Página 9 dos resultados de 8928 itens digitais encontrados em 0.011 segundos

Decylacetate synthesis by enzyme catalysis in SC-CO2

Oliveira, M.V.; Rebocho, Sílvia F.; Ribeiro, Adriano S.; Ferreira, Olga; Vidinha, Pedro; Barreiros, Susana; Macedo, Eugénia A.; Loureiro, José M.
Fonte: Instituto Politécnico de Bragança Publicador: Instituto Politécnico de Bragança
Tipo: Conferência ou Objeto de Conferência
Português
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26.52%
The main purpose of this work was the study of long chain esters production, using decyl acetate as model compound, by enzymatic catalysis in supercritical media, with the aim of developing a sustainable, clean and efficient process as an alternative to the traditional chemical processes. The combination of a sustainable and clean technology, as biocatalysis, with a green/natural solvent, as supercritical CO2, besides allowing the establishment of processes with less environmental costs, leads to products considered as natural, which results in a significant increase of their market value. Decyl acetate is a high added value product. It figures in the “Food and Drug Administration” list of authorized flavoring agents and additives, and it also finds applications in the fragrance industry because of its floral essence. Usually, these kind of esters are obtained by extraction from natural expensive oils and waxes or produced through the esterification of carboxylic acids by acid catalysis. In this work, it was synthesized by the transesterification reaction of vinyl acetate with decanol in a high-pressure experimental set-up, equipped with a variable volume batch reactor, operating isothermally at 35 ºC and 100 bar. Candida antarctica Lipase B (CALB)...

Extending the kinetic solution of the classic Michaelis-Menten model of enzyme action

BISPO, Jose Ailton Conceicao; BONAFE, Carlos Francisco Sampaio; SOUZA, Volnei Brito de; SILVA, Joao Batista de Almeida e; CARVALHO, Giovani Brandao Mafra de
Fonte: SPRINGER Publicador: SPRINGER
Tipo: Artigo de Revista Científica
Português
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The principal aim of studies of enzyme-mediated reactions has been to provide comparative and quantitative information on enzyme-catalyzed reactions under distinct conditions. The classic Michaelis-Menten model (Biochem Zeit 49:333, 1913) for enzyme kinetic has been widely used to determine important parameters involved in enzyme catalysis, particularly the Michaelis-Menten constant (K (M) ) and the maximum velocity of reaction (V (max) ). Subsequently, a detailed treatment of the mechanisms of enzyme catalysis was undertaken by Briggs-Haldane (Biochem J 19:338, 1925). These authors proposed the steady-state treatment, since its applicability was constrained to this condition. The present work describes an extending solution of the Michaelis-Menten model without the need for such a steady-state restriction. We provide the first analysis of all of the individual reaction constants calculated analytically. Using this approach, it is possible to accurately predict the results under new experimental conditions and to characterize and optimize industrial processes in the fields of chemical and food engineering, pharmaceuticals and biotechnology.; Fundacao de Amparo a Pesquisa do Estado da Bahia (FAPESB); Fundacao de Amparo a Pesquisa do Estado de Sao Paulo (FAPESP); Conselho Nacional de Desenvolvimento Cientifico e Tecnologico (CNPq); Coordenacao de Aperfeicoamento de Pessoal de Nivel Superior (CAPES)...

Synthesis of new fluorous modular chiral ligand derivatives from amino alcohols and application in enantioselective carbon-carbon bond-forming alkylation reactions

SEHNEM, Jasquer A.; MILANI, Priscila; NASCIMENTO, Vanessa; ANDRADE, Leandro H.; DORNELES, Luciano; BRAGA, Antonio L.
Fonte: PERGAMON-ELSEVIER SCIENCE LTD Publicador: PERGAMON-ELSEVIER SCIENCE LTD
Tipo: Artigo de Revista Científica
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N-Trifluoracyl beta-chalcogeno amides and N-perfluoracyl beta-thio amide ligands were prepared by a simple and efficient reaction sequence. These new ligands were evaluated in palladium-catalyzed alkylation of rac-(E)-1,3-diphenyl-2-propenyl acetate in the presence of dimethyl malonate and an enantioselectivity of up to 99% was obtained. After catalysis, the fluorous ligand can be easily recovered by liquid-liquid extraction and reused without loss in the activity. (C) 2010 Elsevier Ltd. All rights reserved.; CAPES; Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES); CNPq (INCT-Catalise); Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq); FAPESC; FAPESC; Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP); FAPESP

