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At least 127 records · Page 7Linked to original sources
PubMed2004

Synthesis, structure, and dynamics of nickelacarboranes incorporating the [nido-7,9-C(2)B(9)H(11)](2)(-) ligand.

The nickelacarboranes [NEt(4)][2-(eta(3)-C(3)H(4)R)-closo-2,1,7-NiC(2)B(9)H(11)] (R = H (1a), Ph (1b)) have been synthesized via reaction between [Na](2)[nido-7,9-C(2)B(9)H(11)] and [Ni(2)(micro-Br)(2)(eta(3)-C(3)H(4)R)(2)] in THF (THF = tetrahydrofuran), followed by addition of [NEt(4)]Cl. Protonation of 1a in the presence of a donor ligand L affords the complexes [2,2-L(2)-closo-2,1,7-NiC(2)B(9)H(11)] (L = CO (2), CNBu(t) (3)). Addition of PEt(3) (1 equiv) to 2 produces quantitative conversion to [2-CO-2-PEt(3)-closo-2,1,7-NiC(2)B(9)H(11)], 4. Species 2-4 exhibit in solution hindered rotation of the NiL(2) fragment with respect to the eta(5)-C(2)B(9) cage unit. Protonation of 1a in the presence of a diene affords the neutral complexes [2-(eta(2):eta(2)-diene)-closo-2,1,7-NiC(2)B(9)H(11)] (diene = C(5)Me(5)H (5), dcp (6), cod (7), nbd (8), chd (9), and cot (10a); dcp = dicyclopentadiene, cod = 1,5-cyclooctadiene, nbd = norbornadiene, chd = 1,3-cyclohexadiene, and cot = cyclooctatetraene). Variable temperature (1)H NMR experiments show that the [Ni(diene)] fragments are freely rotating even at 193 K. A small quantity of the di-cage species [2,2'-micro-(1,2:5,6-eta-3,4:7,8-eta-cot)-(closo-2,1,7-NiC(2)B(9)H(11))(2)] (10b) is formed as a coproduct in the synthesis of 10a. This species can be rationally synthesized by protonation of 1a and subsequent addition of 10a.

Journal Article↗
PubMed2004

Hydrothermal syntheses, crystal structures, and properties of a novel class of 3,3',4,4'-benzophenone-tetracarboxylate (BPTC) polymers.

Three novel BPTC complexes, (H(2)BPTC)(4,4'-H(2)bpy)H(2)O (1), [Cd(2)Cu(HBPTC)(2)(mu(2)-4,4'-bpy)(2)(4,4'-bpy)(2)(H(2)O)(2)](n) (2), and [Co(3)(HBPTC)(2)(mu(2)-4,4'-bpy)(3)(H(2)O)(4)](n).2nH(2)O (3) (BPTC = 3,3',4,4'-benzophenone-tetracarboxylate and bpy = bipyridine), were hydrothermally synthesized. Complex 1, which is obtained as a coproduct during the syntheses of complexes 2 and 3, features a 2-D layered strong hydrogen bonding network with 2-fold interpenetration. Complex 2 has an unusual 2-D double-layered motif, which is linked together by Cu atoms in a face-to-face manner. It exhibits nanosized channels filled by 4,4'-bpy ligands. Three six-coordinated Co atoms in 3 are interlinked by HBPTC ligands to form a 2-D grid structure, which is further sustained by rigid 4,4'-bpy ligands into a 3-D open framework similar to CdSO(4) with the BPTC moieties situated in the tunnels. The thermal stabilities of complexes 1-3 were examined. The photoluminescence properties of complexes 1-2 and temperature-dependent magnetic susceptibility for 3 were also studied.

Journal Article↗
PubMed1999

C-(Halide) Oxidative Addition Routes to Ruthenium Carbenes.

