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At least 1,207 records · Page 67Linked to original sources
PubMed2000

Syntheses, characterizations, and single-crystal X-ray structures of soluble titanium alkoxide phosphonates.

Reactions of Ti(OiPr)4 with different phosphonic acids RPO3H2 (R = Ph, 4-CNPh, Me, tBu) in organic solvents have been investigated. In the presence of small amounts of water, the new molecular titanium oxide alkoxide phosphonates [Ti4(mu 3-O)(OiPr)5(mu-OiPr)3(RPO3)3].DMSO [R = Ph (1), Me (2), tBu (3), 4-CNPh (4)] were isolated. The single-crystal X-ray structure analyses of 1 and 2 revealed hexacoordinated titanium atoms and a connectivity of (111) for each phosphonate. Under rigorous exclusion of water, the reaction of Ti(OiPr)4 with tert-butylphosphonic acid in toluene gave the titanium phosphonate tetramer [Ti(OiPr)2(tBuPO3)]4 (5). A single-crystal X-ray structure analysis of 5 revealed a 5 + 1 coordination of the titanium atoms as a result of the (112) connectivity of each phosphonate; such a coordination mode has never been reported for a titanium phosphate, phosphonate, or phosphinate. Compounds 1-5 were characterized by FT-IR, 31P MAS NMR, and solution multinuclear NMR (1H, 13C(1H,) 31P(1H)) spectroscopies. 13C CP MAS NMR experiments were carried out on arylphosphonates 1 and 4. Solution NMR experiments were also used to investigate the exchange reaction between 1 and 2 and the conversion of 5 to [Ti4(mu 3-O)(OiPr)5(mu-OiPr)3(tBuPO3)3].iPrOH by partial hydrolysis in the presence of Ti(OiPr)4. The phosphonate clusters 1-5 are soluble in organic solvents and are likely intermediates in the sol-gel processing of inorganic-organic hybrids based on titanium oxide and phosphonate groups that we are currently developing.

Journal Article↗
PubMed2005

Analysis of reduced phosphorus in samples of environmental interest.

The combination of ion chromatography (IC) and inductively coupled plasma emission spectroscopy (ICP-ES) was used forthe sensitive and specific detection of hypophosphite (PO2), phosphite (PO3), methylphosphonic acid (MPA), and phosphate (PO4). Application of this technique to a wide range of environmental samples proved that reduced phosphorus was present in some situations including process water from thermal phosphorus plants, drinking water contacting cast iron, and phosphorus corrosion inhibitor used in water treatment and in sewage wastewater. Preliminary testing did not detect high concentrations of reduced phosphorus and phosphine in situations where it was previously reported to be very important, including anaerobic digesters in wastewater treatment plants. The new IC-ICP-ES technique is a promising tool for use in corrosion and soil research where phosphites are likely to be present.

Chromatography, Ion Exchange↗
PubMedDate not supplied

Synergistic extraction of cobalt (II) by beta hydroxy naphthaldoxime and neutral donors.

Liquid - liquid extraction of Cobalt (II) from aqueous hydrochloric acid medium by the use of oxime derivative of beta-hydroxy naphthaldehyde in o-xylene medium is being reported. The ligand was synthesized and characterized in our laboratory. The typical pH range of extraction of Co (II) by the ligand - donor combination was seen to be between 8 - 9. The effect of different donors like dimethyl sulphoxide (DMSO), trioctyl phosphine oxide (TOPO) and bis(2-ethylhexyl) phosphonate and the effect of various diluents on the extraction of Co(II) by the present method were studied in detail. The binary adduct formation constant (log kex) in the organic phase was found to be 3.182. The overall equilibrium constant (log K) for the ternary species [Co(A)(DMSO)(Cl)], [Co(A)(TOPO)(Cl)] and [Co(A)(phosphonate)(Cl)] were estimated to be 6.42, 6.22 and 6.25 respectively. The trend in equilibrium constants were in accordance with their basic character, i.e., (DMSO > or = TOPO approximately/= phosphonate). The results were extrapolated in the determination of Cobalt in pharmaceutical drugs and cobalt bearing ore samples.

