The Crystal Structures of Two Calcium(II) Complexes with Pyrazine- 2,6-Dicarboxylate and Water Ligands

The structure of catena-{bis[(μ-aqua)(diaqua)(pyrazine-2,6-dicarboxylato-O,N-μ-O')](calcium(II)} consists of dimeric units composed of two calcium(II) ions, two ligand molecules and six water molecules. The calcium ions are bridged by two bidentate oxygen atoms, each donated by one carboxylic group of the ligand. The Ca(II) ion is also coordinated by one oxygen atom of the second carboxylate group and the hetero-ring nitrogen atom belonging to the same ligand molecule. Both calcium ions in a dimer are bridged to the Ca(II) ions in adjacent dimers by a pair of water molecules forming infinite molecular ribbons. In addition, each Ca(II) ion is coordinated by three water molecules; one of them is used for bridging the adjacent dimer. The coordination polyhedron around the Ca(II) ion is a pentagonal bipyramid with two apices above and one apex below the equatorial plane. The same molecular pattern is observed in the structure of catena-{bis[(μ-aqua)(diaqua)(pyrazine-2,6-dicarboxylato-O,N-μ-O')](calcium(II)} dihydrate which, in addition, contains two solvation water molecules per unit cell. In both compounds the molecular ribbons are held together by extended systems of hydrogen bonds.     

Complex Formation Reactions of Divinyltin(IV) Complexes with Amino Acids,Peptides, Dicarboxylic acids and Related Compounds

Complex formation equilibria of divinyltin(IV) with amino acids, peptides, and dicarboxylic acids have been investigated. Stoichiometry and stability constants for the complexes formed were determined at 25°C and ionic strength 0.1 M NaNO₃. The results showed the formation of ML, MLH, and ML₂ (organotin : ligand : hydrogen) complexes with amino acids. Peptides form ML complexes and the corresponding deprotonated amide species MLH_?1. In the latter species the binding with divinyltin(IV) occurs through the terminal amino group, carboxylate oxygen, and the amide nitrogen atoms (CO^? ₂, N^? _amide, NH₂). The results showed the formation of ML and ML₂ complexes with dicarboxylic acids. The concentration distribution of the complexes in solution was evaluated. The bonding sites of the divinyltin(IV) complex in solid state with oxalic acid was investigated by means of elemental analyses, FTIR, and mass spectra. Non-isothermal decomposition of the above complex has been studied and the result was statistically analyzed. The main steps were identified for the thermal decomposition reaction and each step proved to be a first order reaction. The kinetic parameters E _a and A were calculated for each step in the reaction. The thermodynamic functions H, G, and S* were calculated for each step of the reaction.     

Hydrogen Ion Affinity in Humic Acids: A Coordination Approach

Potentiometric titrations of humic acids (HA) with 0.250 M Ba(OH)₂/BaCl₂ titrant (0.750 M constant ionic strength), have been performed at 25 ± 0.1°C with a calibrated glass electrode for measuring p[H⁺]. The pK′_w for water in this BaCl₂ medium is 12.899 ± 0.006 and 13.712 ± 0.006 in 0.500 M NaCl. Divalent Ba²⁺ cations force the ionization of the acid groups and improve solubility. Under such conditions derivative potentiometric curves show diverse equivalent peaks of the acidic sites of HA. The presence of 0.05% of a neutral detergent such as Triton X-100 is essential for effective dispersion of HA in the working solutions and to obtain very stable potentiometric measurements. Computer programs were used in the treatment of the potentiometric data in order to solve a number of simultaneous equations to obtain overall conditional β _{i H} formation constants, which come from a coordination model of hydrogen ions to the organic matrix and permit calculation of conditional pK data. Conductimetric titrations with Ba(OH)₂ or NaOH give the total acidity. A typical result in BaCl₂ medium for a peat HA presents seven acid groups with the following pK data: pK ₁ = 3.80 ± 0.3, pK ₂ = 4.67 ± 0.02, pK ₃ = 7.57 ± 0.01, pK ₄ = 8.190 ± 0.005, pK ₅ = 8.80 ± 0.01, pK ₆ = 8.91 ± 0.02, and pK ₇ = 8.93 ± 0.01.     

