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81.
Valfredo Azevedo Lemos Douglas Gonalves da Silva Anaildes Lago de Carvalho Dbora de Andrade Santana Geisiane dos Santos Novaes Adenilde Souza dos Passos 《Microchemical Journal》2006,84(1-2):14-21
A new chelating sorbent has been developed using Amberlite XAD-2 resin anchored with pyrocatechol through –N=C– group. This sorbent, characterised by elemental analysis and infrared (IR) spectra, was used as packing for the minicolumn in an on-line system preconcentration system for cadmium, cobalt, copper and nickel determination. Metal ions were sorbed in the minicolumn, from which it could be eluted directly to the nebulizer–burner system of the flame atomic absorption spectrometer (FAAS). Elution of all metals from minicolumn can be made with 0.50 mol L− 1 HCl or HNO3. The enrichment factors obtained were 16 (Cd), 24 (Co), 15 (Cu) and 19 (Ni), for 60 s preconcentration time, and 39 (Cd), 69 (Co), 36 (Cu) and 41 (Ni), if used 180 s preconcentration time. Under the optimum conditions, the proposed procedure allowed the determination of cadmium, cobalt, copper and nickel with detection limits of 0.31, 0.32, 0.39 and 1.64 μg L− 1, respectively, when used preconcentration periods of 180 s. The accuracy of the developed procedure was sufficient and evaluated by the analysis of the certified reference materials NIST 1515 apple leaves and NIST 1570a spinach leaves. The method was applied to the analysis of food samples (spinach, black tea and rice flour). 相似文献
82.
Singlet oxygen generation ability of squarylium cyanine dyes 总被引:1,自引:0,他引:1
P. F. Santos L. V. Reis P. Almeida A. S. Oliveira L. F. Vieira Ferreira 《Journal of photochemistry and photobiology. A, Chemistry》2003,160(3):159-161
The quantum yields for singlet oxygen generation of several squarylium cyanine dyes derived from benzothiazole, benzoselenazole and quinoline, displaying absorption within the so-called “phototherapeutic window” (600–1000 nm), were determined, envisioning their potential usefulness for photodynamic therapy (PDT). The determination was performed by a direct method measuring the luminescence decay of the dyes in the near infrared. Considering the absorption and the quantum yields displayed by some of the dyes, these seemed to be potential candidates as sensitizers for PDT. 相似文献
83.
[reaction: see text] Enantiomerically pure gamma-fluoroalkyl beta-amino sulfones are readily synthesized in three steps starting from fluorinated imidoyl chlorides and arylmethyl sulfones. A complementary two-step sequence starting from chiral fluorinated beta-amino sulfoxides has also been developed. To illustrate the application of this procedure, a new method for the synthesis of alpha-fluoroalkyl allylic amines in optically pure form involving a Julia methylenation-desulfonylation reaction is presented. 相似文献
84.
Cheng X Kelso C Hornak V de los Santos C Grollman AP Simmerling C 《Journal of the American Chemical Society》2005,127(40):13906-13918
The process by which DNA repair enzymes recognize and selectively excise damaged bases in duplex DNA is fundamental to our mechanistic understanding of these critical biological reactions. 8-Oxoguanine (8-oxoG) is the most common form of oxidative DNA damage; unrepaired, this lesion generates a G:C-->T:A mutation. Central to the recognition and repair of DNA damage is base extrusion, a process in which the damaged base lesion or, in some cases, its partner disengages from the helix and is bound to the enzyme's active site where base excision takes place. The conformation adopted by 8-oxoG in duplex DNA is affected by the base positioned opposite this lesion; conformational changes may also take place when the damaged base binds to its cognate repair enzyme. We performed unrestrained molecular dynamics simulations for several 13-mer DNA duplexes. Oligomers containing G:C and 8oxoG:C pairs adopted Watson-Crick geometries in stable B-form duplexes; 8oxoG showed increased local and global flexibility and a reduced barrier to base extrusion. Duplexes containing the G:A mismatch showed much larger structural fluctuations and failed to adopt a well-defined structure. For the 8oxoG:A mismatch that is recognized by the DNA glycosylase MutY, the damaged nucleoside underwent spontaneous and reproducible anti-->syn transitions. The syn conformation is thermodynamically preferred. Steric hindrance and unfavorable electrostatics associated with the 8oxoG O8 atom in the anti conformation were the major driving forces for this transition. Transition events follow two qualitatively different pathways. The overall anti-->syn transition rate and relative probability of the two transition paths were dependent on local sequence context. These simulations indicate that both the dynamic and equilibrium behavior of the duplex change as a result of oxidation; these differences may provide valuable new insight into the selective action of enzymes on damaged DNA. 相似文献
85.
Darío Gmez Marina Dos Santos Fabin Fujiwara Griselda Polla Julieta Marrero Laura Dawidowski Patricia Smichowski 《Microchemical Journal》2007,85(2):276-284
A study was undertaken to evaluate the distribution of Al, As, Cr, Cu, Fe, Mn, Ni, Pb, Ti, V and Zn in fly ashes collected in the electrostatic precipitator of a thermal power plant in San Nicolás (Argentina). Five samples were collected during one week of operation. For the fractionation, the scheme applied consisted in extracting the elements in four fractions namely (i) soluble and exchangeable elements; (ii) carbonates, oxides and reducible elements; (iii) bound to sulfidic metals; and (iv) residual elements. Metals and metalloids at μg g− 1 level were determined in each fraction by inductively coupled plasma optical emission spectrometry (ICP OES). For validation, a standard reference material (SRM 1633 coal fly ash) from NIST was subjected to the same chemical sequential extraction procedure that the samples. X-ray diffraction powder (XRD) analysis and scanning electron microscopy (SEM) were used to characterize the major minerals present in the matrix. Total analyte concentration (in μg g− 1) varied from 10.6 for Pb to 17,622 for Al. Minimum and maximum concentrations (in μg g− 1) found in individual samples in the four fractions were: Al, 92.7–9668; As, < 0.3–143; Cr, 2.0–10.4; Cu, < 0.2–35.6; Fe, < 0.3–4992; Mn, < 0.1–128; Ni, < 0.3–139; Pb, < 0.5–9.1; Ti, < 0.3–2243; V, 17.0–112.9; and Zn, < 0.1–68.2. The leachability of the 11 elements under study proved to be different. Low percentages of Al (1%), V (7%) and Cr (8%) were detected in the most bioavailable fraction. Arsenic was found to be most abundant in the non-silicate phase, represented by the second and third fractions, while Cr, Fe, Ni, Pb and Zn were mostly associated to the residual fraction. 相似文献
86.