Aldehydes↗
PubMed1992

Recent advances in the design of HIV proteinase inhibitors.

Inhibition of HIV proteinase is currently one of the most widely studied approaches for chemotherapeutic intervention in the treatment of AIDS. A range of inhibitors of this essential enzyme has been designed from detailed knowledge of its mechanism of action and cleavage sites. These inhibitors have been classified according to their derivation. All are transition-state analogues and contain a hydroxyethylene, hydroxyethylamine, phosphinate or symmetrical moiety. Many of these inhibitors have high selectivity for the viral enzyme and significant antiviral activity. Advances in the design of HIV proteinase inhibitors that have been reported in the past year are reviewed.

Amino Acid Sequence↗
PubMed2006

Stereoselective synthesis of vinylsilanes by a gold(I)-catalyzed acetylenic sila-cope rearrangement.

Cationic tri-tert-butylphosphinegold(I) serves as a catalyst in the sila-Cope rearrangement of acetylenic allylsilanes. When phenol is employed as a nucleophile, the reaction allows for the stereoselective synthesis of vinylsilanes. Alternatively, use of methanol as a nucleophile leads to cyclic vinylsilanes, which can be viewed as latent vinylsilanes that are revealed on treatment with a mild Lewis acid. Thus, both of these reagents serve as useful reagents for stereoselective synthesis of trisubstituted olefins through transition-metal-catalyzed cross-coupling reactions.

Alkynes↗
PubMed2006

Tricyclohexylphosphine-cyclopalladated ferrocenylimine complexes: synthesis, crystal structures and application in Suzuki and Heck reactions.

A series of novel tricyclohexylphosphine (PCy(3))-cyclopalladated ferrocenylimine complexes 2c-2g have been easily synthesized. These new palladacycles are thermally stable and are not sensitive to air and moisture. Their detailed structures have been determined by single-crystal X-ray analysis and six different types of intermolecular hydrogen bonds are found to be existed in the crystals of these complexes. The use of 2c-2g as catalysts for Suzuki and Heck reactions was examined. They were found to be very efficient for the Suzuki reaction of aryl chlorides with phenylboronic acid. Typically, using 0.1 mol% of catalyst in the presence of 1.5 equivalent of Cs(2)CO(3) as base in dioxane at 100 degrees C provided coupled products in excellent yields. These complexes also displayed good activity in the Heck reaction of a range of aryl bromides with acrylic acid ethyl ester although they were not particularly useful for the activation of aryl chlorides.

Catalysis↗
PubMed2005

Hydrogen elimination from a hydroxycyclopentadienyl ruthenium(II) hydride: study of hydrogen activation in a ligand-metal bifunctional hydrogenation catalyst.

At high temperatures in toluene, [2,5-Ph(2)-3,4-Tol(2)(eta(5)-C(4)COH)]Ru(CO)(2)H (3) undergoes hydrogen elimination in the presence of PPh(3) to produce the ruthenium phosphine complex [2,5-Ph(2)-3,4-Tol(2)-(eta(4)-C(4)CO)]Ru(PPh(3))(CO)(2) (6). In the absence of alcohols, the lack of RuH/OD exchange, a rate law first order in Ru and zero order in phosphine, and kinetic deuterium isotope effects all point to a mechanism involving irreversible formation of a transient dihydrogen ruthenium complex B, loss of H(2) to give unsaturated ruthenium complex A, and trapping by PPh(3) to give 6. DFT calculations showed that a mechanism involving direct transfer of a hydrogen from the CpOH group to form B had too high a barrier to be considered. DFT calculations also indicated that an alcohol or the CpOH group of 3 could provide a low energy pathway for formation of B. PGSE NMR measurements established that 3 is a hydrogen-bonded dimer in toluene, and the first-order kinetics indicate that two molecules of 3 are also involved in the transition state for hydrogen transfer to form B, which is the rate-limiting step. In the presence of ethanol, hydrogen loss from 3 is accelerated and RuD/OH exchange occurs 250 times faster than in its absence. Calculations indicate that the transition state for dihydrogen complex formation involves an ethanol bridge between the acidic CpOH and hydridic RuH of 3; the alcohol facilitates proton transfer and accelerates the reversible formation of dihydrogen complex B. In the presence of EtOH, the rate-limiting step shifts to the loss of hydrogen from B.

