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Hydroxide Ion (hydroxide + ion)

Distribution by Scientific Domains

Chemistry	92%

Selected Abstracts

Methoxycarbonylation of Aliphatic Diamines with Dimethyl Carbonate Promoted by in situ Generated Hydroxide Ion: A Mechanistic Consideration

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 2-3 2010
Dae Won Kim
Abstract The methoxycarbonylation reactions of aliphatic diamines with dimethyl carbonate are accelerated greatly in the presence of water. Theoretical investigations on the mechanistic aspects of the methoxycarbonylation of 1,6-hexanediamine strongly suggest that the hydroxide ion, generated in situ from the interaction of 1,6-hexanediamine with water, is an active catalytic species and plays a pivotal role in the rate-determining hydrogen abstraction step from the amino group. [source]

The Reaction of Ozone with the Hydroxide Ion: Mechanistic Considerations Based on Thermokinetic and Quantum Chemical Calculations and the Role of HO4, in Superoxide Dismutation

CHEMISTRY - A EUROPEAN JOURNAL, Issue 4 2010
Gábor Merényi Prof.
Abstract The reaction of OH, with O3 eventually leads to the formation of . OH radicals. In the original mechanistic concept (J. Staehelin, J. Hoigné, Environ. Sci. Technol.1982, 16, 676,681), it was suggested that the first step occurred by O transfer: OH,+O3,HO2,+O2 and that . OH was generated in the subsequent reaction(s) of HO2, with O3 (the peroxone process). This mechanistic concept has now been revised on the basis of thermokinetic and quantum chemical calculations. A one-step O transfer such as that mentioned above would require the release of O2 in its excited singlet state (1O2, O2(1,g)); this state lies 95.5,kJ,mol,1 above the triplet ground state (3O2, O2(3,g,)). The low experimental rate constant of 70,M,1,s,1 is not incompatible with such a reaction. However, according to our calculations, the reaction of OH, with O3 to form an adduct (OH,+O3,HO4,; ,G=3.5,kJ,mol,1) is a much better candidate for the rate-determining step as compared with the significantly more endergonic O transfer (,G=26.7,kJ,mol,1). Hence, we favor this reaction; all the more so as numerous precedents of similar ozone adduct formation are known in the literature. Three potential decay routes of the adduct HO4, have been probed: HO4,,HO2,+1O2 is spin allowed, but markedly endergonic (,G=23.2,kJ,mol,1). HO4,,HO2,+3O2 is spin forbidden (,G=,73.3,kJ,mol,1). The decay into radicals, HO4,,HO2.+O2.,, is spin allowed and less endergonic (,G=14.8,kJ,mol,1) than HO4,,HO2,+1O2. It is thus HO4,,HO2.+O2., by which HO4, decays. It is noted that a large contribution of the reverse of this reaction, HO2.+O2.,,HO4,, followed by HO4,,HO2,+3O2, now explains why the measured rate of the bimolecular decay of HO2. and O2., into HO2,+O2 (k=1×108,M,1,s,1) is below diffusion controlled. Because k for the process HO4,,HO2.+O2., is much larger than k for the reverse of OH,+O3,HO4,, the forward reaction OH,+O3,HO4, is practically irreversible. [source]

ChemInform Abstract: Hydrated Electron Production by Reaction of Hydrogen Atoms with Hydroxide Ions: A First-Principles Molecular Dynamics Study.

CHEMINFORM, Issue 44 2008
Jean Philippe Renault
Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

Electrochemical Elucidation of the Facilitated Ion Transport Across a Bilayer Lipid Membrane in the Presence of Neutral Carrier Compounds

ELECTROANALYSIS, Issue 11 2010
Jun Onishi
Abstract The ion transport facilitated by neutral carrier compounds (valinomycin, nonactin) has been investigated by cyclic voltammetry in the several electrolyte solutions (KF, KCl, KBr, KNO3, KSCN, KClO4), and we demonstrated the effect of the counter anions on the facilitated transport of K+ from the viewpoint of electroneutrality. Voltammograms for the ion transport were generated at steady state and the current density between W1 and W2, jW1,W2, increased with the absolute value of the applied membrane potential, EW1,W2. Then, the magnitude of jW1,W2 at a certain EW1,W2 increased with the hydrophobicity of the counter anion. It was proved that the logarithm of |jW1,W2|at a certain EW1,W2 is almost proportional to the hydration energy of the counter anion. This indicates that not only K+ but also the counter anion distributes into the BLM. Therefore, the magnitude of jW1,W2 at a certain EW1,W2 increased with an increase of pH, because the hydroxide ion was served as a counter anion. Based on the variation of the zero-current potential in case of various asymmetrical ionic compositions, it is found that the amount of cation transport is much larger than that of anion transport. [source]