Igarashi RY Laryukhin M Dos Santos PC Lee HI Dean DR Seefeldt LC Hoffman BM 《Journal of the American Chemical Society》2005,127(17):6231-6241
We here show that the iron-molybdenum (FeMo)-cofactor of the nitrogenase alpha-70(Ile) molybdenum-iron (MoFe) protein variant accumulates a novel S = (1)/(2) state that can be trapped during the reduction of protons to H(2). (1,2)H-ENDOR measurements disclose the presence of two protons/hydrides (H(+/)(-)) whose hyperfine tensors have been determined from two-dimensional field-frequency (1)H ENDOR plots. The two H(+/)(-) have large isotropic hyperfine couplings, A(iso)( )() approximately 23 MHz, which shows they are bound to the cofactor. The favored analysis for these plots indicates that the two H(+/)(-) have the same principal values, which indicates that they are chemically equivalent. The tensors are further related to each other by a permutation of the tensor components, which indicates an underlying symmetry of binding relative to the cofactor. At present, no model for the structure of the iron-molybdenum (FeMo)-cofactor in the S = (1)/(2) state trapped during the reduction of H(+) can be shown unequivocally to satisfy all of the constraints generated by the ENDOR analysis. The data disfavors any model that involves protonation of sulfides, and thus suggests that the intermediate instead contains two chemically equivalent bound hydrides; it appears unlikely that these are terminal monohydrides. 相似文献
87.
Characterisation of the surface Lewis acid-base properties of poly(butylene terephthalate) by inverse gas chromatography 总被引:3,自引:0,他引:3
Two modes of high-speed counter-current chromatography (HSCCC) were applied to separate 3- and 4-sulfophthalic acid from a mixture. Conventional HSCCC was useful for the separation of up to several hundred milligram quantities of these positional isomers, while pH-zone-refining CCC was implemented successfully to separations at the multigram level. The conventional HSCCC separations were performed with a standard J-type HSCCC system that has a superior resolution but a lower level of retention of the stationary phase of the biphasic solvent system used (acidified n-butanol-water). The pH-zone-refining CCC separations were performed with an X-type HSCCC system (a cross-axis system) that has a higher capability for retention of the stationary phase. The purified positional isomers (over 99% pure as determined by HPLC) were characterized by 1H NMR and negative ion electrospray ionization mass spectrometry. 相似文献
88.
Marcelo O. Santiago Alzir A. Batista Marcio P. de Araújo Claudio L. Donnici Icaro de S. Moreira Eduardo E. Castellano Javier Ellena Sauli dos Santos Jr Salete L. Queiroz 《Transition Metal Chemistry》2005,30(2):170-175
A series of cis-{RuCl2(PPh3)2[4,4-(X)2-2,2-bipy]} [cis-chlorines; X=-H, -Me, -SMe, and (-Cl,-Me)] complexes have had their structures determined by single crystal X-ray diffraction. The geometry of these complexes, also determined in CH2Cl2 solution by 31P{1H}-n.m.r. spectroscopy, showed that the chemical shifts for the phosphorus atoms are slightly dependent on the pKa of the 4,4-(-X)2-2,2-bipy ligands. 相似文献
89.
Érika Pinto Marinho A. G. Souza Danniely S. de Melo Iêda M. G. Santos Dulce M. A. Melo Walquíria J. da Silva 《Journal of Thermal Analysis and Calorimetry》2007,87(3):801-804
Because
of their electrical, magnetic and catalytic properties rare earth and transition
metal mixed oxides are important compounds. Lanthanum chromites have been
extensively used as solid oxide fuel cell (SOFC) interconnect materials. In
this work, lanthanum chromites partially substituted by alkaline earth metals
were synthesized by the urea combustion process. TG and DSC techniques were
used to evaluate the presence of the organic material in the powder after
reaction on the hot plate. The powders were calcinated at 900°C and characterized
by XRD and SEM. The results show that the particles have nanometric dimensions
and the perovskite structure was formed. 相似文献
90.
Palacios F Aparicio D Ochoa De Retana AM de los Santos JM Gil JI Alonso JM 《The Journal of organic chemistry》2002,67(21):7283-7288
A simple and efficient asymmetric synthesis of 2H-azirine-2-phosphine oxides 3 is described. The key step is a solid-phase bound achiral or chiral amine-mediated Neber reaction of beta-ketoxime tosylates derived from phosphine oxides 1. Reaction of 2H-azirines 3 and 11 with carboxylic acids 4 gives phosphorylated ketamides 5 and 12. Ring closure of ketamides 5 and 12 with triphenylphosphine and hexachloroethane in the presence of triethylamine leads to the formation of phosphorylated oxazoles 8 and 13. 相似文献