Catalysis↗
PubMed2005

Characterization of the redox and metal binding activity of BsSco, a protein implicated in the assembly of cytochrome c oxidase.

Members of the Sco protein family are implicated in the assembly of the respiratory complex cytochrome c oxidase. Several possible roles have been proposed for Sco: a copper delivery agent, a site-specific thiol reductase, and an indicator of cellular redox status. Two cysteine residues (C45 and C49) in the sequence CXXXCP and a histidine (H135) approximately 90 residues toward the C-terminus are conserved in Sco from bacteria, yeast, and humans. The soluble domain of Sco has a thioredoxin fold that is suggestive of redox activity for this protein. We have characterized the soluble domain of the Sco protein from Bacillus subtilis (i.e., sBsSco) for its redox reactivity and metal binding capacity. In oxidized sBsSco, the cysteines are present as an intramolecular disulfide. Oxidized sBsSco does not bind metal, but can be reduced in vitro to a metal-binding form. Reduction of the disulfide in sBsSco is accompanied by increased intrinsic fluorescence. The reducibility of the cystine is unchanged when the conserved histidine is mutated to alanine. Tight binding by reduced sBsSco is observed for Cu(II) by electronic absorption, intrinsic fluorescence, and EPR spectroscopies, and isothermal titration calorimetry with an observed stoichiometry of one Cu(II) ion per sBsSco and a KD of approximately 50 nM. Tight binding of Cu(I) and Ag(I) is observed by quenching of intrinsic tryptophan fluorescence. Cobalt(II) exhibits weak binding, whereas Ni(II) and Zn(II) do not appear to bind. The high-affinity binding of metals by BsSco is triggered by its redox state, and this property could be important for its function in vivo.

Amino Acid Substitution↗
PubMed1999

A novel endogenous inhibitor of phenoloxidase from Musca domestica has a cystine motif commonly found in snail and spider toxins.

Phenoloxidase inhibitor (POI), found in the hemolymph of housefly pupae, is a novel dopa-containing and cystine-rich peptide that competitively inhibits phenoloxidase with a Ki in the nanomolar range. [Tyr32]POI is a potential precursor molecule also found in the hemolymph that may be posttranslationally oxidized to the dopa-containing peptide after creation of a rigid structure. By employing both a solid-phase peptide synthesis system based on a 9-fluorenylmethoxycarbonyl strategy and a specific air oxidation technique to ensure correct folding, we have been able to synthesize [Tyr32]POI. The synthetic [Tyr32]POI was confirmed to be identical to the native [Tyr32]POI by coelution high-performance liquid chromatography analysis and by enzymatic analysis using the phenoloxidase inhibition assay. To determine the disulfide pairings within the peptides, a series of enzyme hydrolyses and partial reduction/alkylation steps were performed. Three cystine pairs (Cys11-Cys25, Cys18-Cys29, and Cys24-Cys36) were determined by identification of the resulting peptides. The disulfide pairings of the two adjacent Cys residues (Cys11-Cys25 and Cys24-Cys36) were unambiguously assigned by comparing the derived fragments with the two possible isomers synthesized through a novel disulfide-linking technique. The arrangement of the disulfide bridges in POI was found to be topologically identical to those found for several peptides within the inhibitor cystine knot structural family. Although these peptides share a low primary sequence homology and display a diversity of biological functions, they nonetheless share similarities in their cystine motifs and tertiary structure. The tertiary structure model of POI, which was derived through molecular dynamics and energy minimization studies using restraints with determined disulfide connectivities, suggests that POI is a new class member of the inhibitor cystine-knot structural family.