Evolutionarily Conserved Linkage between Enzyme Fold, Flexibility, and Catalysis

Ramanathan, Arvind; Agarwal, Pratul K.
Fonte: Public Library of Science Publicador: Public Library of Science
Tipo: Artigo de Revista Científica
Português
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Proteins are intrinsically flexible molecules. The role of internal motions in a protein's designated function is widely debated. The role of protein structure in enzyme catalysis is well established, and conservation of structural features provides vital clues to their role in function. Recently, it has been proposed that the protein function may involve multiple conformations: the observed deviations are not random thermodynamic fluctuations; rather, flexibility may be closely linked to protein function, including enzyme catalysis. We hypothesize that the argument of conservation of important structural features can also be extended to identification of protein flexibility in interconnection with enzyme function. Three classes of enzymes (prolyl-peptidyl isomerase, oxidoreductase, and nuclease) that catalyze diverse chemical reactions have been examined using detailed computational modeling. For each class, the identification and characterization of the internal protein motions coupled to the chemical step in enzyme mechanisms in multiple species show identical enzyme conformational fluctuations. In addition to the active-site residues, motions of protein surface loop regions (>10 Å away) are observed to be identical across species...

Substrate-assisted catalysis: molecular basis and biological significance.

Dall'Acqua, W.; Carter, P.
Fonte: Cold Spring Harbor Laboratory Press Publicador: Cold Spring Harbor Laboratory Press
Tipo: Artigo de Revista Científica
Publicado em /01/2000 Português
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Substrate-assisted catalysis (SAC) is the process by which a functional group in a substrate contributes to catalysis by an enzyme. SAC has been demonstrated for representatives of three major enzyme classes: serine proteases, GTPases, and type II restriction endonucleases, as well as lysozyme and hexose-1-phosphate uridylyltransferase. Moreover, structure-based predictions of SAC have been made for many additional enzymes. Examples of SAC include both naturally occurring enzymes such as type II restriction endonucleases as well as engineered enzymes including serine proteases. In the latter case, a functional group from a substrate can substitute for a catalytic residue replaced by site-directed mutagenesis. From a protein engineering perspective, SAC provides a strategy for drastically changing enzyme substrate specificity or even the reaction catalyzed. From a biological viewpoint, SAC contributes significantly to the activity of some enzymes and may represent a functional intermediate in the evolution of catalysis. This review focuses on advances in engineering enzyme specificity and activity by SAC, together with the biological significance of this phenomenon.

Transition State Charge Stabilization and Acid-Base Catalysis of mRNA Cleavage by the Endoribonuclease RelE

Dunican, Brian F.; Hiller, David A.; Strobel, Scott A.
Fonte: PubMed Publicador: PubMed
Tipo: Artigo de Revista Científica
Português
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The bacterial toxin RelE is a ribosome-dependent endoribonuclease. It is part of a type II toxin-antitoxin system that contributes to antibiotic resistance and biofilm formation. During amino acid starvation RelE cleaves mRNA in the ribosomal A-site, globally inhibiting protein translation. RelE is structurally similar to microbial RNases that employ general acid-base catalysis to facilitate RNA cleavage. The RelE active-site is atypical for acid-base catalysis, in that it is enriched for positively charged residues and lacks the prototypical histidine-glutamate catalytic pair, making the mechanism of mRNA cleavage unclear. In this study we use a single-turnover kinetic analysis to measure the effect of pH and phosphorothioate substitution on the rate constant for cleavage of mRNA by wild-type RelE and seven active-site mutants. Mutation and thio-effects indicate a major role for stabilization of increased negative change in the transition state by arginine 61. The wild-type RelE cleavage rate constant is pH-independent, but the reaction catalyzed by many of the mutants is strongly pH dependent, suggestive of general acid-base catalysis. pH-rate curves indicate that wild-type RelE operates with the pKa of at least one catalytic residue significantly downshifted by the local environment. Mutation of any single active-site residue is sufficient to disrupt this microenvironment and revert the shifted pKa back above neutrality. pH-rate curves are consistent with K54 functioning as a general base and R81 as a general acid. The capacity of RelE to effect a large pKa shift and facilitate a common catalytic mechanism by uncommon means furthers our understanding of other atypical enzymatic active sites.

