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Substrate Structure (substrate + structure)
Selected AbstractsInfluence of Substrate Structure on PGA-Catalyzed Acylations.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1 2005Evaluation of Different Approaches for the Enzymatic Synthesis of Cefonicid Abstract The influence of the substrate structure on the catalytic properties of penicillin G acylase (PGA) from Escherichia coli in kinetically controlled acylations has been studied. In particular, the affinity of different ,-lactam nuclei towards the active site has been evaluated considering the ratio between the rate of synthesis (vs) and the rate of hydrolysis of the acylating ester (vh1). 7-Aminocephalosporanic acid (7-ACA) and 7-amino-3-(1-sulfomethyl-1,2,3,4-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (7-SACA) showed a good affinity for the active centre of PGA. The enzymatic acylation of these nuclei with R -methyl mandelate has been studied in order to evaluate different approaches for the enzymatic synthesis of cefonicid. The best results have been obtained in the acylation of 7-SACA. Cefonicid (8) was recovered from the reaction mixture as the disodium salt in 65% yield and about 95% of purity. Furthermore, through acylation of 7-ACA, a "one-pot" chemo-enzymatic synthesis was carried out starting from cephalosporin C using three enzymes in sequence: D -amino acid oxidase (DAO), glutaryl acylase (GA) and PGA. Cefonicid disodium salt was obtained in three steps, avoiding any intermediate purification, in 35% overall yield and about 94% purity. This approach presents several advantages compared with the classical chemical processes. [source] Alternation of Chemoselective Control in Stille,Heck and Heck,Stille Reaction SequencesADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2009Kye-Simeon Masters Abstract Sequential and one-pot Stille,Heck and Heck,Stille reaction processes have been invoked to give divergent access to polycyclic ring systems. Both reaction conditions and substrate structure are important in determining the nature of the reaction products formed. The Heck,Stille reactions have involved a reversal of the usual Heck regioselectivity and both cine- and ipso- substitutions have been observed in the Stille reaction. [source] Influence of Substrate Structure on PGA-Catalyzed Acylations.ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1 2005Evaluation of Different Approaches for the Enzymatic Synthesis of Cefonicid Abstract The influence of the substrate structure on the catalytic properties of penicillin G acylase (PGA) from Escherichia coli in kinetically controlled acylations has been studied. In particular, the affinity of different ,-lactam nuclei towards the active site has been evaluated considering the ratio between the rate of synthesis (vs) and the rate of hydrolysis of the acylating ester (vh1). 7-Aminocephalosporanic acid (7-ACA) and 7-amino-3-(1-sulfomethyl-1,2,3,4-tetrazol-5-yl)thiomethyl-3-cephem-4-carboxylic acid (7-SACA) showed a good affinity for the active centre of PGA. The enzymatic acylation of these nuclei with R -methyl mandelate has been studied in order to evaluate different approaches for the enzymatic synthesis of cefonicid. The best results have been obtained in the acylation of 7-SACA. Cefonicid (8) was recovered from the reaction mixture as the disodium salt in 65% yield and about 95% of purity. Furthermore, through acylation of 7-ACA, a "one-pot" chemo-enzymatic synthesis was carried out starting from cephalosporin C using three enzymes in sequence: D -amino acid oxidase (DAO), glutaryl acylase (GA) and PGA. Cefonicid disodium salt was obtained in three steps, avoiding any intermediate purification, in 35% overall yield and about 94% purity. This approach presents several advantages compared with the classical chemical processes. [source] Effect of the substrate temperature on the properties of the RF sputtered TiO2 thin filmsPHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2010I. Ben Mbarek Abstract Titanium dioxide (TiO2) thin films were deposited by RF magnetron sputtering on glass and silicon substrates at different substrate temperatures (20, 100, 200 and 300 °C). The structural and morphological characteristics of the films were investigated by X ray Diffractometry (XRD) and Atomic Force Microscopy (AFM) while the optical properties of the films were studied by optical spectroscopy. It was shown that at room temperature, TiO2 films grown on glass were amorphous following the substrate structure. At higher temperatures, XRD detected only a nanocrystalline rutile TiO2 structure. This indicated that the transition temperature toward the most stable TiO2 phase was obtained from 100 °C and the crystallinity was enhanced at higher growth temperature. For TiO2 films grown on