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991.
Ruthenium-catalyzed coupling of readily accessible 1-trimethylsilyl-1-alkyn-3-ols to allyl ethyl ether followed by in situ ketalization and Lewis-acid induced cyclization affords 1,5-oxygen-bridged eight- and nine-membered carbocycles, products which are difficult to construct with previously published strategies. 相似文献
992.
Hilal M Parrado MF Rosa M Gallardo M Orce L Massa EM González JA Prado FE 《Photochemistry and photobiology》2004,79(2):205-210
UV-B radiation (280-320 nm) is harmful to living organisms and has detrimental effects on plant growth, development and physiology. In this work we examined some mechanisms involved in plant responses to UV-B radiation. Seedlings of quinoa (Chenopodium quinoa Willd.) were exposed to variable numbers of UV-B radiation doses, and the effect on cotyledons was studied. We analyzed (1) cotyledons anatomy and chloroplasts ultrastructure; (2) peroxidase activity involved in the lignification processes; and (3) content of photosynthetic pigments, phenolic compounds and carbohydrates. Exposure to two UV-B doses induced an increase in the wall thickness of epidermal cells, which was associated with lignin deposition and higher activity of the peroxidase. The chloroplast ultrastructure showed an appearance typical of plants under shade conditions, likely in response to reduced light penetration into the mesophyll cells due to the screening effect of epidermal lignin deposition. Exposure to UV-B radiation also led to (1) enhancement in the level of phenolics, which may serve a protective function; (2) strong increase in the fructose content, a fact that might be related to higher requirement of erythrose-4P as a substrate for the synthesis of lignin and phenolics; and (3) reduction in the chlorophyll concentration, evidencing alteration in the photosynthetic system. We propose that the observed lignin deposition in epidermal tissues of quinoa is a resistance mechanism against UV-B radiation, which allows growing of this species in Andean highlands. 相似文献
993.
An effective one-pot synthesis of 5-substituted tetronic acids 总被引:1,自引:0,他引:1
Aragón DT López GV García-Tellado F Marrero-Tellado JJ de Armas P Terrero D 《The Journal of organic chemistry》2003,68(8):3363-3365
An expeditious one-pot synthesis of 5-substituted tetronic acids from aldehydes and terminal conjugated alkyne as starting materials is described. The entire process embodies two consecutive chemical events: a catalytic domino reaction to build the 1,3-dioxolane scaffolds 5 and a two-step acid-catalyzed trans-acetalization-lactonization reaction to furnish the tetronic acid derivatives 6. 相似文献
994.
Fernando Oliveira Tartarotti Marcelo Firmino de Oliveira Valdir Roberto Balbo Nelson Ramos Stradiotto 《Mikrochimica acta》2006,155(3-4):397-401
A mercury-free electrode chemically modified with carbon paste containing dimethylglyoxime was used for determination of nickel
in fuel ethanol. The instrumental parameters and composition of the modified paste were optimized. The analytical curve for
nickel determination from 5.0 × 10−9 to 5.0 × 10−7 mol L−1 was obtained using 25 min of accumulation time. The detection limit and amperometric sensitivity obtained for this method
were 2.7 × 10−9 mol L−1 and 5.2 × 108 μA mol−1 L, respectively. The values for nickel concentration in four commercial samples of fuel ethanol were obtained in the range
of 1.1 × 10−8 to 6.9 × 10−8 mol L−1. A comparison to graphite furnace atomic absorption spectrometry (GFAAS) was performed for nickel determination in commercial
samples of ethanol. 相似文献
995.