Critical Endpoint and Inverse Magnetic Catalysis for Finite Temperature and Density Quark Matter in a Magnetic Background

Ruggieri, M.; Oliva, L.; Castorina, P.; Gatto, R.; Greco, V.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 04/02/2014 Português
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In this article we study chiral symmetry breaking for quark matter in a magnetic background, $\bm B$, at finite temperature and quark chemical potential, $\mu$, making use of the Ginzburg-Landau effective action formalism. As a microscopic model to compute the effective action we use the renormalized quark-meson model. Our main goal is to study the evolution of the critical endpoint, ${\cal CP}$, as a function of the magnetic field strength, and investigate on the realization of inverse magnetic catalysis at finite chemical potential. We find that the phase transition at zero chemical potential is always of the second order; for small and intermediate values of $\bm B$, ${\cal CP}$ moves towards small $\mu$, while for larger $\bm B$ it moves towards moderately larger values of $\mu$. Our results are in agreement with the inverse magnetic catalysis scenario at finite chemical potential and not too large values of the magnetic field, while at larger $\bm B$ direct magnetic catalysis sets in.; Comment: 6 pages, 2 figures

Magnetic catalysis (and inverse catalysis) at finite temperature in two-color lattice QCD

Ilgenfritz, E. -M.; Muller-Preussker, M.; Petersson, B.; Schreiber, A.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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Two-color lattice QCD with N_f=4 staggered fermion degrees of freedom (no rooting trick is applied) with equal electric charge q is studied in a homogeneous magnetic background field B and at non-zero temperature T. In order to circumvent renormalization as a function of the bare coupling we apply a fixed-scale approach. We study the influence of the magnetic field on the critical temperature. At rather small pseudo-scalar meson mass ($m_{\pi} \approx 175 \mathrm{MeV} \approx T_c(B=0)$) we confirm a monotonic rise of the quark condensate $<\bar{\psi} \psi>$ with increasing magnetic field strength, i.e. magnetic catalysis, as long as one is staying within the confinement or deconfinement phase. In the transition region we find indications for a non-monotonic behavior of $T_c(B)$ at low magnetic field strength ($qB<0.8 \mathrm{GeV}^2$) and a clear rise at stronger magnetic field. The conjectured existence of a minimum value $T_c(B^{*}) < T_c(B=0)$ would leave a temperature window for a decrease of $<\bar{\psi} \psi>$ with rising $B$ (inverse magnetic catalysis) also in the present model.; Comment: 10 pages, 7 figures; revised version accepted by Phys. Rev. D

Inverse magnetic catalysis in field theory and gauge-gravity duality

Preis, Florian; Rebhan, Anton; Schmitt, Andreas
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 02/08/2012 Português
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We investigate the surface of the chiral phase transition in the three-dimensional parameter space of temperature, baryon chemical potential and magnetic field in two different approaches, the field-theoretical Nambu-Jona-Lasinio (NJL) model and the holographic Sakai-Sugimoto model. The latter is a top-down approach to a gravity dual of QCD with an asymptotically large number of colors and becomes, in a certain limit, dual to an NJL-like model. Our main observation is that, at nonzero chemical potential, a magnetic field can restore chiral symmetry, in apparent contrast to the phenomenon of magnetic catalysis. This "inverse magnetic catalysis" occurs in the Sakai-Sugimoto model and, for sufficiently large coupling, in the NJL model and is related to the physics of the lowest Landau level. While in most parts our discussion is a pedagogical review of previously published results, we include new analytical results for the NJL approach and a thorough comparison of inverse magnetic catalysis in the two approaches.; Comment: 37 pages, 11 figures, to appear in Lect. Notes Phys. "Strongly interacting matter in magnetic fields" (Springer), edited by D. Kharzeev, K. Landsteiner, A. Schmitt, H.-U. Yee