Kinetics of Bis(p -nitrophenyl)phosphate (BNPP) Hydrolysis Reactions with Trivalent Lanthanide Complexes of N -Hydroxyethyl(ethylenediamine)- N,N,,N, -triacetate (HEDTA),

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 8 2009
C. Allen Chang
Abstract Kinetic studies of hydrolysis reactions of BNPP [sodium bis(p -nitrophenyl)phosphate] with trivalent lanthanide (Ln3+) complexes of HEDTA [HEDTA = N -hydroxyethyl(ethylenediamine)- N,N,,N, -triacetate] were performed at pH 6.96,11.34 and 25 °C by a spectrophotometric method and by HPLC analysis. The reaction rates increase with increasing atomic number of lanthanide and solution pH from PrHEDTA to EuHEDTA and then decrease for heavier LnHEDTA complexes. Plots of pseudo-first-order rate constants (kobs) vs. pH could be fitted to the equation kobs = kLnL(OH)[LnL]T/{1,+,exp[,2.303(pH,,,pKh)]}, where kLnL(OH) is the rate constant for the reaction of LnHEDTA(OH), with BNPP, Kh is the hydrolysis constant of LnHEDTA, and [LnL]T is the total concentration of LnHEDTA. The pKh values obtained by the kinetic method are in the range 8.2,10.3 and are similar to those measured by potentiometric methods. At [LnL]T = 10,70 mM and pH 10.5, most of the observed pseudo-first-order rate constants could be fitted to a simple saturation kinetic model, kobs = k1K[LnHEDTA(OH),]/{1 + K[LnHEDTA(OH),]}, where K is the equilibrium constant for the formation for LnHEDTA(OH),BNPP and is in the range 2,147 M,1. The k1 values are in the range 1.12,×,10,5,2.71,×,10,3 s,1. The kobs data for TbHEDTA and HoHEDTA were fitted to a quadratic equation. It was observed that the dinuclear species are more reactive. ESI mass spectrometry confirmed that the reaction between BNPP and EuHEDTA is a simple hydrolysis but not a transesterification, presumably because the three inner-sphere coordinated water molecules are far away from the coordinated hydroxyethyl group. Hydrolysis is likely to occur by proton transfer from one inner-sphere coordinated water molecule to the deprotonated ethyl oxide group followed by nucleophilic attack of the resulting hydroxide ion on the bonded BNPP anion.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

Kinetics and mechanism of oxidation of the drug mephenesin by bis(hydrogenperiodato)argentate(III) complex anion

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 8 2007
Shigang Shen
Mephenesin is being used as a central-acting skeletal muscle relaxant. Oxidation of mephenesin by bis(hydrogenperiodato)argentate(III) complex anion, [Ag(HIO6)2]5,, has been studied in aqueous alkaline medium. The major oxidation product of mephenesin has been identified as 3-(2-methylphenoxy)-2-ketone-1-propanol by mass spectrometry. An overall second-order kinetics has been observed with first order in [Ag(III)] and [mephenesin]. The effects of [OH,] and periodate concentration on the observed second-order rate constants k, have been analyzed, and accordingly an empirical expression has been deduced: k, = (ka + kb[OH,])K1/{f([OH,])[IO,4]tot + K1}, where [IO,4]tot denotes the total concentration of periodate, ka = (1.35 ± 0.14) × 10,2M,1s,1 and kb = 1.06 ± 0.01 M,2s,1 at 25.0°C, and ionic strength 0.30 M. Activation parameters associated with ka and kb have been calculated. A mechanism has been proposed to involve two pre-equilibria, leading to formation of a periodato-Ag(III)-mephenesin complex. In the subsequent rate-determining steps, this complex undergoes inner-sphere electron transfer from the coordinated drug to the metal center by two paths: one path is independent of OH, whereas the other is facilitated by a hydroxide ion. In the appendix, detailed discussion on the structure of the Ag(III) complex, reactive species, as well as pre-equilibrium regarding the oxidant is provided. © 2007 Wiley Periodicals, Inc. Int J Chem Kinet 39: 440,446, 2007 [source]

Methoxycarbonylation of Aliphatic Diamines with Dimethyl Carbonate Promoted by in situ Generated Hydroxide Ion: A Mechanistic Consideration

Molecular dynamics simulations of the detoxification of paraoxon catalyzed by phosphotriesterase

