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At least 1,099 records · Page 61Linked to original sources
PubMed2004

Phosphine-mediated [4 + 2] annulation of bis(enones): a Lewis base catalyzed "mock Diels-Alder" reaction.

Lewis-base-catalyzed cycloisomerization of bis(enones) to decalins has been demonstrated as an alternative to the traditional Lewis acid catalyzed Diels-Alder cycloaddition. In this process, a trialkylphosphine mediates both bond formation steps in two distinct catalytic cycles. The single-pot operation generates two carbon-carbon bonds and up to five contiguous stereocenters in one step, starting from achiral, aliphatic substrates; eight examples are provided. [reaction: see text]

Journal Article↗
PubMed1994

On-line procedures for alkylation of cysteine residues with 3-bromopropylamine prior to protein sequence analysis.

We have previously shown that 3-bromopropylamine offers several advantages over other alkylating reagents in the modification and subsequent identification of cysteine residues by protein sequencing. We describe here simple on-sequencer procedures for alkylating cysteines in proteins which employ the reduction of cystines in proteins with tri-n-butylphosphine and concomitant alkylation of the resulting cysteines with 3-bromopropylamine. Addition of an aqueous acetone wash to a modified reaction cycle on the Applied Biosystems 477A sequencer removes excess 3-bromopropylamine. As a result, very little background in the first step of the sequence analysis is seen. Under these conditions, cysteines are readily modified and identified during sequencing. Moreover, very little preview of the next amino acid is observed, which indicates that the N-terminal amino acid is not appreciably alkylated by 3-bromopropylamine. On-sequencer methods have been developed for proteins spotted onto glass fiber filters and proteins electroblotted onto polyvinylidene difluoride membranes.

Alkylation↗
PubMed1996

Cloning, high-yield expression in Escherichia coli, and purification of biologically active HIV-1 Tat protein.

We have established an expression system for full-length HIV-1 transactivator (Tat) protein in Escherichia coli. By constructing a synthetic gene for high level expression in enteric bacteria, the recombinant protein can be obtained in high yield. Fusion of the Tat sequence to an N-terminal histidine tag allows the rapid purification of the fusion protein through a single chromatographic step. After cleavage of the fusion protein with CNBr, pure Tat can be obtained through the use of a MonoS column. Reduction of the protein with Tris(2-carboxyethyl)phosphine-HCl and subsequent stepwise refolding yields biologically active Tat. Sample purity and the identity of the protein mass with the mass expected from the amino acid sequence was demonstrated by mass spectrometry. Nuclear magnetic resonance spectroscopy showed the identity of bacterially expressed and chemically synthesized Tat protein (P. Bayer et al., 1995, J. Mol. Biol. 247, 529-535). The expression of Tat in E. coli enables isotope labeling as a prerequisite for multidimensional NMR experiments toward the elucidation of the structure of the Tat-trans-activation response element complex.

Amino Acid Sequence↗
PubMed2005

The initiating radical yields and the efficiency of polymerization for various dental photoinitiators excited by different light curing units.

OBJECTIVES: To evaluate the efficiency of the photopolymerization of dental resins it is necessary to know to what extent the light emitted by the light curing units is absorbed by the photoinitiators. On the other hand, the efficiency of the absorbed photons to produce species that launch the polymerization process is also of paramount importance. Therefore, the previously determined PAE (photon absorption efficiency) is used in conjunction with the polymerization quantum yields for the photoinitiators, in order to be able to compare the total process on an equivalent basis. This parameter can be used to identify the best performance for the photochemical process with specific photoinitiators. METHODS: The efficiency of LED (Ultrablue IS) and QTH (Optilux 401) lamps were tested comparing their performances with the photoinitiators camphorquinone (CQ); phenylpropanedione (PPD); monoacylphosphine oxide (Lucirin TPO); and bisacylphosphine oxide (Irgacure 819). The extent of photopolymerization per absorbed photon was determined from the polymerization quantum yields obtained by using the photoinitiators to polymerize methyl methacrylate, and afterwards combined with the previously determined PAEs. RESULTS: Although CQ presents a rather low polymerization quantum yield, its photopolymerization efficiency is practically the highest when irradiated with the Ultrablue LED. On the other hand, Lucirin is much more efficient than the other photoinitiators when irradiated with a QTH lamp, due to its high quantum yield and the overlap between its absorption spectrum and the output of the visible lamp light. SIGNIFICANCE: Difference in photopolymerization efficiencies arise when combinations of photoinitiators are used, and when LED sources are used in preference to QTH. Mechanistic understanding is essential to optimal initiator formulation.