Inverse magnetic catalysis induced by sphalerons

Chao, Jingyi; Chu, Pengcheng; Huang, Mei
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 06/05/2013 Português
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The recently discovered inverse magnetic catalysis around the critical temperature indicates that some important information is missing in our current understanding of conventional chiral dynamics of QCD, which is enhanced by the magnetic field. In this work, we provide a mechanism to explain that the inverse magnetic catalysis around the critical temperature is induced by sphalerons. At high temperatures, sphaleron transitions between distinct classical vacua cause an asymmetry between the number of right- and left-handed quarks due to the axial anomaly of QCD. In the presence of a strong magnetic field, the chiral imbalance is enhanced and destroys the right- and left-handed pairings, which naturally induces a decreasing critical temperature of the chiral phase transition for increasing magnetic field. The inverse magnetic catalysis at finite baryon density, and the critical end point in the presence of a strong magnetic field is also explored in this work.; Comment: 5 pages, 3 figures

Geometrically induced magnetic catalysis and critical dimensions

Flachi, Antonino; Fukushima, Kenji; Vitagliano, Vincenzo
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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We discuss the combined effect of magnetic fields and geometry in interacting fermionic systems. At leading order in the heat-kernel expansion, the infrared singularity (that in flat space leads to the magnetic catalysis) is regulated by the chiral gap effect, and the catalysis is deactivated by the effect of the scalar curvature. We discover that an infrared singularity is found in higher-order terms that mix the magnetic field with curvature, and these lead to a novel form of geometrically induced magnetic catalysis. The dynamical mass squared is then modified not only due to the chiral gap effect by an amount proportional to the curvature, but also by a magnetic shift $\propto (4-D)eB$, where $D$ represents the number of space-time dimensions. We argue that $D=4$ is a critical dimension across which the behavior of the magnetic shift changes qualitatively.; Comment: 5 pages; minor changes

On the magnetic catalysis and inverse catalysis of phase transitions in the linear sigma model

Ayala, Alejandro; Loewe, M.; Villavicencio, C.; Zamora, R.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 04/09/2014 Português
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We consider the evolution of critical temperature both for the formation of a pion condensate as well as for the chiral transition, from the perspective of the linear sigma model, in the background of a magnetic field. We developed the discussion for the pion condensate in one loop approximation for the effective potential getting magnetic catalysis for high values of B, i.e. a raising of the critical temperature with the magnetic field. For the analysis of the chiral restoration, we go beyond this approximation, by taking one loop thermo-magnetic corrections to the couplings as well as plasma screening effects for the boson's masses, expressed through the resumation of ring diagrams. Here we found the opposite behavior, i.e. inverse magnetica catalysis, i.e. a decreasing of the chiral critical temperature as function of the intensity of the magnetic field, which seems to be in agreement with recent results form the lattice community.; Comment: Contribution to the QCD14 conference, Montpellier, France. To appear in the proceedings

Predicting template-based catalysis rates in a simple catalytic reaction model

Hordijk, Wim; Steel, Mike
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Português
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26.64%
We show that in a particular model of catalytic reaction systems, known as the binary polymer model, there is a mathematical invariance between two versions of the model: (1) random catalysis and (2) template-based catalysis. In particular, we derive an analytical calculation that allows us to accurately predict the (observed) required level of catalysis in one version of the model from that in the other version, for a given probability of having self-sustaining autocatalytic sets exist in instances of both model versions. This provides a tractable connection between two models that have been investigated in theoretical origin-of-life studies.; Comment: 16 pages, 6 figures, 2 tables

Atomic Gold and Palladium Anion-Catalysis of Water to Peroxide: Fundamental Mechanism