Biondini F Brunetti BG Candori P De Angelis F Falcinelli S Tarantelli F Moix Teixidor M Pirani F Vecchiocattivi F 《The Journal of chemical physics》2005,122(16):164307
The energetics of [Rg... N2O]* autoionizing collision complexes (where Rg=He or Ne) and their dynamical evolution have been studied in a crossed beam apparatus, respectively, by Penning ionization electron spectroscopy (PIES) and by mass spectrometry (MS) techniques in the thermal energy range. The PIES spectra, detected by an electron energy analyzer, were recorded for both complexes at four different collision energies. Such spectra allowed the determination of the energy shifts for Penning electron energy distributions, and the branching ratios for the population of different electronic states and for the vibrational population in the molecular nascent ions. For the [Ne...N2O]* collision complex it was found, by MS, that the autoionization leads to the formation of N2O+, NO+, O+, and NeN2O+ product ions whose total and partial cross sections were measured in the collision energy range between 0.03 and 0.2 eV. The results are analyzed exploiting current models for the Penning ionization process: the observed collision energy dependence in the PIES spectra as well as in the cross sections are correlated with the nature of the N2O molecule orbitals involved in the ionization and are discussed in term of the Rg-N2O interaction potentials, which are estimated by using a semiempirical method developed in our laboratory. 相似文献
996.
A new concept for the amplified electrochemical detection of the 7229-base viral DNA of M13phi is developed. A thiolated 27-base nucleic acid (1) is assembled on an Au-electrode. Hybridization between the sensing interface and the M13phi DNA is followed by the polymerase-induced replication of the analyte DNA in the presence of dCTP, dGTP, dATP, and ferrocene-tethered-dUTP (2). The generated redox-active replica mediates electron transfer between the enzyme glucose oxidase (GOx) and the electrode and activates the bioelectrocatalyzed oxidation of glucose. The bioelectrocatalyzed oxidation of glucose provides a biocatalytic amplification path for the formation of the redox-active replica. The electrochemical techniques to follow the replication and the bioelectrocatalytic amplification are differential pulse voltammetry and cyclic voltammetry. The electrical responses from the system relate to the bulk concentration of the M13phi DNA, thus enabling the quantitative analysis of the viral gene. 相似文献
997.
Raitman OA Patolsky F Katz E Willner I 《Chemical communications (Cambridge, England)》2002,(17):1936-1937
A novel method to generate an integrated electrically contacted glucose dehydrogenase electrode by the surface reconstitution of the apo-enzyme on a pyrroloquinoline quinone (PQQ)-modified polyaniline is described. In situ electrochemical surface plasmon resonance (SPR) is used to characterize the bioelectrocatalytic functions of the system. 相似文献
998.
Fernando Sartillo-Piscil Silvano CruzMario Sánchez Herbert HöpflCecilia Anaya de Parrodi Leticia Quintero 《Tetrahedron》2003,59(23):4077-4083
Twelve 2-phenoxy-2-oxo-1,3,2-dioxaphosphorinanes fused with a 1,2-O-isopropylidene-α-d-xylofuranose moiety in cis orientation and substituted at the C′5 position were prepared in two steps from commercially available diacetone-α-d-glucose. Their conformations, and configurations were determined by 1H and 31P NMR and X-ray crystallographic techniques. Both, chair-twisted-chair and chair-boat equilibria were observed in solution. We observed that the strong anisotropic shielding effect of the benzene ring in the phenoxy group generates an upfield shift of the H1 hydrogen atom, when the cyclic phosphates adopt a boat conformation. This is due to a relative cis-orientation of the P-phenoxy group and the H1 proton of the 1,2-O-isopropylidene-α-d-xylofuranose moiety. Therefore, the configuration of the phosphorus center (SP or RP) can be determined by 1H NMR spectroscopy. Interestingly, the crystal structure of one of the cyclic phosphates exhibits two independent molecules in the asymmetric unit, one with a chair and the other one with a boat conformation. 相似文献
999.
Beatriz Fernandez-Band Fernando Lázaro M.D. Luque de Castro Miguel Valcarcel 《Analytica chimica acta》1990
A method for the determination of 2,4-dinitrophenylhydrazine and 2- and 4-nitrophenylhydrazine in mixtures based on their different spectral features is proposed. The sample containing the amines is injected into an FIA manifold and passed through the flow cell of a diode-array spectrophotometer where a support (C18 bonded silica) retains the analytes. The amines are concentrated in situ, thereby achieving a determination limit of 5 × 10?7 M. The carrier (phosphate buffer-methanol) rapidly elutes the retained compounds, so a sampling frequency of 40 h?1 could be obtained. The resolution of mixtures of these amines can be carried out with acceptable low errors. 相似文献
1000.