Ru(H)(2)(H(2))(2)L(2) (L = PCy(3)) reacts with CHRCl(2) (R = H, Ph) to give Ru(CHR)Cl(2)L(2) and H(2). Using Cl(2)C=CH(2) as the gem-dihalide gives Ru(CHCH(3))Cl(2)L(2), due to hydrogenation of the C=C bond of the presumed vinylidene primary product by released H(2). Released H(2) also reacts with Ru(CHR)Cl(2)L(2) (R = H, Ph) to give H(3)CR, HCl and RuHCl(H(2))L(2). This undesirability of H(2) as a coproduct can be diminished by using Ru(H)(2)(N(2))(2)L(2) as the reagent, giving Ru(CHR)Cl(2)L(2) and 1H(2) and 2N(2) as products. Reaction of Ru(H)(2)(N(2))(2)L(2) with Cl(2)CHEt gives RuCl(2)(CHEt)L(2) and RuHCl(N(2))L(2), the latter apparently by competitive beta-H migration from an intermediate RuHCl(CHClEt)L(2) species. When Ru(H)(2)(N(2))(2)L(2) is reacted with the monochloride PhCH(2)Cl, the primary product RuCl(CH(2)Ph)(H(2))L(2) slowly (hours) evolves further to give RuHCl(N(2))L(2) and PhCH(3). Reaction of Ru(H)(2)(N(2))(2)L(2) with C(6)F(6), BrHC=CHPh, and CH(3)I give RuHX(N(2))L(2) (X = F, Br, I, respectively). The N(2) ligand in RuHCl(N(2))L(2) can be displaced by H(2) and by CO, while H(2) converts RuHF(N(2))L(2) to Ru(H)(2)(H(2))(2)L(2) and HF.

Journal Article↗
PubMed2002

Development of bifunctional salen catalysts: rapid, chemoselective alkylations of alpha-ketoesters.

Lewis acid-Lewis base salen complexes have been identified as highly efficient catalysts for the addition of dialkylzincs to alpha-ketoesters. In contrast to aldehydes or ketones, the reaction between diethylzinc and alpha-ketoesters is significant in the absence of catalyst. In the presence of catalyst, the reaction rate is increased over 100-fold relative to the background. Furthermore, the reduction product, which is a major coproduct with other catalysts, is not observed with these bifunctional salens. As a result, high yields of the addition products can be obtained (57-99%). Both the Lewis acid and Lewis base portions of the catalyst are critical to the reactivity and selectivity. The two separate portions of the catalyst have been shown to function in a cooperative manner.

Aldehydes↗
PubMed2004

The Brønsted acid-catalyzed direct aza-Darzens synthesis of N-alkyl cis-aziridines.

A mild protocol for the synthesis of cis-aziridines is described that employs a catalytic amount of Brønsted acid. Despite the potential for diazo compound decomposition via alkylation or homocoupling upon exposure to a proton source, these pathways are slow relative to [2 + 1] annulation in the presence of a Schiff base, leading to aziridine product. The process uses no metals or reagents that must be removed chromatographically, exhibits rapid turnover rates, and produces only atomic nitrogen as a coproduct. High levels of relative stereocontrol are also possible when forming the Schiff base from a chiral nonracemic aldehyde.

Journal Article↗
PubMed2005

Bronsted acid-promoted olefin aziridination and formal anti-aminohydroxylation.

A straightforward synthesis of aziridines is reported from an electron-rich azide (alkyl or aryl azide), electron-deficient olefin, and triflic acid in cold acetonitrile. The only coproduct of the reaction is dinitrogen (N2). Active ester substrates bearing a nucleophilic carbonyl engage the putative protonated aziridine intermediate to produce the product of olefin aminohydroxylation in which the nitrogen is benzyl protected and the oxygen is acylated. The possibility that a triazoline need not be an intermediate in aziridine production is advanced.

Journal Article↗
PubMed2006

Reaction progress of chromophore biogenesis in green fluorescent protein.

The mature form of green fluorescent protein (GFP) is generated by a spontaneous self-modification process that is essentially irreversible. A key step in chromophore biosynthesis involves slow air oxidation of an intermediate species, in which the backbone atoms of residues 65-67 have condensed to form a five-membered heterocycle. We have investigated the kinetics of hydrogen peroxide evolution during in vitro GFP maturation and found that the H2O2 coproduct is generated prior to the acquisition of green fluorescence at a stoichiometry of 1:1 (peroxide/chromophore). The experimental progress curves were computer-fitted to a three-step mechanism, in which the first step proceeds with a time constant of 1.5 (+/-1.1) min and includes protein folding and peptide cyclization. Kinetic data obtained by HPLC analysis support a rapid cyclization reaction that can be reversed upon acid denaturation. The second step proceeds with a time constant of 34.0 (+/-1.5) min and entails rate-limiting protein oxidation, as supported by a mass loss of 2 Da observed for tryptic peptides derived from species that accumulate during the reaction. The final step in GFP maturation proceeds with a time constant of 10.6 (+/-1.2) min, suggesting that this step may contribute to overall rate retardation. We propose that under highly aerobic conditions, the dominant reaction path follows a cyclization-oxidation-dehydration mechanism, in which dehydration of the heterocycle is facilitated by slow proton abstraction from the Tyr66 beta-carbon. In combination, the results presented here suggest a role for molecular oxygen in trapping the cyclized form of GFP.

Amino Acid Sequence↗
PubMed2006

A simple tagging system for protein encapsulation.

