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At least 1,315 records · Page 73Linked to original sources
PubMed2006

Elucidation of carotenoid patterns in citrus products by means of comprehensive normal-phase x reversed-phase liquid chromatography.

A novel approach for carotenoid analysis has been developed. Orange essential oil and juice carotenoids were separated by means of comprehensive dual-gradient elution HPLC, using normal phase with a microbore silica column in the first dimension (first D), reversed phase with a monolithic C18 column in the second dimension (second D), and a 10-port switching valve as an interface. An on-line photodiode array detector was used in order to obtain absorption spectra. Peak identification was obtained by combining retention data with the UV-visible spectra.

Beverages↗
PubMed1976

Evidence for phase boundary lipid. Permeability of Tempo-choline into dimyristoylphosphatidylcholine vesicles at the phase transition.

The existence of distinct regions of mismatch in molecular packing at the interfaces of the fluid and ordered domains during the phase transition of dimyristoylphosphatidylcholine vesicles has been demonstrated by measuring the temperature dependence of the permeability to a spin-label cation and comparing this with a statistical mechanical calculation of the fraction of interfacial lipid. The kinetics of uptake and release of the 2,2,6,6-tetramethylpiperidinyl-1-oxycholine (Tempo-choline) spin label by single-bilayer dimyristoylphosphatidylcholine vesicles were measured using electron spin resonance spectroscopy to quantitate the amount of spin label present within the vesicles after removal of the external spin-label by ascorbate at 0 degrees C. Both the uptake and release experiments show that the Tempo-choline permeability peaks to a sharp maximum at the lipid-phase transition, the vesicles being almost impermeable to Tempo-choline below the transition and having a much reduced permeability above. The temperature profile of the permeability is in reasonable quantitative agreement with calculations of the fraction of interfacial boundary lipid from the Zimm and Bragg theory of cooperative transitions, which use independent spin-label measurements of the degree of transition to determine the cooperativity parameter. The relatively high intrinsic permeability of the interfacial regions (P approximately 0.2-1.0 X 10(-8) cm/s) is attributed to the mismatch in molecular packing of the lipid molecules at the ordered-fluid boundaries, which could have important implications not only for permeability in natural membranes (e.g., in transmitter release), but also for the function of membrane-bound enzymes and transport proteins.

Binding Sites↗
PubMedDate not supplied

Solid phase extraction purification of carboxylic acid products from 96-well format solution phase synthesis with DOWEX 1x8-400 formate anion exchange resin.

The anion exchange resin DOWEX 1x8-400 formate has been developed for the isolation or resin capture of carboxylic acids from solution phase reactions in a 96-well format using a batchwise solid phase extraction technique. Eleven different anion exchange resins (formate forms) were evaluated for their efficiency at scavenging aryl and aliphatic carboxylic acids from solution. The model carboxylic acids had pK(a)s ranging from 3.40 to 4.89. Exchange efficiency onto the resin was pK(a) dependent with the carboxylic acids but not with their diisopropylethylammonium salts. Exchange off of the resin also showed pK(a) dependence with the stronger acids requiring more concentrated solvent acid for exchange. DOWEX 1x8-400 formate was determined to have superior capacity and the fastest exchange rate. Solvents suitable for exchanging the acids onto the resin were CH2Cl2, methanol, and various solvent/water mixtures. Solvents suitable for exchanging the carboxylic acids off of the resin were TFA/solvent or HCO2H/solvent mixtures. The resin was found to swell best in CH2Cl2 and in polar protic solvents such as water, alcohols, and acids. Application of this technique to the crude product mixtures from an arrayed reductive amination and an arrayed Stille reaction provided product carboxylic acids in yields averaging 57% and purities averaging 89%.

Journal Article↗
PubMedDate not supplied

Hydrophilic polymer supports for solid-phase synthesis: preparation of poly(ethylene glycol) methacrylate polymer beads using "classical" suspension polymerization in aqueous medium and their application in the solid-phase synthesis of hydantoins.

Lightly cross-linked hydrophilic polymer beads representing new types of supports for solid-phase synthesis have been prepared from commercial oligoethylene glycol monomethacrylates using an aqueous suspension polymerization process and specifically designed polymerization mixtures. These beads swell extensively in solvents with a wide range of polarities from dichloromethane, tetrahydrofuran, and water to dimethylformamide, and they enable high functional loadings of 1.2-1.8 mmol g(-1). Their ability to serve as supports was demonstrated in the model solid-phase synthesis of a small library of hydantoins. This four-step synthesis using primary amines readily affords yields of over 70%.

Amines↗
PubMedDate not supplied

Parallel solution-phase synthesis of (Z)-3-(arylamino)-2,3-dehydroalanine derivatives and solid-phase synthesis of fused pyrimidones.