Amino Acid Sequence↗
PubMed2003

Novel class of tertiary phosphine ligands based on a phospha-adamantane framework and use in the Suzuki cross-coupling reactions of aryl halides under mild conditions.

[reaction: see text] A new class of sterically hindered phosphines based on a phospha-adamantane framework is described. Arylation or alkylation of the 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phospha-adamantane system allows for the preparation of tertiary phosphines suitable for use in palladium-catalyzed cross-coupling reactions. For example, use of a catalytic system incorporating Pd(2)(dba)(3) and 1,3,5,7-tetramethyl-2,4,8-trioxa-6-phenyl-6-phospha-adamantane is shown to promote the Suzuki cross-coupling of aryl iodides, bromides, and activated chlorides with a variety of aryl boronic acids at room temperature in a few hours with high yields.

Journal Article↗
PubMed2006

Key role of the Lewis base position in asymmetric bifunctional catalysis: design and evaluation of a new ligand for chiral polymetallic catalysts.

New chiral ligands for asymmetric polymetallic catalysts were designed on the basis of the assumption that the higher-order assembly structure is stabilized by modifying the modular unit. The designed ligands 6 and 7 contained a scaffolding cyclohexane ring with a Lewis base phosphine oxide directly attached to the scaffold. A module in the polymetallic complex contains two metals per ligand, and a stable 6-, 5-, 5-membered fused chelation ring system should be generated. Synthesis of these ligands is simple and high yielding, using a catalytic dynamic kinetic resolution promoted by the Trost catalyst as a key step. Ligand function was assessed in a catalytic asymmetric ring-opening reaction of meso-aziridines with TMSCN, a useful reaction for the synthesis of optically active beta-amino acids. The Gd complex generated from Gd(OiPr)3 and the ligand was a highly active and enantioselective catalyst in this reaction. Enantioselectivity was reversed compared to the previously reported d-glucose-derived catalyst containing the same chirality of the individual module. ESI-MS analysis and X-ray crystallographic studies indicate that the assembly state of the modules in the polymetallic catalysts differs depending on the chiral ligand. The difference in the higher-order structure stems from a subtle change (one carbon) in the position of the Lewis base relative to the Gd metal. The change in the higher-order structure of the polymetallic complex led to a dramatic reversal of the enantioselectivity and increased catalyst activity.

Aziridines↗
PubMed1993

Modification of catalytic properties of chicken liver fructose 1,6-bisphosphatase by allicin.

The activity of chicken liver fructose 1,6-bisphosphatase increases dramatically after incubation with allicin, a major biologically active compound produced by garlic. Activation is more pronounced when the enzyme is assayed with Mn2+ than Mg2+. Maximum activation is accompanied by the disappearance of 4 highly reactive sulfhydryl groups per molecule of enzyme. This modification also leads to loss of activation by K+, and reduced sensitivity to inhibition by AMP, fructose 2,6-bisphosphate, and high concentration of fructose 1,6-bisphosphate. All the altered properties induced by allicin can be reversed by dithiothreitol or tris(2-carboxyethyl)phosphine, the latter being much more effective.

Animals↗
PubMed1985

Reactivity and activation of dioxygen-derived species in aprotic media (a model matrix for biomembranes).