Molecular containers that encapsulate specific cargo can be useful for many natural and non-natural processes. We report a simple system, based on charge complementarity, for the encapsulation of appropriately tagged proteins within an engineered, proteinaceous capsid. Four negative charges per monomer were added to the lumazine synthase from Aquifex aeolicus (AaLS). The capsids formed by the engineered AaLS associate with green fluorescent protein bearing a positively charged deca-arginine tag upon coproduction in Escherichia coli. Analytical ultracentrifugation and scanning force microscopy studies indicated that the engineered AaLS retains the ability to form capsids, but that their average size was substantially increased. The success of this strategy demonstrates that both the container and guest components of protein-based encapsulation systems can be convergently designed in a straightforward manner, which may help to extend their versatility.

Amino Acid Substitution↗
PubMed2006

Protein distribution in commercial wet- and dry-milled corn germ.

To identify high-valued coproducts from commercially processed corn germ, it was necessary to determine the effect of processing conditions on corn germ proteins. We found that significantly less protein was extracted from commercial wet-milled as compared to dry-milled corn germ using Tris, sodium dodecyl sulfate (SDS) buffer containing 14 mM 2-mercaptoethanol at 100 degrees C for 10 min. SDS-polyacrylamide gel electrophoresis (PAGE) revealed a number of proteins with molecular masses ranging from approximately 10 to 66 kDa for the dry-milled corn germ as compared to only a few significant protein bands centered around 23 kDa in the wet-milled corn germ. The protein content of the wet- and dry-milled corn germ was approximately the same; however, nonprotein nitrogen values were significantly greater for the wet-milled than for the dry-milled germ. The distribution of fractionated germ protein freshly excised from the embryo of yellow dent corn kernels was more similar to that of dry-milled than wet-milled corn. SDS-PAGE of laboratory preparations of wet-milled corn germ more closely resembled commercial dry- than wet-milled corn germ, which could be attributed to limited microbial growth during steeping in the laboratory preparations.

Agriculture↗
PubMed1982

10-Propargylaminopterin and alkyl homologues of methotrexate as inhibitors of folate metabolism.

Reported antifolate activity against leukemia L1210 by N-[14-[[(2-amino-4-hydroxy-6-quinazolinyl)methyl]-propargylamino]benzoyl]]-L-glu tamic acid through potent inhibition of thymidylate synthase (EC 2.1.1.45) prompted us to include the propargyl group in a study of the effect on folate metabolism and membrane transport of replacing the 10-methyl group of methotrexate with other groups. Along with the propyl (8a) and octyl (8b) homologues of methotrexate, the propargyl compound 8c was prepared for evaluation. Syntheses of 8a,b were achieved by a standard multistep sequence involving preparation of the side-chain precursors via tosylated intermediates and then their alkylation with 6-(bromomethyl)-2,4-pteridinediamine hydrobromide. The side-chain precursor to 8c was prepared by direct alkylation of diethyl N-(4-aminobenzoyl)-L-glutamate with propargyl bromide and was separated from unchanged amine and dipropargyl coproduct by a combination of methods, including dry-column chromatography and recrystallization. Subsequent steps leading to 8c were like those used to prepare 8a,b. Biological evaluations of the three compounds consisted of studies of their effects on enzyme inhibition [(dihydrofolate reductase (EC 1.5.1.3) and thymidylate synthase)], L1210 cell growth inhibition, cellular membrane transport with various murine cell types (L210, S180, Ehrlich, and epithelial), in vivo (mice) activity vs. L1210 leukemia and S180 ascites, and plasma clearance in mice. The in vivo results vs. S180 ascites offered evidence that 8c might have a better therapeutic index against this tumor than methotrexate, but no other result from either of these compounds suggested significant superiority over methotrexate.

Aminopterin↗
PubMed2006

Chain mechanism in the photocleavage of phenacyl and pyridacyl esters in the presence of hydrogen donors.