Tesfamichael, Aron; Suggs, Kelvin; Felfli, Zineb; Wang, Xiao-Qian; Msezane, Alfred Z.
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 10/01/2012 Português
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We have performed dispersion-corrected density-functional transition state calculations on atomic Au- and Pd- catalysis of water conversion to peroxide. The Au- ion is found to be an excellent catalyst; however, atomic Pd- has a higher catalytic effect on the formation of peroxide. The Au^{-}(H2O)2 and Pd^{-}(H2O)2 anion molecular complexes formation in the transition state is identified as the fundamental mechanism for breaking the hydrogen bonding strength during the water catalysis. Our theoretical results provide crucial insight into the mechanism of the atomic Au- and Pd- catalysis in good agreement with recent experimental observations.; Comment: 8 Pages,3 Figures

Without Spectroscopy at the Beginning, Catalysis Research Proceeded in the Wrong Direction for More Than 100 Years

Gardner-Chavis, Ralph A.; Reye, John T.; Selover Jr, Theodore B.; Zhang, Huixiong
Fonte: Universidade Cornell Publicador: Universidade Cornell
Tipo: Artigo de Revista Científica
Publicado em 08/01/2008 Português
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A study by infrared spectroscopy of the physisorbed region of catalysis demonstrated that the intermediates of catalysis exist on the surface as a two dimensional gas. Data in the Atomic Energy Level tables show that of the thousands of positive ions tabulated only approximately one hundred have the low-lying excited states that produce surface electric fields with a fractional charge. The specific catalyst for a reaction has the electric field with the fractional charge which when imparted to the two reactants changes the frequency of the fields at the sites of reaction on each so that they are harmonically equal, that is their ratio is a power of two. When the two reactants meet in the electric field of the catalyst resonance occurs. It is during resonance that electrons are shared, paired and exchanged and bonds are broken and made. This analysis of catalysis explains the most extraordinary observation that a catalyst is Not consumed when used because the catalyst is the electric fields. These discoveries are applied to explain such diverse reactions as the oxidation and chlorination of carbon monoxide and the destruction of nitric oxide in automobile exhaust. The use of electric fields to produce reactions may have application not only in chemistry but in biology and mechanics as well.; Comment: 23 pages

Force-clamp spectroscopy detects residue co-evolution in enzyme catalysis.

Pérez-Jímenez, Raul; Kosuri, Pallav; Rodríguez-Larrea, David; Gavira Gallardo, J. A.; Fernández Soler, Julio M.; Sánchez-Ruiz, José M.; Wiita, Arun P.
Fonte: American Society for Biochemistry and Molecular Biology Publicador: American Society for Biochemistry and Molecular Biology
Tipo: Artículo Formato: 10752 bytes; application/octet-stream
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22 pages, 6 figures, 1 table.; Understanding how the catalytic mechanisms of enzymes are optimized through evolution remains a major challenge in molecular biology. The concept of co-evolution implicates that compensatory mutations occur to preserve the structure and function of proteins. We have combined statistical analysis of protein sequences with the sensitivity of single molecule force-clamp spectroscopy to probe how catalysis is affected by structurally distant correlated mutations in Escherichia coli thioredoxin. Our findings show that evolutionary anti-correlated mutations have an inhibitory effect on enzyme catalysis, whereas positively correlated mutations rescue the catalytic activity. We interpret these results in terms of an evolutionary tuning of both the enzyme-substrate binding process and the chemistry of the active site. Our results constitute a direct observation of distant residue co-evolution in enzyme catalysis.; We thank Dr. Sergi Garcia-Manyes and Dr. Robert Szoszkiewicz for careful reading of the manuscript and all the Fernandez laboratory members for helpful discussions.; Peer reviewed

Spiers Memorial Lecture: Interplay of theory and computation in chemistry—examples from on-water organic catalysis, enzyme catalysis, and single-molecule fluctuations

Marcus, R. A.
Fonte: Royal Society of Chemistry Publicador: Royal Society of Chemistry
Tipo: Article; PeerReviewed Formato: application/pdf
Publicado em //2010 Português
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In this lecture, several examples are considered that illustrate the interplay of experiment, theory, and computations. The examples include on-water catalysis of organic reactions, enzymatic catalysis, single molecule fluctuations, and some much earlier work on electron transfer and atom or group transfer reactions. Computations have made a major impact on our understanding and in the comparisons with experiments. There are also major advantages of analytical theories that may capture in a single equation an entire field and relate experiments of one type to those of another. Such a theory has a generic quality. These topics are explored in the present lecture.