N-Protected (Z)-3-(arylamino)-2,3-dehydroalanine esters 5 and 10 were prepared in one step from methyl (Z)-2-acylamino-3-(dimethylamino)prop-2-enoates 3 and 9 and anilines 4 employing a parallel solution-phase synthetic approach. In most cases, analytically pure products 5 and 10 were obtained. On the other hand, a three-step parallel solid-phase synthesis of 2-acetylamino-4H-azino[1,2-x]pyrimidin-4-ones 15 via the polymer-bound methyl (Z)-2-acetylamino-3-(dimethylamino)prop-2-enoate (12) was also developed.

Journal Article↗
PubMedDate not supplied

New synthetic pathway to diverse 2-substituted quinolines based on a multicomponent reaction: solution-phase and solid-phase applications.

Using Kobayashi's modification of the Grieco reaction, we were able to synthesize diverse 4-phenylthio-1,2,3,4-tetrahydroquinolines. These intermediates were oxidized and subsequently pyrolized to provide the corresponding quinolines. This new approach to 2-substituted quinolines was exemplified by liquid-phase production of a 25-member library. This was extended to solid-phase chemistry, starting from (l)-4-nitrophenylalanine on Wang resin, for production of a 16-member library. The latter compounds possess potentially interesting VLA-4 antagonist properties.

Journal Article↗
PubMed2002

Source reconciliation of atmospheric gas-phase and particle-phase pollutants during a severe photochemical smog episode.

A comprehensive organic compound-based receptor model is developed that can simultaneously apportion the source contributions to atmospheric gas-phase organic compounds, semivolatile organic compounds, fine particle organic compounds, and fine particle mass. The model is applied to ambient data collected at four sites in the south coast region of California during a severe summertime photochemical smog episode, where the model determines the direct primary contributions to atmospheric pollutants from 11 distinct air pollution source types. The 11 sources included in the model are gasoline-powered motor vehicle exhaust, diesel engine exhaust, whole gasoline vapors, gasoline headspace vapors, organic solvent vapors, whole diesel fuel, paved road dust, tire wear debris, meat cooking exhaust, natural gas leakage, and vegetative detritus. Gasoline engine exhaust plus whole gasoline vapors are the predominant sources of volatile organic gases, while gasoline and diesel engine exhaust plus diesel fuel vapors dominate the emissions of semivolatile organic compounds from these sources during the episode studied at all four air monitoring sites. The atmospheric fine particle organic compound mass was composed of noticeable contributions from gasoline-powered motor vehicle exhaust, diesel engine exhaust, meat cooking, and paved road dust with smaller but quantifiable contributions from vegetative detritus and tire wear debris. In addition, secondary organic aerosol, which is formed from the low-vapor pressure products of gas-phase chemical reactions, is found to be a major source of fine particle organic compound mass under the severe photochemical smog conditions studied here. The concentrations of secondary organic aerosol calculated in the present study are compared with previous fine particle source apportionment results for less intense photochemical smog conditions. It is shown that estimated secondary organic aerosol concentrations correlate fairly well with the concentrations of 1,2-benzenedicarboxylic acid in the atmospheric fine particle mass, indicating that aromatic diacids may be useful in the quantification of certain sources of secondary organic aerosol in the atmosphere.

Atmosphere↗
PubMed1998

Gas-Phase Reaction of Tetraborane(10) and Ethyne: Molecular Structure of nido-1,2-C(2)B(3)H(7) in the Gas Phase.

The molecular structure of nido-1,2-C(2)B(3)H(7), 1, the principal volatile carborane generated in the quenched gas-phase reaction of B(4)H(10) and ethyne at 70 degrees C, has been determined by a combined analysis of gas-phase electron-diffraction data and rotation constants restrained by ab initio computations at the CCSD(T)/TZP' level. The structure is consistent with a geometry having C(s)() symmetry, similar to that of pentaborane(9). The apical position is occupied by a carbon atom, displaced toward B(4) from a position directly above the B(5).B(3) vector, and hydrogen atoms asymmetrically bridge the B-B bonds. The basal atoms are almost coplanar, C(2) lying ca. 2 degrees below the B(3)-B(4)-B(5) plane. Important experimental structural parameters (r(alpha) degrees /pm, angle(alpha)/ degrees ) are r[C(1)-C(2)] = 162.6(6); r[C(1)-B(3)] = 161.4(3); r[C(2)-B(3)] = 154.3(2); r[C(1)-B(4)] = 157.4(5); r[B(3)-B(4)] = 185.7(3); <B(3)-B(4)-B(5) = 80.9(1). In addition to this and the other previously reported carboranes, 2,3-C(2)B(4)H(8), 2-Me-2,3,4-C(3)B(3)H(6), and 4-Me-2-CB(5)H(8), several new derivatives have been identified among the volatile products. These include the dicarbahexaboranes 2,4-Me(2)-2,3-C(2)B(4)H(6) and 5-Et-2,3-C(2)B(4)H(7) (the major volatile products obtained when the reaction is allowed to go to completion, previously reported as tricarbahexaboranes) and the derivatives 2,5-Me(2)-2,3-C(2)B(4)H(6), 4-Et-2,3-C(2)B(4)H(7), 1-Me-2,3,4-C(3)B(3)H(6), 2-Me-2-CB(5)H(8), 3-Me-2-CB(5)H(8), and 2,3,4,5-C(4)B(2)H(6). The complex mechanism of the reaction is discussed in light of these new results.

