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Ionic Form (ionic + form)
Selected AbstractsPyridoxal 5,-phoshate Schiff base in Citrobacter freundii tyrosinephenol-lyaseFEBS JOURNAL, Issue 6 2000Ionic, tautomeric equilibria Spectral properties of the internal Schiff base in tyrosine phenol-lyase have been investigated in the presence of an activating cation K+ and a cation-inhibitor Na+. The holoenzyme absorption spectra in the pH range 6.5,8.7 were recorded in the presence of K+. No apparent pKa value of the coenzyme chromophore was found in this pH range, indicating that the internal Schiff base does not change its ionic form on going from pH 6.5 to 8.7. To determine the ionic state and tautomeric composition of the Schiff base in tyrosine phenol-lyase, the absorption and circular dichroism spectra were analyzed using lognormal distribution curves. The predominant form of the internal Schiff base is that with protonated pyridinium and aldimine nitrogen atoms and deprotonated 3,-hydroxy group, i.e. the ketoenamine. This form is in prototropic equilibrium with its enolimine tautomer. The internal aldimine ionic form is changed upon replacement of K+ with Na+. This replacement leads to a significant decrease in the pKa value of pyridinium nitrogen of the pyridoxal- P. [source] A mechanistic model for the water-gas shift reaction over noble metal substituted ceriaAICHE JOURNAL, Issue 5 2010Parag A. Deshpande Abstract The water-gas shift (WGS) reaction was carried out in the presence of Pd and Pt substituted nanocrystalline ceria catalysts synthesized by solution combustion technique. The catalysts were characterized by powder XRD and XPS. The noble metals were found to be present in ionic form substituted for the cerium atoms. The catalysts showed high activity for the WGS reaction with high conversions below 250°C. The products of reaction were only carbon dioxide and hydrogen, and no hydrocarbons were observed even in trace quantities. The reactions were carried out with different amounts of noble metal ion substitution and 2% Pt substituted ceria was found to be the best catalyst. The various possible mechanisms for the reaction were proposed and tested for their consistency with experimental data. The dual site mechanism best described the kinetics of the reaction and the corresponding rate parameters were obtained. © 2009 American Institute of Chemical Engineers AIChE J, 2010 [source] Mononuclear Co(III)-complex promoted phosphate diester hydrolysis: dependence of reactivity on the leaving group,JOURNAL OF PHYSICAL ORGANIC CHEMISTRY, Issue 6-7 2004Michela Padovani Abstract The TrpnCo(III)(OH)(OH2)-promoted hydrolysis of a range of methyl aryl phosphate diesters was investigated at 37°C and I,=,0.1,M (NaClO4). The pH,rate profile confirms that the aqua-hydroxy form of the complex is the only kinetically significant ionic form. At pH 6.9, all the reactions are first order in both diester and Co(III) complex. Plotting the second-order rate constant for Co(III) complex-promoted hydrolysis against the pKa of the leaving aryloxy group revealed a bent LFER indicating a change in rate-limiting step. This is discussed in terms of either a change from rate-limiting hydrolysis to rate-limiting binding or the presence of a phosphorane intermediate. Copyright © 2004 John Wiley & Sons, Ltd. [source] Non-destructive Raman study of the glazing technique in lustre potteries and faience (9,14th centuries): silver ions, nanoclusters, microstructure and processingJOURNAL OF RAMAN SPECTROSCOPY, Issue 3 2004Philippe Colomban Abstract The oldest known nanotechnology dates back to the fabrication of the first lustre potteries. A lustre is a thin film formed just below the surface of medieval Islamic glazed potteries which contains silver and/or copper in the metallic and ionic form. Raman studies of the lustre films of different ceramics excavated from Fustāt (near Cairo, Egypt, 11,12th century) or from the Silk Road (Termez, 13,14th centuries) showed that they associate many layers of different compositions (with or without cassiterite). Energy-dispersive spectroscopic analysis shows that all studied glazes are Ca- (and K)-rich, nearly free of Al silicates, with some addition of lead. Comparison is made with a copy of three-colour Tang ceramics made in Bassorah or Baghdad, in the 9th century, which is among the first known ,faiences', i.e. ceramics enamelled with an Sn-containing glaze. Surprisingly, Sn is not present in the form of a cassiterite (SnO2) precipitate but as a Ca,K-rich salt. Composition analysis and Raman spectra show that all glazes have been processed with similar technology. The distribution of elemental Ag and Cu is very heterogeneous in the lustre decor. The main Raman signature (50,100 cm,1 peaks) of the lustre film is assigned to Ag+ ions. The additional low-wavenumber features could be due to the Ag0 [or (Agn)m+] nanocluster modes. It is clear that the lustre colour arises from the combination of iridescence (diffraction) and absorption/diffusion. Raman criteria are proposed for a sample classification as a function of processing (cassiterite content, processing temperature). The glazing technique is discussed on the basis of experimental evidence and ancient potters' reports. Exothermic burning of acetate residus is proposed as the key step for the preparation of polychrome lustre. Copyright © 2004 John Wiley & Sons, Ltd. [source] An efficient one-pot synthesis of 6-aryl-5-cyano-2-thiopyrimidinone derivatives and their piperidinium ionic forms, x-ray crystal structuresJOURNAL OF HETEROCYCLIC CHEMISTRY, Issue 4 2006Saeed Balalaie Three-component condensation of benzaldehyde derivatives, alkyl cyanoacetates and thiourea in the presence of piperidine in reflux condition provides a direct route to piperidinium 6-aryl-5-cyano 2-thiopyrimidonate salts in good yields. These reactions were also carried out under microwave irradiation. The yields of products under the microwave condition were better as compared to the reflux media. The acidification of these ionic forms resulted in the formation of 6-aryl-5-cyano-2-thiopyrimidone derivatives. The X-ray structures of the ionic forms (4, 5, and 7) show that there are anionic thiopyrimidinone skeletons hydrogen bridged with piperidinium cations. [source] Influence of Ionization State on the Activation of Temocapril by hCES1: A Molecular-Dynamics StudyCHEMISTRY & BIODIVERSITY, Issue 11 2009Giulio Vistoli Abstract Temocapril is a prodrug whose hydrolysis by carboxylesterase 1 (CES1) yields the active ACE inhibitor temocaprilat. This molecular-dynamics (MD) study uses a resolved structure of the human CES1 (hCES1) to investigate some mechanistic details of temocapril hydrolysis. The ionization constants of temocapril (pK1 and pK3) and temocaprilat (pK1, pK2, and pK3) were determined experimentally and computationally using commercial algorithms. The constants so obtained were in good agreement and revealed that temocapril exists mainly in three ionic forms (a cation, a zwitterion, and an anion), whereas temocaprilat exists in four major ionic forms (a cation, a zwitterion, an anion, and a dianion). All these ionic forms were used as ligands in 5-ns MS simulations. While the cationic and zwitterionic forms of temocapril were involved in an ion-pair bond with Glu255 suggestive of an inhibitor behavior, the anionic form remained in a productive interaction with the catalytic center. As for temocaprilat, its cation appeared trapped by Glu255, while its zwitterion and anion made a slow departure from the catalytic site and a partial egress from the protein. Only its dianion was effectively removed from the catalytic site and attracted to the protein surface by Lys residues. A detailed mechanism of product egress emerges from the simulations. [source] | ||||||||||