In aprotic media the electrochemical reduction of dioxygen yields superoxide ion (O2-), which is an effective Brønsted base, nucleophile, one-electron reductant, and one-electron oxidant of reduced transition metal ions. With electrophilic substrates (organic halides and carbonyl carbons) O2- displaces a leaving group to form a peroxy radical (ROO.) in the primary process. Superoxide ion oxidizes the activated hydrogen atoms of ascorbic acid, catechols, hydrophenazines and hydroflavins. Combination of O2- with 1,2-diphenylhydrazine yields the anion radical of azobenzene, which reacts with O2 to give azobenzene and O2- (an example of O2--induced autoxidation). With phenylhydrazine, O2- produces phenyl radicals. The in situ formation of HO2. (O2- plus a proton source) results in H-atom abstraction from allylic and other groups with weak heteroatom--H bonds (binding energy (b.e.) less than 335 kJ). This is a competitive process with the facile second-order disproportionation of HO2. to H2O2 and O2 (kbi approximately equal to 10(4) mol-1 s-1 in Me2SO). Addition of [FeII(MeCN)4] (ClO4)2 to solutions of hydrogen peroxide in dry acetonitrile catalyses a rapid disproportionation of H2O2 via the initial formation of an adduct [FeII(H2O2)2+----Fe(O)(H2O)2+], which oxidizes a second H2O2 to oxygen. In the presence of organic substrates such as 1,4-cyclohexadiene, 1,2-diphenylhydrazine, catechols and thiols the FeII-H2O2/MeCN system yields dehydrogenated products; with alcohols, aldehydes, methylstyrene, thioethers, sulphoxides, and phosphines, the FeII(H2O2)2+ adduct promotes their monoxygenation. The product from the FeO2+-H2O2 reaction, [FeII(H2O2)22+], exhibits chemistry that is closely similar to that for singlet oxygen (1O2), which has been confirmed by the stoichiometric dioxygenation of diphenylisobenzofuran, 9,10-diphenylanthracene, rubrene and electron-rich unsaturated carbon-carbon bonds (Ph2C = CPh2, PhC = CPh and cis-PhCH = CHPh). In dry ligand-free acetonitrile (MeCN), anhydrous ferric chloride (FeIIICl3) activates hydrogen peroxide for the efficient epoxidation of alkenes. The FeIIICl3 further catalyses the dimerization of the resulting epoxides to dioxanes. These observations indicate that strong Lewis acids that are coordinatively unsaturated, [FeII(MeCN)4]2+ and [FeIIICl3], activate H2O2 to form an effective oxygenation and dehydrogenation agent.(ABSTRACT TRUNCATED AT 400 WORDS)

Cell Membrane↗
PubMed1996

Synthesis and Characterization of Palladium(II) and Platinum(II) Complexes Containing Water-Soluble Hybrid Phosphine-Phosphonate Ligands.

Water-soluble phosphonate-functionalized triaryl phosphine ligands Na(2)[Ph(2)P(4-C(6)H(4)PO(3))].1.5H(2)O (4a), Na(2)[Ph(2)P(3-C(6)H(4)PO(3))].2H(2)O (4b), and Na(2)[Ph(2)P(2-C(6)H(4)PO(3))].2H(2)O (4c), were prepared in 54-56% yields by the transesterification and hydrolysis of the appropriate phosphonic acid diethyl ester precursors. The solubilities of 4a-c in water are compared and the spectroscopic properties studied in detail. The crystal structure of Na(2)[Ph(2)P(4-C(6)H(4)PO(3))(H(2)O)(3)(CH(3)OH)].CH(3)OH (monoclinic, P2(1)/n, a = 6.4457(8) Å, b = 8.1226(8) Å, c = 46.351(3) Å, beta = 92.902(8) degrees, Z = 4) shows a dimeric association via two bridging water molecules and four sodium ions. Reaction of 4a with PtCl(2)(PPh(3))(2) in a biphasic H(2)O/CH(2)Cl(2) mixture gives cis- and trans-Na(4)[PtCl(2){Ph(2)P(4-C(6)H(4)PO(3))}(2)]. 3H(2)O. Palladium dichloride and 4a in H(2)O/benzene catalyzes the carbonylation of benzyl chloride to give phenylacetic acid (91%).

Journal Article↗
PubMed1985

A rapid synthesis of a DNA fragment using an unprotected nucleoside and a phosphine derivative.

A rapid synthesis of DNA fragment from unprotected nucleoside and phosphine derivative, morpholinophosphordichloridite, has been studied, demonstrating a d(T-T) and its amino-phosphonate derivative syntheses. A high selectivity of this reagent eliminates the protection of nucleoside hydroxyl groups. The P-N bond in the resulting dinucleoside phosphite can readily be converted to a phosphite triester with alcohol and to the corresponding aminophosphonate by a non-aqueous oxidation with m-chlorobenzoic acid. The P-N bond in the phosphate link is very stable and so provides a protection for the phosphoryl group which has many potential uses. Deprotection can be achieved by a simple treatment with NH2OH.