Kinetics of the Ligand Exchange of Bipyridine in Tetradentate Nickel Complexes

An investigation into the ligand exchange reaction of bipyridine with Ni(EDDA) and Ni(trien) has led to proposed mechanisms based on steric and electronic effects. Kinetic studies show that the rate order dependence of bipyridine is different for the two reactions implying a slightly different mechanism for the bulkier trien. Both mechanisms involve unwrapping of the tetradentate ligand as the rate-determining step. This unwrapping is faster in acidic solution, because protonation of the amine groups inhibits recoordination of the ligand to the nickel. The rates for bipyridine exchange were significantly faster than for phenanthroline, suggesting an electronic ligand effect.     

Stability of caffeic acid phenethyl amide (CAPA) in rat plasma

A validated C₁₈ reverse‐phase HPLC method with UV detection at 320 nm was developed and used for the stability evaluation of caffeic acid phenethyl amide (CAPA) and caffeic acid phenethyl ester (CAPE) in rat plasma. CAPA is the amide derivative of CAPE, a naturally occurring polyphenolic compound that has been found to be active in a variety of biological pathways. CAPA has been shown to protect endothelial cells against hydrogen peroxide‐induced oxidative stress to a similar degree to CAPE. CAPE has been reported to be rapidly hydrolyzed in rat plasma via esterase enzymes. CAPA is expected to display a longer half‐life than CAPE by avoiding hydrolysis via plasma esterases. The stability of CAPA and CAPE in rat plasma was investigated at three temperatures. The half‐lives for CAPA were found to be 41.5, 10 and 0.82 h at 25, 37 and 60 °C, respectively. The half‐lives for CAPE were found to be 1.95, 0.35 and 0.13 h at 4, 25 and 37 °C, respectively. The energy of activation was found to be 22.1 kcal/mol for CAPA and 14.1 kcal/mol for CAPE. A more stable compound could potentially extend the beneficial effects of CAPE. Copyright © 2011 John Wiley & Sons, Ltd.     

The Crystal Structures of Two Polymorphic Forms of a Calcium(II) Complex with Pyridine-2,6-Dicarboxylate,Water and Nitrate Ligands

The calcium (II) complex: catena-mono(μ-pyridine-2,6-dicarboxylato-O:O:N;O') (diaqua-O)mono (nitrato-O:O)calcium(II) exists in two polymorphic forms. Each contains molecular ribbons in which adjacent Ca(II) ions are bridged by monodentate oxygen atoms donated by one carboxylate group of the pyridine-2,6-carboxylate ligand. Apart from this bridging oxygen atom, the Ca(II) ion is coordinated by two carboxylate oxygen atoms contributed by a different carboxylate group of the ligand molecule, the heteroring nitrogen atom, two water oxygen atoms and two oxygen atoms of a nitrate group giving rise to a distorted pentagonal bipyramid as a coordination polyhedron. The structures of the polymorphic modifications differ in the way in which the nitrate ligands are oriented with respect to the equatorial planes of the adjacent Ca(II) coordination polyhedra: the trans mode in the α-form; the cis mode in the β-form. In both forms, hydrogen bonds operate between the carboxylate oxygen atoms, water oxygen atoms and nitrate oxygen atoms.     