4-Aminobenzoic Acid↗
PubMed2002

Fabrication of ordered arrays of InP microstructures by wet chemical etching with Au masks.

Ordered arrays of InP microstructures have been fabricated on InP(001) substrates by wet chemical etching in aqueous HCl with patterned Au masks. The masks were produced by Au deposition through copper grids or a monolayer of polystyrene microspheres. Square InP mesas (20 x 20 microns) and pillars (approximately 100 nm in both diameter and height) were both produced and characterized by scanning electron microscopy and atomic force microscopy.

Crystallization↗
PubMed2006

Derivatization in mass spectrometry--8. Soft ionization mass spectrometry of small molecules.

This is the first of two reviews devoted to derivatization approaches for "soft" ionization mass spectrometry (FAB, MALDI, ESI, APCI) and deals, in particular, with small molecules. The principles of the main "soft" ionization mass spectrometric methods as well as the reasons for derivatizing small molecules are briefly described. Derivatization methods for modification of amines, carboxylic acids, amino acids, alcohols, carbonyl compounds, monosaccharides, thiols, unsaturated and aromatic compounds etc. to improve their ionizability and to enhance structure information content are discussed. The use of "fixed"-charge bearing derivatization reagents is especially emphasized. Chemical aspects of derivatization and "soft" ionization mass spectrometric properties of derivatives are considered.

Amines↗
PubMed1980

3-nitro-2-pyridinesulfenyl (Npys) group. A novel selective protecting group which can be activated for peptide bond formation.

The novel 3-nitro-2-pyridinesulfenyl (Npys) group, which is useful for the protection and the activation of amino and hydroxyl groups for peptide synthesis, is reported. The Npys group is readily introduced by treatment of amino acids with 3-nitro-2-pyridinesulfenyl chloride. The Npys group is easily removed by treatment with very dilute HCl, e.g. 0.1-0.2 N HCl in dioxane, but is is resistant to trifluoroacetic acid and 88% formic acid. Npys is also selectively removed under neutral conditions using triphenylphosphine or 2-pyridinethiol 1-oxide without affecting benzyloxycarbonyl (Z), tert-butyloxycarbonyl (Boc), 2-(4-biphenylyl)propyl(2)oxycarbonyl (Bpoc), 9-fluorenylmethyloxycarbonyl (Fmoc), benzyl (Bzl) or tert-butyl (tBu) protecting groups. The N-Npys and O-Npys groups when activated in the presence of RCOOH by the addition of tertiary phosphine form peptide or ester bonds via oxidation-reduction condensation. The important features of the Npys group are demonstrated through the synthesis of peptides in solution and by solid phase methodology without a formal deprotection procedure. In solid phase synthesis, 4-(Npys-oxymethyl) phenylacetic acid is used as the key intermediate for the introduction of the trifluoroacetic acid resistant 4-(oxymethyl) phenylacetamido linking group to the resin.

Amino Acid Sequence↗
PubMed2004

Remarkably diastereoselective synthesis of a chiral biphenyl diphosphine ligand and its application in asymmetric hydrogenation.

Essentially complete atropdiastereoselectivity was realized in the preparation of biaryl diphosphine dioxide by asymmetric intramolecular Ullmann coupling and oxidative coupling with central-to-axial chirality transfer. A bridged C(2)-symmetric biphenyl phosphine ligand possessing additional chiral centers on the linking unit of the biphenyl groups was synthesized. No resolution step was required for the preparation of the enantiomerically pure chiral ligand. These findings offer a general and practical tool for the development of previously uninvestigated atropdiastereomeric biaryl phosphine ligands. The diphosphine ligand was found to be highly effective in the asymmetric hydrogenation of alpha- and beta-ketoesters, 2-(6'-methoxy-2'-naphthyl)propenoic acid, beta-(acylamino)acrylates, and enol acetates.

