Search results for "Catalysis"

Current views on the mechanism of catalytic cracking

2000

Abstract The cracking mechanisms of hydrocarbons have been reviewed and the kinetic and thermodynamic implications of the different steps, i.e. initiation, chain propagation, and termination, have been discussed. Although the cracking mechanism of olefins and alkylaromatics is well established, the initiation step for the cracking of paraffins is still under debate. The role of Bronsted-type active sites and also the possible influence of extra-framework Al species in the case of zeolite catalysts, especially when commercial feeds and industrial conditions are employed, are presented. The product distribution is determined by the number of propagation events occurring per initiation step, a…

Snapshots of the Formation of Inorganic MoS2 Onion-Type Fullerenes: A “Shrinking Giant Bubble” Pathway

2010

Comparative computational analysis of different active site conformations and substrates in a chalcone isomerase catalyzed reaction.

2006

Chalcone isomerase catalyzes the transformation of chalcones to flavanones. We present a computational study of the rate-limiting chemical step, an intramolecular Michael addition of a 2'-oxyanion to the alpha,beta-double bound. By using quantum mechanical/molecular mechanical hybrid methods we traced the free-energy profiles associated with the reaction of two different substrates (chalcone and 6'-deoxychalcone) in two different conformations of the active site that are described in the different crystallographic structures available. We have obtained significant differences (about 4 kcal/mol) in the free-energy barriers calculated for the two active sites. According to our results, the ac…

Enzymatic effects on reactant and transition states. The case of chalcone isomerase.

2007

Chalcone isomerase catalyzes the transformation of chalcone to naringerin as a part of flavonoid biosynthetic pathways. The global reaction takes place through a conformational change of the substrate followed by chemical reaction, being thus an excellent example to analyze current theories about enzyme catalysis. We here present a detailed theoretical study of the enzymatic action on the conformational pre-equilibria and on the chemical steps for two different substrates of this enzyme. Free-energy profiles are obtained in terms of potentials of mean force using hybrid quantum mechanics/molecular mechanics potentials. The role of the enzyme becomes clear when compared to the counterpart eq…

A Novel Strategy to Study Electrostatic Effects in Chemical Reactions: Differences between the Role of Solvent and the Active Site of Chalcone Isomer…

2015

The electrostatic behavior of active site residues in enzyme catalysis is quite different from that of water molecules in solution. To highlight the electrostatic differences between both environments, we propose a QM/MM strategy to study the role of the environment in chemical reactions. The novelty of the present communication is that free energy surfaces are generated by means of two distinguished reaction coordinates: a solute coordinate and the electrostatic potential created by the environment. This is applied to analyze the origin of catalysis in the transformation of a chalcone into a flavanone, a Michael addition that requires the desolvation of the nucleophile.

Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2-Phenylation of 1-Methylindole

2012

New dinuclear palladium(III) compounds of general formula Pd2[(C6H4)PPh2]2[O–O]2Cl2, O–O being chelating phenolates C6H4OC(O)R (R = CH3, 3a; R = C2H5, 3b; R = OPh, 3c) or acetylacetonates RC(O)CHC(O)R (R = CH3, 4a; R = CF3, 4b; R = C(CH3)3, 4c), have been obtained by oxidation with PhICl2 of the corresponding palladium(II) compounds. The stability of the new compounds has been studied by 31P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability of the complexes have also been performed. In agreement with these calculations, only compound Pd2[(C6H4)PPh2]2[(CF3C(O)CHC(O)CF3]2Cl2, 6b, showed the highest thermal stability. 6b was characterized by X-ray diffraction methods, prese…

Chemical and catalytical properties of thermal polymers of amino acids (proteinoids)

1974

The significance of thermal polyamino acids (proteinoids) as abiotic predecessors of proteins is reviewed on the basis of new experimental results. Most proteinoids yield only 50% to 80% amino acid upon acid hydrolysis. They contain 40% to 60% less peptide links than typical proteins, whereas their average nitrogen content is like that of proteins. The arrangement of amino acid residues is nonrandom. The degree of nonrandomness is difficult to determine because unusual crosslinks disturb most of the sequencing methods typically applied in protein chemistry. The products obtained in a polymerization experiment are heterogeneous. They can be separated into a limited number of related fraction…

Nature-Inspired Effects of Naturally Occurring Trace Element-Doped Hydroxyapatite Combined with Surface Interactions of Mineral-Apatite Single Crysta…

2022

Innovative engineering design for biologically active hydroxyapatites requires enhancing both mechanical and physical properties, along with biocompatibility, by doping with appropriate chemical elements. Herein, the purpose of this investigation was to evaluate and elucidate the model of naturally occurring hydroxyapatite and the effects of doped trace elements on the function of normal human fibroblasts, representing the main cells of connective tissues. The substrates applied (geological apatites with hexagonal prismatic crystal habit originated from Slyudyanka, Lake Baikal, Russia (GAp) and from Imilchil, The Atlas Mountains, Morocco (YAp)) were prepared from mineral natural apatite wit…

On the role of interfacial hydrogen bonds in "on-water" catalysis.

2014

Numerous experiments have demonstrated that many classes of organic reactions exhibit increased reaction rates when performed in heterogeneous water emulsions. Despite enormous practical importance of the observed "on-water" catalytic effect and several mechanistic studies, its microscopic origins remains unclear. In this work, the second generation Car-Parrinello molecular dynamics method is extended to self-consistent charge density-functional based tight-binding in order to study "on-water" catalysis of the Diels-Alder reaction between dimethyl azodicarboxylate and quadricyclane. We find that the stabilization of the transition state by dangling hydrogen bonds exposed at the aqueous inte…

Chromogenic and fluorogenic reagents for chemical warfare nerve agents' detection

2008

The ease of production, the extreme toxicity of organophosphorus-containing nerve agents, and their facile use in terrorism attacks underscores the need to develop accurate systems to detect these chemicals. Among different technologies we review here recent advances in the design of chromo-fluorogenic methods for the specific detection of nerve agents. Optical sensing (especially colorimetric detection) requires usually low-cost and widely used instrumentation and offers the possibility of so-called “naked eye detection”. Recent reported examples suggest that the application of chromo-fluorogenic supramolecular concepts for the chromogenic or fluorogenic sensing of nerve agents might be an…