[reaction: see text] Excited phenacyl and 3-pyridacyl esters of benzoic acid react with an excess of aliphatic alcohols in a chain reaction process involving hydrogen transfer from the ketyl radical intermediates, leading to benzoic acid in addition to acetophenone and 3-acetylpyridine, respectively, as the byproducts. While the maximum quantum yields reached 4 in both cases, the 2- or 4-pyridacyl ester photoreduction proceeded with the efficiency below 100% under the same conditions. The investigation indicates that a radical coupling between ketyl radicals, both formed from the excited ester by hydrogen abstraction from an alcohol, is accompanied by the elimination of benzoic acid from the ester ketyl radical itself. A partitioning between two reactions was found to be remarkably sensitive to the chromophore nature, such as a position of the nitrogen atom in the pyridacyl moiety. The magnitude of a radical chain process is dependent on the efficiency of consecutive steps that produce free radicals capable of a subsequent ester reduction. The driving force of a possible electron transfer from the ketyl radicals to the ester has been excluded on the basis of cyclic voltametry measurements. The observed quantum yields of photoreduction were found to be diminished by formation of relatively long-lived light absorbing transients, coproducts obtained apparently by secondary photochemical reactions. Additionally, it is shown that basic additives such as pyridine can further increase the efficiency of the photoreduction by a factor of 4. A radical nature of the reduction mechanism was supported by finding a large kinetic chain length of an analogous reaction initiated by free radicals generated thermally yet again when phenacyl or 3-pyridacyl benzoate was used. Both phenacyl and pyridacyl chromophores are pronounced to be valuable as the photoremovable protecting groups when high quantum and chemical yields of carboxylic acid elimination are important, but higher concentrations of the hydrogen atom donors are not destructive for a reaction system or are experimentally impractical.

Benzoates↗
PubMed2006

Oxidations of secondary alcohols to ketones using easily recyclable bis(trifluoroacetate) adducts of fluorous alkyl iodides, CF3(CF2)(n-1)I(OCOCF3)2.

Reactions of commercial fluorous alkyl iodides RfnI (1-Rfn; Rfn = CF3(CF2)(n-1); n = 7, 8, 10, 12) with 80% H2O2 and trifluoroacetic anhydride give RfnI(OCOCF3)2 (2-Rfn; 97-89%). These efficiently oxidize aliphatic and benzylic secondary alcohols to the corresponding ketones (92-57%) in the presence of aqueous KBr and absence of organic or fluorous solvents. Bromide ion activates the reagents and/or generates a relay oxidant such as a functional equivalent of Br+. Oxidations are much more rapid (<30 min, 2-R(f8); <70 min, 2-R(f10)) than with other iodine(III) compounds under similar conditions. The coproducts 1-Rfn can be recovered by adding 3-5 volumes of methanol to the reaction mixtures. Fluorous/methanolic liquid/liquid (1-R(f8)) or solid/liquid (1-R(f10)) biphase systems result. The recovered 1-Rfn can be reoxidized to 2-Rfn and reused. Three cycles are conducted with 1-phenyl-1-propanol and 2-R(f10). The propiophenone yields range from 92% to 83% per cycle, and after the final cycle 59-57% of the original charge of the fluorous iodide species is recovered.

Journal Article↗
PubMed2000

Convenient enzymatic resolution of alcohols using highly reactive, nonharmful acyl donors, 1-ethoxyvinyl esters

1-Ethoxyvinyl esters 3, a new type of acyl donors for enzymatic resolution of racemic alcohols, were disclosed to be superior to the contemporary major reagents, vinyl esters 1 and isopropenyl esters 2. Three features of 3 are noticeable: (1) 3 generates ethyl acetate as a single coproduct, which does not affect any enzymes, while acetaldehyde liberated from 1 deactivates some kinds of lipases. (2) The reactivity of 3 was not less than that of 1 and much higher than that of 2, and the optical purity of the products was as high as that of 1 and 2. Especially, it was generally observed that 3 showed higher reactivity than 1 for reactions using Candida rugosa lipases, one of the most commonly employed lipases, having liberal applicability to substrates but sensitive to acetaldehyde. Twelve examples of the kinetic resolution of racemic secondary alcohols (5 and 10) and one desymmetrization of meso-alcohol 7 were presented employing the acetate 3a or the octanoate 3b and four types of lipases. (3) A one-pot procedure for the preparation of 3 from the corresponding carboxylic acid and the subsequent enzymatic resolution of alcohols, which has not been reported using 1 or 2, was elucidated. The chemical and optical yields of the products by this procedure were similar to those obtained using isolated 3.

Journal Article↗
PubMed2005

Photochemical sources of organic acids. 1. Reaction of ozone with isoprene, propene, and 2-butenes under dry and humid conditions using SPME.

