower HAuCl4 concentration compared with the other two types of AuNPs, which may reduce the process cost. The AuNPs-II was selected to analyze melamine in pre-treated milk samples, and the recovery percentage was in the range of 91-106%. Thus, the efficient detection of melamine was possible using AuNPs-II for the on-site detection without the aid of expensive instruments.

Eu3+-activated SrMoO4 phosphors for white LEDs applications: Synthesis and structural characterization

We report the synthesis of Eu3+-activated SrMoO4 phosphors by the facile nitrate-citrate gel combustion method. Powder XRD and Rietveld refinement data confirmed that these phosphors have a monophasic scheelite-type tetragonal structure with space group I4(1)/a (No. 88). FESEM micrographs indicate the agglomerated spherical particles. FTIR spectra showed four stretching and bending vibrational modes (2A(u) and 2E(u)). UV-Visible absorption spectroscopy illustrated that the optical band gap energy (E-g) values increase with increase in Eu3+ concentration. The host SrMoO4 phosphor exhibited an intense blue emission under UV excitation (368 nm). The Eu3+-activated SrMoO4 phosphors revealed characteristic luminescence due to Eu3+ ion corresponding to D-5(1) -> F-7(J) (J = 1,2) and D-5(0) -> F-7(J) (J = 1,2,3,4) transitions upon 465 nm excitation. The electric dipole transition located at 615 nm (D-5(0) -> F-7(2)) was stronger than the magnetic dipole transition located at 592 nm (D-5(0) -> F-7(1)). Intensity parameters (Omega(2), Omega(4)) and radiative properties such as transition probabilities (A(T)), radiative lifetime (tau(rad)) and branching ratio (beta) of Eu3+-activated SrMoO4 phosphors were calculated using the Judd-Ofelt theory. Based on the CIE chromaticity diagram, these phosphors can be promising materials for the development of blue and orange-red component in white LEDs. (C) 2015 Elsevier B.V. All rights reserved.

Synthesis, characterization and low temperature electrical conductivity of Polyaniline/NiFe2O4 nanocomposites

Conducting polymer/ferrite nanocomposites with an organized structure provide a new functional hybrid between organic and inorganic materials. The most popular among the conductive polymers is the polyaniline (PANI) due to its wide application in different fields. In the present work nickel ferrite (NiFe2O4) nanoparticles were prepared by sol-gel citrate-nitrate method with an average size of 21.6nm. PANI/NiFe2O4 nanoparticles were synthesized by a simple general and inexpensive in-situ polymerization in the presence of NiFe2O4 nanoparticles. The effects of NiFe2O4 nanoparticles on the dc-electrical properties of polyaniline were investigated. The structural components in the nanocomposites were identified from Fourier Transform Infrared (FTIR) spectroscopy. The crystalline phase of nanocomposites was characterized by X-Ray Diffraction (XRD). The Scanning Electron Micrograph (SEM) reveals that there was some interaction between the NiFe2O4 particles and polyaniline and the nanocomposites are composed of polycrystalline ferrite nanoparticles and PANI. The dc conductivity of polyaniline/NiFe2O4 nanocomposites have been measured as a function of temperature in the range of 80K to 300K. It is observed that the room temperature conductivity sigma(RT) decreases with increase in the relative content of NiFe2O4. The experimental data reveals that the resistivity increases for all composites with decrease of temperature exhibiting semiconductor behaviour.

An ultrastable conjugate of silver nanoparticles and protein formed through weak interactions

In recent years, silver nanoparticles (AgNPs) have attracted significant attention owing to their unique physicochemical, optical, conductive and antimicrobial properties. One of the properties of AgNPs which is crucial for all applications is their stability. In the present study we unravel a mechanism through which silver nanoparticles are rendered ultrastable in an aqueous solution in complex with the protein ubiquitin (Ubq). This involves a dynamic and reversible association and dissociation of ubiquitin from the surface of AgNP. The exchange occurs at a rate much greater than 25 s(-1) implying a residence time of <40 ms for the protein. The AgNP-Ubq complex remains stable for months due to steric stabilization over a wide pH range compared to unconjugated AgNPs. NMR studies reveal that the protein molecules bind reversibly to AgNP with an approximate dissociation constant of 55 mu M and undergo fast exchange. At pH > 4 the positively charged surface of the protein comes in contact with the citrate capped AgNP surface. Further, NMR relaxation-based experiments suggest that in addition to the dynamic exchange, a conformational rearrangement of the protein takes place upon binding to AgNP. The ultrastability of the AgNP-Ubq complex was found to be useful for its anti-microbial activity, which allowed the recycling of this complex multiple times without the loss of stability. Altogether, the study provides new insights into the mechanism of protein-silver nanoparticle interactions and opens up new avenues for its application in a wide range of systems.

Synthesis and characterization of magnetic and conductive nickel ferrite-polyaniline nanocomposites

Conducting polymer/ferrite nanocomposites with an organized structure provide a new functional hybrid between organic and inorganic materials. The most popular among the conductive polymers is the polyaniline due to its wide application in different fields. In the present work nickel ferrite nanoparticles were prepared by sol-gel citrate-nitrate method. Polyaniline/nickel ferrite nanocomposites were synthesized by a simple general and inexpensive in-situ polymerization in the presence of nickel ferrite nanoparticles. The effects of nickel ferrite nanoparticles on the DC-electrical and magnetic properties of polyaniline were investigated. The structural, morphological and thermal stability of nanocomposites were characterized by X-ray diffraction, FTIR, scanning electron micrograph and TGA. The DC conductivity of polyaniline/nickel ferrite nanocomposites have been measured as a function of temperature in the range of 80K to 300K. The magnetic properties of the nanocomposites were measured using vibrating sample magnetometer in the temperature range 300-10K up to 30 kOe magnetic field.