DNA↗
PubMed1997

Redox modulation of synaptic responses and plasticity in rat CA1 hippocampal neurons.

Effects of redox reagents on excitatory and inhibitory synaptic responses as well as on the bidrectional plasticity of alpha-amino-3-hydroxy-5-methylisoxazolepropionic acid (AMPA) and N-methyl-D-aspartate (NMDA) receptor-mediated synaptic responses were studied in CA1 pyramidal neurons in rat hippocampal slices. The oxidizing agent 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB, 200 microM) did not affect AMPA, GABAA or GABAB receptor-mediated synaptic responses or the activation of presynaptic metabotropic receptors. However, DTNB irreversibly decreased (by approximately 50%) currents evoked by focal application of NMDA. DTNB also decreased the NMDA component of the EPSC. The reversal potential of NMDA currents and the Mg2+ block were not modified. In the presence of physiological concentrations of Mg2+ (1.3 mM), DTNB did not affect the NMDA receptor-dependent induction of long-term potentiation (LTP) or long-term depression (LTD) expressed by AMPA receptors. In contrast, DTNB fully prevented LTP and LTD induced and expressed by NMDA receptors. Plasticity of NMDA receptor-mediated synaptic responses could be reinstated by the reducing agent tris-(2-carboxyethyl) phosphine (TCEP, 200 microM). These results suggest that persistent, bidirectional changes in synaptic currents mediated by NMDA receptors cannot be evoked when these receptors are in an oxidized state, whereas NMDA-dependent LTP and LTD are still expressed by AMPA receptors. Our observations raise the possibility of developing therapeutic agents that would prevent persistent excitotoxic enhancement of NMDA receptor-mediated events without blocking longterm modifications of AMPA receptor-mediated synaptic responses, thought to underlie memory processes.

Animals↗
PubMed2000

Asymmetric 1,4-addition of phenylboronic acid to 2-cyclohexenone catalyzed by Rh(I)/binap complexes.

Reaction of 2-cyclohexenone with phenylboronic acid in the presence of 3 mol% of a rhodium(I)/(S)-binap catalyst in dioxane/H2O (10/1) at 100 degrees C proceeded with high enantioselectivity to give a high yield of (S)-3-phenylcyclohexanone of up to 99% ee. The high enantioselectivity was achieved by use of a catalyst generated in situ from Rh(acac)(C2H4)2 and (S)-binap or Rh(acac)((S)-binap) as an isolated rhodium-phosphine complex.

Journal Article↗
PubMed2004

Electron spin resonance study of phosphorus-nitroxides from 1,3-additions of silicon-phosphorus reagents to nitrones.

This article explores a new, convenient route to beta-phosphorus nitroxides. Specifically, the reaction sequence involves the novel 1,3-addition of trimethylsilyl phosphites (e.g. diethyl) or trimethylsilyl phosphines (e.g. diphenyl) to aldo-nitrones [e.g. alpha-phenyl-N-tert-butylnitrone (PBN) or 5,5-dimethyl-l-pyrroline-N-oxide (DMPO)] or keto-nitrones [e.g. 2-ethyl-5,5-dimethyl-1 pyrroline-N-oxide (2-Et-DMPO) or 2-phenyl-5,5-dimethyl-l-pyrroline-N-oxide (2-Ph-DMPO)] to form alpha-phosphityl- or alpha-phosphinyl-O-silylhydroxylamines. Acidic hydrolysis provides the corresponding hydroxylamines that are easily oxidized to the title beta-phosphorus-nitroxides. ESR spectroscopic analysis revealed some very large beta-phosphorus hyperfine splittings (i.e. in excess of 5 mT). For this reason and their remarkable stability (persistence) some of these nitroxides show promise as integral components in new, improved weak-field dynamic nuclear polarization (DNP) magnetometers.

Cyclic N-Oxides↗