Search results for " Catalysis"
showing 10 items of 585 documents
Do dynamic effects play a significant role in enzymatic catalysis? A theoretical analysis of formate dehydrogenase.
2010
A theoretical study of the protein dynamic effects on the hydride transfer between the formate anion and nicotinamide adenine dinucleotide (NAD + ), catalyzed by formate dehydrogenase (FDH), is presented in this paper. The analysis of free downhill molecular dynamic trajectories, performed in the enzyme and compared with the reaction in aqueous solution, has allowed the study of the dynamic coupling between the reacting fragments and the protein or the solvent water molecules, as well as an estimation of the dynamic effect contribution to the catalytic effect from calculation of the transmission coefficient in the enzyme and in solution. The obtained transmission coefficients for the enzyme…
Isomerization and rearrangement of (E)- and (Z)-phenylhydrazones of 3-benzoyl-5-phenyl-1,2,4-oxadiazole: evidence for a “new” type of acid-catalysis …
2008
A kinetic investigation in methanol of the title reaction has evidenced the occurrence of two processes: the 1-E1-Z isomerization and the rearrangement of the (Z)-isomer into the relevant 4-benzoylamino-2,5-diphenyl-1,2,3-triazole (1-Z T). The latter reaction is in line with the ability of the (Z)-phenylhydrazones of 3-benzoyl-1,2,4-oxadiazoles to undergo the so called mononuclear rearrangement of heterocycles (MRH). The occurrence of both the examined reactions is dependent on a Lewis-acid-catalysis. The obtained results have shown the possibility of a ‘new’ type of acid-catalysis (bifunctional catalysis by Lewis salts) in the MRH. This catalysis operates through a completely different me…
A deep insight into the mechanism of the acid-catalyzed rearrangement of the Z-phenylhydrazone of 5-amino-3-benzoyl-1,2,4-oxadiazole in a non-polar s…
2010
The conversion of the Z-phenylhydrazone of 5-amino-3-benzoyl-1,2,4-oxadiazole (1a) into the relevant 1,2,3-triazole (2a) has been quantitatively studied in toluene in the presence of several halogenoacetic acids (HAAs, 3a–h). Again, the occurrence of two reaction pathways has been pointed out: they require one or two moles of acid, respectively, thus repeating the situation previously observed in the presence of trichloroacetic acid. The observed rate constant ratios (kIII/kII) are only slightly affected by the nature of the acid used. To gain a deeper insight into the action of the acids used we have measured the association constants of the HAAs (3a–h) with 4-nitroaniline (4) in toluene. …
Are there dynamical effects in enzyme catalysis? Some thoughts concerning the enzymatic chemical step.
2015
Highlights • The possible role of enzymatic reaction dynamical effects is examined. • Solution reactions usefully inform the issue of dynamical effects in enzymes. • Division into regions containing and away from the transition state is important. • Motions in passage to/from the transition state need not lead to dynamical effects. • Transition State Theory is usually a reasonable description of enzyme kinetics.
Hydrolysis of Phosphotriesters: A Theoretical Analysis of the Enzymatic and Solution Mechanisms
2012
A theoretical study on the alkaline hydrolysis of paraoxon, one of the most popular organophosphorus pesticides, in aqueous solution and in the active site of Pseudomonas diminuta phosphotriesterase (PTE) is presented. Simulations by means of hybrid quantum mechanics/molecular mechanics (QM/MM) potentials show that the hydrolysis of paraoxon takes place through an A(N)D(N) or associative mechanism both in solution and in the active site of PTE. The results correctly reproduce the magnitude of the activation free energies and can be used to rationalize the observed kinetic isotope effects (KIEs) for the hydrolysis of paraoxon in both media. Enzymatic hydrolysis of O,O-diethyl p-chlorophenyl …
Structural Basis and Enzymatic Mechanism of the Biosynthesis of C9- from C10-Monoterpenoid Indole Alkaloids
2009
Cutting carbons: The three-dimensional structure of polyneuridine aldehyde esterase (PNAE) gives insight into the enzymatic mechanism of the biosynthesis of C(9)- from C(10)-monoterpenoid indole alkaloids (see scheme). PNAE is a very substrate-specific serine esterase. It harbors the catalytic triad S87-D216-H244, and is a new member of the alpha/beta-fold hydrolase superfamily. Its novel function leads to the diversification of alkaloid structures.
Synthesis and application of new iminopyridine ligands in the enantioselective palladium-catalyzed allylic alkylation
2014
A variety of iminopyridines were obtained by condensation of chiral amines with pyridine-2-carboxaldehyde and quinoline-8-carbaldehyde, or of aminoalkylpyridine derivatives with chiral ketones. These ligands were assessed in the enantioselective palladium catalyzed allylic substitution of 1,3-diphenylprop-2-enyl acetate with dimethyl malonate affording the product dimethyl 1,3-diphenylprop-2-enylmalonate in good yields and moderate enantioselectivities (up to 62% ee). Catalytic activity and enantioselectivity were found to be highly dependent upon the steric properties of the ligands. The best enantioselectivity (62% ee) was obtained by an iminopyridine based on a camphane skeleton. © 2014 …
Azoxybenzene rearrangement catalyzed by solid acids
2008
Abstract For the first time, the potential of acidic cation-exchange resin (sulfonated polystyrene) to catalyze the Wallach rearrangement of azoxybenzene into 4-hydroxyazobenzene has been proved. This finding reveals an alternative reaction path possible in a heterogeneous process using solid acids and may help to clear some doubts concerning the rearrangement mechanism postulated so far. The resin-induced reaction was found to proceed exclusively in a non-polar medium. Reasonable yield was obtained particularly in isooctane due to favorable distribution of azoxybenzene throughout the resin's matrix. On the contrary, the HY type zeolite did not activate the rearrangement, most probably beca…
First Principles Calculations for Hydrogenation of Acrolein on Pd and Pt: Chemoselectivity Depends on Steric Effects on the Surface
2016
The chemoselective hydrogenation of acrolein on Pt(111) and Pd(111) surfaces is investigated employing density functional theory calculations. The computed potential energy surfaces together with the analysis of reaction mechanisms demonstrate that steric effects are an important factor that governs chemoselectivity. The reactions at the C=O functionality require more space than the reactions at the C=C functionality. Therefore the formation of allyl alcohol is more favorable at low coverage, while the reduction of the C=C bond and the formation of propanal becomes kinetically more favorable at higher coverage. The elementary reaction steps are found to follow different reaction mechanisms,…
Hydrogenation of 2,4-Dinitro-toluene on Pd/C Catalysts: Computational Study on the Influence of the Molecular Adsorption Modes and of Steric Hindranc…
2002
Abstract A new time-dependent Monte Carlo algorithm was developed to simulate isobar and isotherm three-phase batch hydrogenation of 2,4-dinitro-toluene on Pd/C catalysts. A new reaction mechanism was formulated, involving 9 and 27 toluene derivatives, in solution and adsorbed on the surface, respectively. In fact, three different ways of adsorption were considered for all surface derivatives. Microscopic mechanistic hypotheses were formulated analyzing the mimicked surface populations able to reproduce the experimental catalytic activity–selectivity patterns at different times, temperatures, reagent concentrations, and catalyst particle morphologies. The three different adsorption modes, g…