Si substrates, only a nanocrystalline anatase TiO2 structure was obtained at room temperature. At higher temperatures, we noticed the appearance of other secondary phases related to rutile, anatase and brookite structures. From AFM images, we noticed that at room temperature, the films were porous. With increasing the temperature, the structure of the films became crystallized showing a columnar structure. Film growth and structural properties were discussed in terms of the Thornton model. From optical analysis, the films were transparent with an indirect band gap and a refraction index which reached 3.09 eV and 2.7, respectively. The reflectance and transmittance spectra showed, not only that there was a little translation from UVB to UVA and near-visible range, but also a decrease of reflection with a temperature increase indicating that the films could be used as anti-reflection coatings. (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Stereoselectivity of Pseudomonas cepacia lipase toward secondary alcohols: A quantitative modelPROTEIN SCIENCE, Issue 6 2000Tanja Schulz Abstract The lipase from Pseudomonas cepacia represents a widely applied catalyst for highly enantioselective resolution of chiral secondary alcohols. While its stereopreference is determined predominantly by the substrate structure, stereoselectivity depends on atomic details of interactions between substrate and lipase. Thirty secondary alcohols with published E values using P. cepacia lipase in hydrolysis or esterification reactions were selected, and models of their octanoic acid esters were docked to the open conformation of P. cepacia lipase. The two enantiomers of 27 substrates bound preferentially in either of two binding modes: the fast-reacting enantiomer in a productive mode and the slow-reacting enantiomer in a nonproductive mode. Nonproductive mode of fast-reacting enantiomers was prohibited by repulsive interactions. For the slow-reacting enantiomers in the productive binding mode, the substrate pushes the active site histidine away from its proper orientation, and the distance d(HN, , Oalc) between the histidine side chain and the alcohol oxygen increases. d(HN, , Oalc) was correlated to experimentally observed enantioselectivity: in substrates for which P. cepacia lipase has high enantioselectivity (E > 100), d(HN, , Oalc) is>2.2 Å for slow-reacting enantiomers, thus preventing efficient catalysis of this enantiomer. In substrates of low enantioselectivity (E < 20), the distance d(HN, , Oalc) is less than 2.0 Å, and slow- and fast-reacting enantiomers are catalyzed at similar rates. For substrates of medium enantioselectivity (20 < E< 100), d(HN, , Oalc) is around 2.1 Å. This simple model can be applied to predict enantioselectivity of P. cepacia lipase toward a broad range of secondary alcohols. [source] Synthesis of a Nitro Analogue of Plakoric Acid,CHINESE JOURNAL OF CHEMISTRY, Issue 9 2006Qi Zhang Abstract Synthesis of a nitro analogue of plakoric acid is presented. The peroxy bond was incorporated into the substrate structure through a boron trifluoride etherate catalyzed methoxy-hydroperoxy group partial exchange reaction in diethyl ether with urea-hydrogen peroxide complex (UHP, a commercially available solid reagent) as the source of the hydrogen peroxide. Under the given conditions, only one of the two methoxyl groups underwent the MeO,OOH exchange and the resulting hydroperoxy hemiketal proceeded directly to the end product through an intramolecular Michael addition of the hydroperoxyl group to the nitro group activated carbon-carbon double bond. [source] Facile Synthesis of Enantiopure 4-Substituted 2-Hydroxy-4- butyrolactones using a Robust Fusarium LactonaseADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 17 2009Bing Chen Abstract A facile chemo-enzymatic process has been developed for producing stereoisomers of 4-substituted 2-hydroxy-4-butyrolactones with good to excellent enantioselectivity. This process involves an easy separation of the diastereoisomers by column chromatography and efficient enzymatic resolution by whole cells of Escherichia coli JM109 expressing Fusarium proliferatum lactonase gene. This biocatalyst shows strong tolerance towards different substrate structures and at least three out four possible isomers could be obtained in excellent enantiomeric purity. Different substrate concentrations (10,mM,200,mM) were examined, giving a substrate to catalyst ratio of up to 26:1. This general and efficient enzymatic process provides access to stereoisomers of 4-substituted 2-hydroxy-4-butyrolactones readily and cost-effectively. The stereochemical assignments were conducted systematically based on NMR, X-ray diffraction and circular dichroism, leading to further understanding of the enzyme's stereoselectivity. [source] | ||||||||||