Formation of C4 and smaller carboxylic acids from gas-phase ozonolysis of several alkenes under dry (relative humidity (RH) < 1%) and humid (RH = 65%) conditions have been investigated. We have developed a technique based on solid-phase microextraction (SPME) and gas chromatography/mass spectrometry (GC/MS) to quantify the acids, as well as other products, and applied it to the reactions of ozone with propene, trans-2-butene, 2,3-dimethyl-2-butene, and isoprene. Acetic acid yields from propene and trans-2-butene ozonolysis in the presence of an OH scavenger were 2.7 +/- 0.6 and 2.9 +/- 0.6%, respectively, under dry conditions and 1.8 +/- 0.4 and 2.3 +/- 0.5% at 65% RH. Isoprene ozonolysis produced methacrylic and propenoic acids with yields of 5.5 +/- 1 and 3.0 +/- 1%, under dry conditions and 4.1 +/- 1 and 1.5 +/- 0.3% under wet conditions, respectively. That water inhibits acid formation indicates that the water reaction with stabilized Criegee intermediates is at most a minor source of acids. Acids that may form as coproducts of the OH radical elimination pathway, acetic acid from 2,3-dimethylbutene and isoprene, and propenoic acid from isoprene were also observed with significant yields (up to 10%), although the production of acetic acid was not a linear function of the alkene reacted. Carbonyl products are also reported.

Journal Article↗
PubMed2005

How active is the bend excitation of methane in the reaction with O(3P)?

The role of bending excitation of methane in the reaction with O(3P) is investigated in a crossed-beam experiment. Previous theories predicted that either stretch or bend excitation of the reactant promotes chemical reactivity and that initial bending excitation of CH4 preferentially yields umbrella-excited CH3 products. Experimentally, both predictions for bend-excited reagent were not borne out in this investigation. We found instead that compared to the ground-state reagent, bend-excited methane yields more vibrational excitation of the hydroxyl coproduct. The first reported product angular distributions show predominant backward scatterings for both ground-state and bend-excited methanes, which corroborate well with a direct rebound reaction mechanism.

Letter↗
PubMed2005

Imaging the reaction dynamics of OH + CD4. 3. Isotope effects.

Isotope effects in the reaction of hydroxyl radical with methane are investigated in a crossed-beam experiment. By exploiting different combinations of OH/OD + CH4/CD4/CHD3, a total of eight isotopically variant reactions are examined to decipher the dynamical consequence of both primary and secondary isotope effects. The most prominent observation is from isotopic substitution of the transferred atom (primary isotope effect), which yields more stretch-excited water product for the D atom case than H atom. The opposite is found, however, for excitations in bending and combination modes. The secondary isotope effects, from either substituted hydroxyl radical or the three H atoms that are not being abstracted, are relatively minor and manifest themselves in spreading the vibrational distribution of the water coproducts.

Journal Article↗
PubMed2006

Dynamics of CO in mesoporous silica monitored by time-resolved step-scan and rapid-scan FT-IR spectroscopy.

Carbon monoxide molecules generated in the channels of mesoporous MCM-41 silica sieve from a precursor (diphenyl cyclopropenone) by photodissociation with a nanosecond laser pulse were monitored by time-resolved Fourier transform infrared (FTIR) spectroscopy using the step-scan and rapid-scan methods. A very broad absorption of CO is observed in the region 2200-2080 cm(-1) at room temperature that decays in a biphasic mode. Two-thirds of the band intensity decays on the hundreds of microsecond scale (lifetime 344 +/- 70 micros). The process represents the escape of the molecules through the mesopores into the surrounding gas phase, and a diffusion constant of 1.5 x 10(-9) m(2)/s is derived (assuming control by intra-MCM-41 particle diffusion). The broad profile of the absorption is attributed to contact of the random hopping CO with siloxane and silanol groups of the pore surface. Measurements using MCM-41 with the silanols partially capped by trimethyl silyl groups gave further insight into the nature of the IR band profile. These are the first observations on the diffusion behavior of carbon monoxide in a mesoporous material at room temperature. The residual carbon monoxide remains much longer in the pores and features distinct peaks at 2167 and 2105 cm(-1) characteristic for CO adsorbed on SiOH groups C end on and O end on, respectively. The bands decrease with time constants of 113 +/- 3 ms (2167 cm(-1)) and 155 +/- 15 ms (2105 cm(-1)) suggesting that CO in these sites is additionally trapped by surrounding diphenyl acetylene coproduct and/or precursor molecules.

Absorption↗
PubMed2003

Photochemical cleavage and release of carboxylic acids from alpha-keto amides.

Carboxylate groups incorporated at the position alpha to the keto carbonyl of alpha-keto amides 1 were photochemically cleaved in aqueous media to give carboxylic acids in 70-90% yields with quantum yields of 0.3. The cleavage coproducts were diastereomeric hemiacetals 2. Prompt release of acetate and gamma-aminobutyrate (GABA) in buffer was observed by difference FT-IR spectroscopy upon 355 nm laser flash photolysis. The time-constant for release of GABA was <30 ms. [reaction--see text]

Acetates↗