Determination of polar impurities in biodiesels using solid-phase extraction and gas chromatography-mass spectrometry

This paper reports on a method for development and validation for simultaneous characterization and determination of oxygenated polar impurities--free fatty carboxylic acids (FFAs), partial glycerides (monoacylglycerides, MGs), residual glycerol and free sterols--in various biodiesels based on the combination of solid-phase extraction (SPE), silylation and GC/MS technologies. The effects of various SPE and silylation conditions on the method recoveries were evaluated. Using this integrated SPE-GC/MS method, 38 target polar compounds (13 FFAs, 17 glycerides and 8 sterols) in 9 biodiesels derived from 4 different feedstocks were successfully separated and quantified. It was found that the carbon chain length of FFAs was ranged from C(6) to C(24), with C(16) and C(18) being the most abundant in all biodiesels. The total FFAs concentration was consistent with the acid values (AVs) measured by standard method ASTM D974-04. MG congeners with carbon number of 18 (mono-C18) were most abundant in the biodiesel samples, followed by mono-C(16) and free glycerol. β-Sitosterol and campesterol were found to be the prevailing phytosterols in all pure vegetable oil-based biodiesels, while brassicasterol and stigmasterol was only significant in the biodiesel from canola oil and soybean oil, respectively, and abundant cholesterol was only detected in animal fat-based biodiesels.     

Study on fluorescence and DNA-binding of praseodymium(III) complex containing 2,2'-bipyridine

Bentonite samples collected from vicinity of Petrovac and Aleksinac were treated with different sulfuric acid molarities. Acid attack dissolved the octahedral sheets by interlayer and edge attack. The effects of the H₂SO₄ acid caused an exchange of Al³⁺, Fe³⁺ and Mg²⁺ with H⁺ ions leading to a modification of the smectite crystalline structure. The Mg and Fe substitution in the octahedral sheets promoted the dispersion of corresponding layers and formation of amorphous silicon. The activated bentonites, after the treatment of sulfuric acid, exhibited a lower cation-exchange capacity (CEC) and significant increase of specific surface area from 6 to 387 m² g⁻¹ (bentonite from Petrovac) and from 11 to 306 m² g⁻¹ (bentonite from Aleksinac). The acid reaction caused a splitting of particles within the octahedral sheet which led to an increase in specific surface area and decrease in CEC in both bentonites.     

Poly(pyridine‐3‐boronic acid)/Multiwalled Carbon Nanotubes Modified Glassy Carbon Electrodes for Simultaneous Determination of Ascorbic Acid, 3,4‐Dihydroxyphenylacetic Acid and Uric Acid

Poly(pyridine‐3‐boronic acid) (PPBA)/multiwalled carbon nanotubes (MWCNTs) composite modified glassy carbon electrode (GCE) was used for the simultaneous determination of ascorbic acid (AA), 3,4‐dihydroxyphenylacetic acid (DOPAC) and uric acid (UA). The anodic peaks for AA, DOPAC and UA at the PPBA/MWCNTs/GCE were well resolved in phosphate buffer solution (pH 7.4). The electrooxidation of AA, DOPAC and UA in the mixture solution was investigated. The peak currents increase with their concentrations increasing. The detection limits (S/N=3) of AA, DOPAC and UA are 5 µM, 3 µM and 0.6 µM, respectively.     

Modeling solubility of acid gases in alkanolamines using the nonelectrolyte Wilson-nonrandom factor model

The nonelectrolyte Wilson-nonrandom factor local composition model (N-Wilson-NRF) by Haghtalab and Mazloumi is applied for modeling the vapor–liquid equilibrium of the acid gases (CO₂ and H₂S)–alkanolamine–water systems. The model is used to calculate the nonideality of species in liquid phase through the activity coefficient equations. In this work, we use the N-Wilson-NRF model for short-range forces in the aqueous electrolyte system of alkanolamines by using the concept of ion-pair. For the long-range interaction the Pitzer–Debye–Hückel theory is applied. The model is used to correlation of the solubility data of CO₂ and H₂S in aqueous monoethanolamine (MEA), diethanolamine (DEA), methyldiethanolamine (MDEA) and 2-amino-2methyl-1-propanol (AMP) systems over wide range of temperature (0–140 °C), partial pressure (0.001–1000 kPa) and acid gases loading (0.001–1.0 mol gas/mol amine). To show the predictability of the model, the interaction parameters without any additional adjustable parameters are used to predict the solubility of CO₂ in aqueous AMP solution at different conditions. The results of the model show a very good agreement with the experimental data.