JOURNAL OF COMPUTATIONAL CHEMISTRY, Issue 15 2009
Xin Zhang
Abstract Combined QM(PM3)/MM molecular dynamics simulations together with QM(DFT)/MM optimizations for key configurations have been performed to elucidate the enzymatic catalysis mechanism on the detoxification of paraoxon by phosphotriesterase (PTE). In the simulations, the PM3 parameters for the phosphorous atom were reoptimized. The equilibrated configuration of the enzyme/substrate complex showed that paraoxon can strongly bind to the more solvent-exposed metal ion Zn,, but the free energy profile along the binding path demonstrated that the binding is thermodynamically unfavorable. This explains why the crystal structures of PTE with substrate analogues often exhibit long distances between the phosphoral oxygen and Zn,. The subsequent SN2 reaction plays the key role in the whole process, but controversies exist over the identity of the nucleophilic species, which could be either a hydroxide ion terminally coordinated to Zn, or the ,-hydroxo bridge between the ,- and ,-metals. Our simulations supported the latter and showed that the rate-limiting step is the distortion of the bound paraoxon to approach the bridging hydroxide. After this preparation step, the bridging hydroxide ion attacks the phosphorous center and replaces the diethyl phosphate with a low barrier. Thus, a plausible way to engineer PTE with enhanced catalytic activity is to stabilize the deformed paraoxon. Conformational analyses indicate that Trp131 is the closest residue to the phosphoryl oxygen, and mutations to Arg or Gln or even Lys, which can shorten the hydrogen bond distance with the phosphoryl oxygen, could potentially lead to a mutant with enhanced activity for the detoxification of organophosphates. © 2009 Wiley Periodicals, Inc. J Comput Chem, 2009 [source]

Stability and hydrolysis kinetics of spirosuccinimide type inhibitors of aldose reductase in aqueous solution and retardation of their hydrolysis by the target enzyme

JOURNAL OF PHARMACEUTICAL SCIENCES, Issue 4 2008
Masuo Kurono
Abstract The stability and the hydrolysis kinetics of spirosuccinimide type aldose reductase (AR) inhibitors, SX-3030 (racemate) and its optical enantiomers (R - and S -isomers), were investigated in aqueous solution. The hydrolysis followed pseudo-first-order kinetics and showed significant pH dependence. Maximum solution stability was observed below pH 2.4, whereas the hydrolysis was gradually catalyzed by hydroxide ion at neutral to alkaline pH while the compounds exhibiting moderate pH-independent stability at acidic to neutral conditions (pH 4,7) to enable oral administration. A pK of 3.7 was obtained from the pH-rate profile, but this kinetically derived pK is approximately 2 pH units below the pK of the parent compounds, suggesting the presence of an acidic intermediate involved in the hydrolysis process. These findings, together with structural analysis, support the notion that the hydrolysis would proceed via nucleophilic attack of a water molecule or hydroxide ion on the scissile carbonyl bond of the succinimide ring to form a succinamic acid intermediate that has a ,-keto acid structure, followed by decarboxylation to give a racemized succinimide ring-opened product. On the other hand, the interconversion of the R - and S -isomers did not occur during hydrolysis; however, the hydrolysis of the R -isomer was markedly suppressed by the target enzyme AR whereas that of the S -isomer was not, indicating a high degree of complementarity of interacting surfaces between the R -isomer and the enzyme. The results in the present study could provide useful clues for facilitating the appropriate stabilization strategies as well as for evaluating the pharmacological effects on target tissues in vivo, and suggested that the R -isomer may be a suitable candidate as AR inhibitor. © 2007 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 97:1468,1483, 2008 [source]

Refined structure of bovine carboxypeptidase A at 1.25,Å resolution

ACTA CRYSTALLOGRAPHICA SECTION D, Issue 2 2003
Alexandra Kilshtain-Vardi
The crystal structure of the bovine zinc metalloproteinase carboxypeptidase A (CPA) has been refined to 1.25,Å resolution based on room-temperature X-ray synchrotron data. The significantly improved structure of CPA at this resolution (anisotropic temperature factors, R factor = 10.4%, Rfree = 14.5%) allowed the modelling of conformational disorders of side chains, improved the description of the protein solvent network (375 water molecules) and provided a more accurate picture of the interactions between the active-site zinc and its ligands. The calculation of standard uncertainties in individual atom positions of the refined model of CPA allowed the deduction of the protonation state of some key residues in the active site and confirmed that Glu72 and Glu270 are negatively charged in the resting state of the enzyme at pH 7.5. These results were further validated by theoretical calculations that showed significant reduction of the pKa of these side chains relative to solution values. The distance between the zinc-bound solvent molecule and the metal ion is strongly suggestive of a neutral water molecule and not a hydroxide ion in the resting state of the enzyme. These findings could support both the general acid/general base mechanism, as well as the anhydride mechanism suggested for CPA. [source]