Journal Article↗
PubMed2002

Observation of conserved solution-phase secondary structure in gas-phase tryptic peptides.

Results from ion mobility studies of tryptic peptides suggest that, in some cases, the gas-phase structures can be related to the solution-phase structure of the parent protein. The interpretation of ion mobility measurements is supported by results from molecular modeling and H/D exchange experiments on the same peptides. This study clearly illustrates the utility of IM-MS for screening complex mixtures for peptides having intrinsically stable secondary/tertiary structures, and/or posttranslational modification.

Amino Acid Sequence↗
PubMed2004

Gas-phase hydrogen/deuterium exchange and conformations of deprotonated flavonoids and gas-phase acidities of flavonoids.

Gas-phase hydrogen/deuterium (H/D) exchange was used to probe the conformations, gas-phase acidities, and sites of deprotonation of isomeric flavonoid aglycons and glycosides. The flavonoids in each isomer series were differentiated on the basis of their relative rate constants and total numbers of exchanges. For example, flavonoids that possess neohesperidose-type disaccharides may undergo faster and far more extensive exchange than isomeric rutinoside flavonoids. The structural factors that promote or prevent H/D exchange were identified and correlated with collisionally activated dissociation (CAD) patterns and/or molecular modeling data (both high-level ab initio acidity calculations and conformational analysis with molecular dynamics (MD) simulations), thus providing a framework for the use of H/D exchange reactions in the structural elucidation of new flavonoids.

Carbohydrates↗
PubMed2004

Gas-phase kinetics and mechanism of the reactions of protonated hydrazine with carbonyl compounds. Gas-phase hydrazone formation: kinetics and mechanism.

The gas-phase reactions of protonated hydrazine (hydrazinium) with organic compounds were studied in a selected ion flow tube-chemical ionization mass spectrometer (SIFT-CIMS) at 0.5 Torr pressure and approximately 300 K and with hybrid density functional calculations. Carbonyl and other polar organic compounds react to form adducts, e.g., N(2)H(5)(+)(CH(3)CH(2)CHO). In the presence of neutral hydrazine, aldehyde adducts react further to form protonated hydrazones, e.g., CH(3)CH(2)CH[double bond]HNNH(2)(+) from propanal. Using deuterated hydrazine (N(2)D(4)) and butanal, we demonstrate that the gas-phase ion chemistry of hydrazinium and carbonyls operates by the same mechanisms postulated for the reactions in solution. Calculations provide insight into specific steps and transition states in the reaction mechanism and aid in understanding the likely reaction process upon chemical or translational activation. For most carbonyls, rate coefficients for adduct formation approach the predicted maximum collisional rate coefficients, k approximately 10(-9) cm(3) molecule(-1) s(-1). Formaldehyde is an exception (k approximately 2 x 10(-11) cm(3) molecule(-1) s(-1)) due to the shorter lifetime of its collision complex. Following adduct formation, the process of hydrazone formation may be rate limiting at thermal energies. The combination of fast reaction rates and unique chemistry shows that protonated hydrazine can serve as a useful chemical-ionization reagent for quantifying atmospheric carbonyl compounds via CIMS. Mechanistic studies provide information that will aid in optimizing reaction conditions for this application.

Journal Article↗
PubMed2006

Photochemical phase transition versus photochemical phase separation.

When a nematic liquid crystal is doped with a large amount of a liquid crystalline azobenzene compound (15 wt %), the photoisomerization of azobenzene under UV and visible light irradiation can give rise to reversible and interconvertible photochemical phase transition and photochemical phase separation. Both normal and reverse modes of photocontrolled switching of transmittance through crossed polarizers can be realized with the same mixture of liquid crystal/azobenzene dopant.

Journal Article↗
PubMed2006

Ethanol/Water extraction combined with solid-phase extraction and solid-phase microextraction concentration for the determination of chlorophenols in cork stoppers.