Journal Article↗
PubMed2006

Reduction of proteins during sample preparation and two-dimensional gel electrophoresis of woody plant samples.

Protein extraction procedure and the reducing agent content (DTT, dithioerythritol, tributyl phosphine and tris (2-carboxyethyl) phosphine (TCEP)) of the sample and rehydration buffers were optimised for European beech leaves and roots and Norway spruce needles. Optimal extraction was achieved with 100 mM DTT for leaves and needles and a mixture of 2 mM TCEP and 50 mM DTT for roots. Performing IEF in buffers containing hydroxyethyldisulphide significantly enhanced the quality of separation for all proteins except for acidic root proteins, which were optimally focused in the same buffer as extracted.

Buffers↗
PubMed2000

Synthesis of chiral hydroxyl phospholanes from D-mannitol and their use in asymmetric catalytic reactions.

Chiral hydroxyl monophosphane 3 [(2S,3S,4S,5S)-3,4-dihydroxy-2, 5-dimethyl-1-phenylphospholane] and bisphospholanes 5a [1,2-bis[(2S, 3S,4S,5S)-3,4-dihydroxy-2,5-dimethylphospholanyl]benzene] and 5b [1, 2-bis[(2S,3S,4S,5S)-2,5-diethyl-3,4-dihydroxyphospholanyl]benzene] were synthesized from readily available D-mannitol in high yields. Strategies for protection and deprotection of OH-groups in the presence of phosphines have been explored. Rate acceleration in the Baylis-Hillman reaction was observed when a hydroxyl phosphine was used as the catalyst. Rhodium complexes with chiral bisphospholanes are highly enantioselective catalysts for the asymmetric hydrogenation of various kinds of functionalized olefins such as dehydroamino acid derivatives, itaconic acid derivatives, and enamides. An interesting feature of the hydroxyl phospholane system is that hydrogenation of some substrates can be carried out in water with >99% ee and 100% conversion (e.g., itaconic acid).

Catalysis↗
PubMed2005

Concise synthesis of ether analogues of lysobisphosphatidic acid.

We describe a versatile, efficient method for the preparation of ether analogues of (S,S)-lysobisphosphatidic acid (LBPA) and its enantiomer from (S)-solketal. Phosphorylation of a protected sn-2-O-octadecenyl glyceryl ether with 2-cyanoethyl bis-N,N-diisopropylamino phosphine and subsequent deprotection generated the bisether LBPA analogues. By simply changing the sequence of deprotection steps, we obtained the (R,R)- and (S,S)-enantiomers of 2,2'-bisether LBPA. An ELISA assay with anti-LBPA monoclonal antibodies showed that the bisether LBPAs were recognized with the same affinity as the natural 2,2'-bisoleolyl LBPA. [reaction: see text]

Antibodies, Monoclonal↗
PubMed2004

Determination of free and total cyst(e)ine in plasma of dogs and cats.

BACKGROUND: In human blood, the amino acid cysteine forms disulfide bonds with itself and with other sulfhydryl compounds in their free form and with sulfhydryls in protein. Protein-bound cysteine is lost when plasma proteins are removed before amino acid analysis. OBJECTIVE: The purpose of this study was to assess the time course and extent of cyst(e)ine (cysteine + half-cystine) loss in dog and cat plasma. METHODS: An equal volume of 6% sulfosalicylic acid was added to plasma aliquots at 0, 2, 4, 10, 16, 24, 36, 48, 60, and 72 hours after separation of blood cells. Tris-2-carboxyethyl-phosphine hydrochloride (TCEP - HCl), a reducing agent, was used to regenerate total plasma cyst(e)ine after 3 months of sample storage (-20 degrees C). RESULTS: Initial free cyst(e)ine concentrations (mean +/- SEM) were higher in canine plasma (77 +/- 4 micromol/L) than in feline plasma (37 +/- 3 micromol/L). Free plasma cyst(e)ine concentrations in dogs and cats decreased after first-order kinetics, with a half-life of 23 and 69 hours, respectively. Total plasma cysteine after TCEP - HCl treatment was similar for dogs (290 micromol/L) and cats (296 micromol/L), but the percentage of free cysteine was higher (P = .02) in dogs (27%) than in cats (13%). Over half of the cyst(e)ine, homocysteine, cysteinylglycine, and glutathione were bound in vivo to plasma proteins. CONCLUSION: These results emphasize the importance of removing plasma proteins within 1 hour after blood collection for reliable assay of free plasma cyst(e)ine.