Determination of the Catalytic Pathway of a Manganese Arginase Enzyme Through Density Functional Investigation

CHEMISTRY - A EUROPEAN JOURNAL, Issue 32 2009
Monica Leopoldini Dr.
Abstract The catalytic mechanism of dimanganese-containing arginase enzyme has been investigated by DFT calculations. Two exchange-correlation functionals, B3,LYP and MPWB1,K, have been used to construct the potential energy profiles for the hydrolysis of an arginine substrate performed by an arginase active site model system. Two reaction mechanisms have been investigated, one involving a water molecule (mechanism,1) and the other involving a hydroxide ion (mechanism,2) as nucleophilic agent. Results obtained in the gas phase and in the protein environment have indicated that mechanism,1 involving a water molecule entails structural features as well as an activation energy for the rate-determining step that are inconsistent with experimental data available for the arginase enzyme. On the other hand, when a hydroxide ion is present at the Mn2 site, a lower activation energy and a structural arrangement closer to the experimental indication are obtained. [source]

Kinetic characterization of a transient reaction by degeneration of the precursor mechanism: Application to the synthesis of 3,4-diazabicyclo[4.3.0]- non-2-ene

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 5 2006
H. Delalu
The rate of the oxidation of N -amino-3-azabicyclo[3.3.0]octane by chloramine has been studied by GC and HPLC between pH 10.5 and 13.5. The second-order reaction exhibits specific acid catalysis. The formation of N,N,-azo-3-azabicyclo[3.3.0]octane or 3,4-diazabicyclo[4.3.0]non-2-ene is pH, concentration, and temperature dependent. In alkaline media, the exclusive formation of 3,4-diazabicyclo[4.3.0]non-2-ene is observed. Kinetic studies show that the oxidation of N -amino-3-azabicyclo[3.3.0]octane by chloramine is a multistep process with the initial formation of a diazene-type intermediate, which is converted by hydroxide ions into 3,4-diazabicyclo[4.3.0]non-2-ene. Because it was not possible to follow the rate of change of the intermediate concentration, to determine the kinetics of 3,4-diazabicyclo[4.3.0]non-2-ene formation, a procedure based on the degeneration of the precursor process was adopted. An appropriate mathematical treatment allowed a quantitative interpretation of all the phenomena observed over the given pH interval. The activation parameters were determined. © 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 327,338, 2006 [source]

Highly Ordered Interstitial Water Observed in Bone by Nuclear Magnetic Resonance,

JOURNAL OF BONE AND MINERAL RESEARCH, Issue 4 2005
Erin E Wilson
Abstract NMR was used to study the nanostructure of bone tissue. Distance measurements show that the first water layer at the surface of the mineral in cortical bone is structured. This water may serve to couple the mineral to the organic matrix and may play a role in deformation. Introduction: The unique mechanical characteristics of bone tissue have not yet been satisfactorily connected to the exact molecular architecture of this complex composite material. Recently developed solid-state nuclear magnetic resonance (NMR) techniques are applied here to the mineral component to provide new structural distance constraints at the subnanometer scale. Materials and Methods: NMR dipolar couplings between structural protons (OH, and H2O) and phosphorus (PO4) or carbon (CO3) were measured using the 2D Lee-Goldburg Cross-Polarization under Magic-Angle Spinning (2D LG-CPMAS) pulse sequence, which simultaneously suppresses the much stronger proton-proton dipolar interactions. The NMR dipolar couplings measured provide accurate distances between atoms, e.g., OH and PO4 in apatites. Excised and powdered femoral cortical bone was used for these experiments. Synthetic carbonate (,2-4 wt%)-substituted hydroxyapatite was also studied for structural comparison. Results: In synthetic apatite, the hydroxide ions are strongly hydrogen bonded to adjacent carbonate or phosphate ions, with hydrogen bond (O-H) distances of ,1.96 Å observed. The bone tissue sample, in contrast, shows little evidence of ordered hydroxide. Instead, a very ordered (structural) layer of water molecules is identified, which hydrates the small bioapatite crystallites through very close arrangements. Water protons are ,2.3-2.55 Å from surface phosphorus atoms. Conclusions: In synthetic carbonated apatite, strong hydrogen bonds were observed between the hydroxide ions and structural phosphate and carbonate units in the apatite crystal lattice. These hydrogen bonding interactions may contribute to the long-range stability of this mineral structure. The biological apatite in cortical bone tissue shows evidence of hydrogen bonding with an ordered surface water layer at the faces of the mineral particles. This structural water layer has been inferred, but direct spectroscopic evidence of this interstitial water is given here. An ordered structural water layer sandwiched between the mineral and the organic collagen fibers may affect the biomechanical properties of this complex composite material. [source]