The appearance of 2,4,6-trichloroanisole (TCA) in cork stoppers is of great concern because it can cause off-flavors in bottled wine. To prevent this sensorial defect, there should not be any traces of 2,4,6-trichlorophenol (TCP), 2,3,4,6-tetrachlorophenol (TeCP), or pentachlorophenol (PCP) in the finished corks, because they are the direct precursors of TCA. In the course of this study two methodologies based upon an extraction with ethanol/water mixtures to determine the chlorophenolic content in cork matrices were developed. The cork extract is preconcentrated using both solid-phase extraction and solid-phase microextraction methodologies. The latter was optimized by applying a full two-level factorial design. Finally, spiked ground corks at nanogram per gram levels of each chlorophenol were analyzed under optimal conditions and by applying both procedures. The obtained results demonstrate that chlorophenols can be detected in corks contaminated at the nanogram per gram level and, thus, these approaches can be successfully applied as quality control measures in the cork industry.

Chlorophenols↗
PubMed2002

Comparison of solid-phase and solution-phase chiral auxiliaries in the alkylation/iodolactonization sequence to gamma-butyrolactones.

Five prolinol-based chiral auxilaries have been compared for the stereoselective synthesis of gamma-butyrolactones via the sequence of N-acylation, C(alpha)()-allylation, and iodolactonization under both solution-phase and solid-phase conditions. Comparisons of stereoselectivity of both the C(alpha)()-allylation and iodolactonization processes indicate that incorporation of a non-C(2)-symmetric auxiliary as a polymer cross-link gives results superior to those obtained either in solution or with other non-C(2)-symmetric auxiliaries and comparable to those observed using a polymer-supported pseudo-C(2)-symmetric auxiliary.

Journal Article↗
PubMed1999

Organic-Fluorous Phase Switches: A Fluorous Amine Scavenger for Purification in Solution Phase Parallel Synthesis.

The synthesis of the fluorous amine scavenger [(C(6)F(13)CH(2)CH(2))(3)SiCH(2)CH(2)CH(2)](2)NH and its successful application in the automated solution phase parallel synthesis of a urea library are described. Ureas were made by robotic synthesis from organic amines and excess isocyanates. The amine scavenger reacts with excess isocyanate, and the fluorous tag serves to solubilize the resulting adduct in the fluorous phase so it can be removed by fluorous-organic extraction. Organic urea products are isolated in high yields and purities after liquid-liquid extraction. Preliminary biological evaluation shows that several of the ureas have ion channel modulation abilities. In contrast to polymer and ionic quenching methods, the fluorous quench works whether the product is soluble or insoluble in the reaction medium, and ionizable functional groups are tolerated in the products.

Journal Article↗
PubMed2005

A new phase of the c(4 x 2) superstructure of alkanethiols grown by vapor phase deposition on gold.

A self-assembled monolayer of dodecanethiol is grown onto (111) oriented gold by vacuum phase deposition and studied by ultrahigh vacuum scanning tunneling microscopy (STM). The films consist of domains that exhibit the c(4 x 2) over-structure of the hexagonal (square root of 3 x square root of 3)R30 of alkanethiols on gold. The domain size is only limited by the terrace size of the underlying gold. By higher resolution scans a new phase of the c(4 x 2) structure consisting of four inequivalent molecules that display different heights in the STM images is discovered.

Letter↗
PubMed2001

Analysis of nucleotide sugars from cell lysates by ion-pair solid-phase extraction and reversed-phase high-performance liquid chromatography.

Analysis of nucleotide sugar metabolism is essential in studying glycosylation in cells. Here we describe practical methods for both extraction of nucleotide sugars from cell lysates and for their analytical separation. Solid-phase extraction cartridges containing graphitized carbon can be used for the purification of nucleotide sugars by using triethylammonium acetate buffer as a ion-pairing reagent for decreasing retention. After that they are separated by high-performance liquid chromatography using a C18 reversed-phase column and the same ion-pairing reagent for increasing retention. These new sample preparation and analysis methods enable good separation of structurally similar sugar nucleotides, compatibility with rapid evaporative concentration, and possibility to automation. Monitoring the production of GDP-deoxyhexoses in genetically engineered yeast and native bacterial cells are described here as specific applications.

Adsorption↗
PubMed2000

Room-temperature electronic phase transitions in the continuous phase diagrams of perovskite manganites

Highly correlated electronic systems--such as transition-metal oxides that are doped Mott insulators--are complex systems which exhibit puzzling phenomena, including high-temperature superconductivity and colossal magnetoresistivity. Recent studies suggest that in such systems collective electronic phenomena are important, arising from long-range Coulomb interactions and magnetic effects. The qualitative behaviour of these systems is strongly dependent on charge filling (the level of doping) and the lattice constant. Here we report a time-efficient and systematic experimental approach for studying the phase diagrams of condensed-matter systems. It involves the continuous mapping of the physical properties of epitaxial thin films of perovskite manganites (a class of doped Mott insulator) as their composition is varied. We discover evidence that suggests the presence of phase boundaries of electronic origin at room temperature.

Journal Article↗