Synthesis of Eu3+-activated BaMoO4 phosphors and their Judd-Ofelt analysis: Applications in lasers and white LEDs

Eu3+-activated BaMoO4 phosphors were synthesized by the nitrate citrate gel combustion method. The Rietveld refinement analysis confirmed that all the compounds were crystallized in the scheelite-type tetragonal structure with I4(1)/a (No. 88) space group. Photoluminescence (PL) spectra of BaMoO4 phosphor reveals broad emission peaks at 465 and 605 nm, whereas the Eu3+-activated BaMoO4 phosphors show intense 615 nm (D-5(0) -> F-7(2)) emission peak. Judd-Ofelt theory was applied to evaluate the intensity parameters (Omega(2), Omega(4)) of Eu3+-activated BaMoO4 phosphors. The transition probabilities (A(T)), radiative lifetime (tau(rad)), branching ratio (beta), stimulated emission cross-section (sigma(e)), gain bandwidth (sigma(e) x Delta lambda(eff)) and optical gain (sigma(e) x tau(rad)) were investigated by using the intensity parameters. CIE color coordinates confirmed that the BaMoO4 and Eu3+-activated BaMoO4 phosphors exhibit white and red luminescence, respectively. The obtained results revealed that the present phosphors can be a potential candidate for red lasers and white LEDs applications. (C) 2015 Elsevier B.V. All rights reserved.

Synthesis and Photoluminescence Properties of Eu3+ activated Ce0.5Al0.5O2-delta Nanophosphors for WLEDs Application

A series of Eu3+ activated Ce0.5Al0.5O2-delta nanophosphors have been synthesized by the nitrate - citrate gel combustion method. All the compounds crystallized in the cubic fluorite CeO2 structure with space group Fm-3m (No. 225). FESEM revealed the flakes-like morphology. The average particle size was estimated from TEM studies and found to be in the range 15-25 nm. The values were in good agreement with the Scherer's method. In photoluminescence (PL) spectra, the D-5(0) -> F-7(2) (612 nm) transition dominates than other transitions which indicates that the Eu3+ ions occupy a site without inversion center. CIE chromaticity diagram confirmed that these nanophosphors can be useful in the fabrication of red component in white light emitting diodes (WLEDs).

Investigations into the structural and down-shifting and up-conversion luminescence properties of Ba2Na1-3xErxNb5O15 (0 <= x <= 0.06) nanocrystalline phosphor synthesized via sol-gel route

The present work deals with the structural and efficient down-shifting (DS) and up-conversion (UC) luminescence properties of erbium ion (Er3+) doped nanocrystalline barium sodium niobate (Ba2Na1-3xErxNb5O15, where x = 0, 0.02, 0.04 and 0.06) powders synthesized via novel citrate-based sol-gel route. The monophasic nature of the title compound was confirmed via x-ray powder diffraction followed by FT-IR studies. High-resolution transmission electron microscopy (HRTEM) facilitated the establishment of the nanocrystalline phase and the morphology of the crystallites. The Kubelka-Munk function, based on diffused reflectance studies and carried out on nano-sized crystallites, was employed to obtain the optical band-gap. The synthesized nanophosphor showed efficient DS/PL-photoluminescence and UC luminescence properties, which have not yet been reported so far in this material. The material emits intense DS green emission on excitation with 378 nm radiation. Interestingly, the material gives intense UC emission in the visible region dominated by green emission and relatively weak red emission on 976 nm excitation (NIR laser excitation). Such a dual-mode emitting nanophosphor could be very useful in display devices and for many other applications.

Structural, Optical, and Piezoelectric Response of Lead-Free Ba0.95Mg0.05Zr0.1Ti0.9O3 Nanocrystalline Powder

Nanocrystalline powders of Ba1-xMgxZr0.1Ti0.9O3 (x = 0.025-0.1) were synthesized via citrate assisted sol-gel method. Interestingly, the one with x = 0.05 in the system Ba1-xMgxZr0.1Ti0.9O3 exhibited fairly good piezoelectric response aside from the other physical properties. The phase and structural confirmation of synthesized powder was established by X-ray powder diffraction (XRD) and Raman Spectroscopic techniques. Two distinct Raman bands i.e., 303 and 723 cm(-1) characteristic of tetragonal phase were observed. Thermogravimetric analysis (TGA) was performed to evaluate the phase decomposition of the as-synthesized Ba0.95Mg0.05Zr0.1Ti0.9O3 sample as a function of temperature. The average crystallite size associated with Ba0.95Mg0.05Zr0.1Ti0.9O3 was calculated using Scherrer formula based on the XRD data and was found to be 25 nm. However, Scanning and Transmission Electron Microscopy studies revealed the average crystallite size to be in the range of 30-40 nm, respectively. Kubelka-Munk function was employed to determine the optical band gap of these nanocrystallites. A piezoelectric response of 26 pm/V was observed for Ba0.95Mg0.05Zr0.1Ti0.9O3 nanocrystal by Piezoresponse Force Microscopy (PFM) technique. Photoluminescence (PL) study carried out on these nanocrystals exhibited a blue emission (470 nm) at room temperature.