Catalysis by mutants of methylmalonyl-CoA mutase: a theoretical rationalization for a change in the rate-determining step

Wetmore, Stacey; Trebeck, Katherine; Radom, Leo
Fonte: Wiley-VCH Verlag GMBH Publicador: Wiley-VCH Verlag GMBH
Tipo: Artigo de Revista Científica
Português
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26.64%

Clarification of the role of key active site residues of glutathione transferase Zeta/maleylacetoacetate isomerase by a new spectrophotometric technique

Board, Philip; Taylor, Matthew; Coggan, Marjorie; Parker, Michael William; Lantum, Hoffman; Anders, Michael
Fonte: Portland Press Publicador: Portland Press
Tipo: Artigo de Revista Científica
Português
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hGSTZ1-1 (human glutathione transferase Zeta 1-1) catalyses a range of glutathione-dependent reactions and plays an important role in the metabolism of tyrosine via its maleylacetoacetate isomerase activity. The crystal structure and sequence alignment of hGSTZ1 with other GSTs (glutathione transferases) focused attention on three highly conserved residues (Ser-14, Ser-15, Cys-16) as candidates for an important role in catalysis. Progress in the investigation of these residues has been limited by the absence of a convenient assay for kinetic analysis. In this study we have developed a new spectrophotometric assay with a novel substrate [(± )-2-bromo-3-(4-nitrophenyl)propionic acid]. The assay has been used to rapidly assess the potential catalytic role of several residues in the active site. Despite its less favourable orientation in the crystal structure, Ser-14 was the only residue found to be essential for catalysis. It is proposed that a conformational change may favourably reposition the hydroxyl of Ser-14 during the catalytic cycle. The Cys16 → Ala (Cys-16 mutated to Ala) mutation caused a dramatic increase in the Km for glutathione, indicating that Cys-16 plays an important role in the binding and orientation of glutathione in the active site. Previous structural studies implicated Arg-175 in the orientation of α-halo acid substrates in the active site of hGSTZ1-1. Mutation of Arg-175 to Lys or Ala resulted in a significant lowering of the kcat in the Ala-175 variant. This result is consistent with the proposal that the charged side chain of Arg-175 forms a salt bridge with the carboxylate of the α-halo acid substrates.

Uma Visão Geral dos Diferentes Tipos de Catálise em Síntese Orgânica; An Overview of the Different Types of Catalysts in Organic Synthesis

Flaviana R. F. Dias; Universidade Federal Fluminense; Vitor F. Ferreira; Universidade Federal Fluminense; Anna Claudia Cunha; Universidade Federal Fluminense
Fonte: Revista Virtual de Química Publicador: Revista Virtual de Química
Tipo: ; Formato: binary/octet-stream
Publicado em 25/12/2012 Português
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Desde meados do século XX, por razões econômicas e ambientais, diversos pesquisadores e algumas indústrias investem no desenvolvimento de tecnologias sintéticas que sejam ambientalmente favoráveis, gerando menos resíduos tóxicos e com economia de energia. Além disso, os processos catalíticos e biocatalíticos associados ao uso de biomassas renováveis são de fundamental importância para a produção de matérias-primas quirais e aquirais com altos rendimentos e elevadas seletividades. Esse conjunto de paradigmas é um dos grandes desafios da atualidade para os pesquisadores nos diversos ramos da síntese orgânica. Este trabalho descreve a importância da catálise e da biocatálise na síntese orgânica de moléculas complexas, destacando-se os setores de química fina e fármacos.  Além disso, a utilização crescente dos diversos processos catalíticos em diferentes setores industriais, tais como petroquímico, de papel e celulose, plásticos, e muitos outros também são assuntos de interesse desse trabalho de revisão. DOI: 10.5935/1984-6835.20120060; Since the mid-twentieth century, by economic and environmental factors, many researchers and some industries are investing in the development of syntheses and technologies that are environmentally friendly...