Animals↗
PubMed2006

Evidence that plasmalogen is protective against oxidative stress in the rat brain.

The antioxidant capabilities of phosphatidylethanolamine plasmalogen (PlsEtn), in vivo, against lipid peroxidation were investigated via acute phosphine (PH(3)) administration in rats. Oxidative stress was assessed from measures of malondialdehyde and various enzyme activities, while NMR analyses of lipid and aqueous tissue extracts provided metabolic information in cerebellum, brainstem, and cortex. Brainstem had the highest basal [PlsEtn], and showed only moderate PH(3)-induced oxidative damage with no loss of ATP. The lowest basal [PlsEtn] was observed in cortex, where PH(3) caused a 51% decrease in [ATP]. The largest oxidative effect occurred in cerebellum, but [ATP] was unaffected. Myo-inositol+ethanolamine pretreatment attenuated all PH(3) effects. Specifically, the pretreatment attenuated the ATP decrease in cortex, and elevated brain [PlsEtn] in the cerebellum, nearly abolishing the cerebellar oxidative effects. Our data suggest a high basal [PlsEtn], or the capacity to synthesize new ethanolamine lipids (particularly PlsEtn) may protect against PH(3) toxicity.

Aldehydes↗
PubMed2001

The effect of pH on the reactions of catalytically important RhI complexes in aqueous solution: reaction of [RhCl(tppms)3] and trans-[RhCl(CO)(tppms)2] with hydrogen (TPPMS = mono-sulfonated triphenylphosphine). .

Hydrolysis and hydrogenation of [RhCl(tppms)3] (1) and trans-[RhCl(CO)(tppms)2] (2) was studied in aqueous solutions in a wide pH range (2 < pH < 11) in the presence of excess TPPMS (3-diphenylphosphinyl-benzenesulfonic acid sodium salt). In acidic solutions hydrogenation of 1 yields a mixture of cis-mer- and cis-fac-[RhClH2(tppms)3] (3a,b) while in strongly basic solutions [RhH(H2O)(tppms)3] (4) is obtained, the midpoint of the equilibrium between these hydride species being at pH 8.2. The paper gives the first successful 1H and 31P NMR spectroscopic characterization of a water soluble rhodium(I)-monohydride (4) bearing only monodentate phosphine ligands. Hydrolysis of 2 is negligible below pH 9 and its hydrogenation results in formation of [Rh(CO)H(tppms)3] (5), which is an analogue to the well known and industrially used hydroformylation catalyst [Rh(CO)H(tppts)3] (6) (TPPTS = 3,3',3''-phosphinetriyltris(benzenesulfonic acid) trisodium salt). It was shown by pH-potentiometric measurements that formation of 5 is strongly pH dependent in the pH 5-9 range, this gives an explanation for the observed but previously unexplained pH dependence of several hydroformylation reactions. Conversely, the effect of pH on the rate of hydrogenation of maleic and fumaric acid catalyzed by 1 in the 2 < pH < 7 range can be adequately described by considering solely the changes in the ionization state of these substrates. All these results warrant the use of buffered (pH-controlled) solutions for aqueous organometallic catalysis.

Journal Article↗
PubMed2003

Reactions of arylaldehydes and N-sulfonated imines with dimethyl acetylenedicarboxylate catalyzed by nitrogen and phosphine lewis bases.

[reaction: see text] In the reaction of arylaldehydes or N-sulfonated imines (0.5 mmol) with dimethyl acetylenedicarboxylate (DMAD) (0.6 mmol) catalyzed by pyridine or DMAP (20 mol %), we found that (E)-2-aryl-but-2-enedioic acid dimethyl ester 1 or (E)-2-[aryl-(toluene-4-sulfonylimino)methyl]-but-2-enedioic acid dimethyl ester 2 was formed in good yields at 60 degrees C in THF. A plausible mechanism has been proposed.

Journal Article↗