Intermediate Formation Rates Research Articles

Boosting Electrocatalytic N2 Reduction to NH3 by Enhancing N2 Activation via Interaction between Au Nanoparticles and MIL-101(Fe) in Neutral Electrolytes.

Electrocatalytic nitrogen reduction reaction (NRR) has attracted much attention as a sustainable ammonia production technology, but it needs further exploration due to its slow kinetics and the existence of competitive side reactions. In this research, xAu/MIL-101(Fe) catalysts were obtained by loading gold nanoparticles (Au NPs) onto MIL-101(Fe) using a one-step reduction strategy. Herein, MIL-101(Fe), with high specific surface area and strong N2 adsorption capacity, is used as a support to disperse Au NPs to increase the electrochemical active surface area. Au NPs, with a high NRR activity, is introduced as the active site to promote charge transfer and intermediate formation rates. More importantly, the strong interaction between Au NPs and MIL-101(Fe) enhances the electron transfer between Au NPs and MIL-101(Fe), thereby enhancing the activation of N2 and achieving efficient NRR. Among the prepared catalysts, 15 %Au/MIL-101(Fe) has the highest NH3 yield of 46.37 μg h-1 mg-1 cat and a Faraday efficiency of 39.38 % at -0.4 V (vs. RHE). In-situ FTIR reveals that the NRR mechanism of 15 %Au/MIL-101(Fe) follows the binding alternating pathway and also indicates that the interaction between Au NPs and MIL-101(Fe) strengthens the activation of the N≡N bond in the rate-limiting process, thereby accelerating the NRR process.

HYDRODENITROGENATION OF 2-METHYL QUINOLINE - EFFECT OF STERIC HINDRANCE IN LIQUID PHASE CATALYTIC HYDROGENATION

Hydrodenitrognation (HDN) of 2-methyl quinoline (quinaldine) and quinoline in liquid phase on S-NiMo/a-Al2O3 catalyst was carried out in a batch reactor at 3.0 - 10.0 MPa and 130 � 360oC in Poly alpha-olefin solvent. Hydrogenolysis and Hydrogenation results in formation of products such as 1,2,3,4-treta hydro Quinaldine (1-THQ), 5,6,7,8-terta hydro Quinaldine (5-THQ), Deca hydro Quinaldine (DHQ), Butyl cyclohexane (BCH), Butyl cyclo hexene (BCHE) and butyl benzene (BB). Under studied condition, amine rings of quinaldine was rapidly hydrogenated and it forms relevant quasi equilibrium with 1,2,3,4-treta hydro quinaldine (1-THQ), 5,6,7,8-terta hydro quinaldine(5-THQ) and Deca hydro quinaldine (DHQ). The quinaldine reaction path either 1-THQ or 5-THQ indicated dependent of temperature and independent of partial pressure. Over which amine adsorption on metal sites reduced due to steric hindrance of methyl-group adjacent to N-bonding and increases the rate of intermediate formation through benzene ring hydrogenation. Distinguished HDN hydrogenolysis mechanism of quinaldine and its intermediate compounds were studied and compared with quinoline HDN. Thus, 5-THQ to DHQ and C-N cleavage identified as rate limiting step through steric hindrance HDN mechanism.

Effect of distal His mutation on the peroxynitrite reactivity of Leishmania major peroxidase

The conserved distal histidine in peroxidases has been considered to play a major role as a general acid-base catalyst for heterolytic cleavage of an OO bond in H2O2. However, heme peroxidases react with peroxynitrite to form transient intermediates but the role of the distal histidine in this reaction is still unknown. In order to investigate catalytic roles of the histidine at the distal cavity, two Leishmania major peroxidase (LmP) mutants (H68E, H68V) were prepared. The rate of transition from ferric H68V to Compound ES by H2O2 is decreased by approximately five orders of magnitude relative to wild type, which is consistent with electron donor oxidation data where the H68V is ~1000 fold less active than wild type. In the reaction with peroxynitrite, the formation rate of intermediates in the mutants is not significantly lower than that for the wild type, indicating that the His68 has no major role in homolytic cleavage of an OO bond in peroxynitrite. EPR spectroscopic data suggest that the transient intermediates formed by the reaction of LmP with H2O2 exhibits an intense and stable signal similar to CCP Compound ES whereas in case of the reaction with peroxynitrite, this signal disappears, indicating that the transient intermediate is Compound II. Rapid kinetics data suggest that the distal His68 mutants display higher decay rates of Compound II than wild type. Thus, His68 mutations minimize Compound II formation (inactive species in peroxynitrite scavenging cycles) by increasing decay rates during the steady state and results in higher peroxynitrite degrading activity.

VSV Trans-Membrane Domain Promotes Content Mixing to occur Early in the Fusion Process

Polyethylene glycol (PEG)-mediated fusion of 25 nm vesicles was studied at different temperatures in the presence of the trans-membrane domain (TMD) of the G protein (fusion protein) of vesicular stomatitis virus (VSV). Vesicles were composed of dioleyol-phosphatidylcholine (DOPC), dioleyol-phosphatidylethanolamine (DOPE), bovine brain sphingomyelin (SM), and cholesterol (CH) in a molar ratio of 35:30:15:20. Kinetic parameters of the fusion process were determined by fitting lipid mixing (LM), content mixing (CM), and content leakage (L) time courses globally to a three state sequential model that allowed for leakage from the final fusion pore state. This yielded the rate constants for conversion between different states as well as the probabilities of the occurrence of LM and CM in each state. The TMD enhanced the initial rate of lipid mixing and content mixing in agreement with our previous report (Biochemistry, 2002, 14925). This resulted partly from enhanced extent of CM, although the extent of LM remained unaltered. In addition, the rate of intermediate formation was increased slightly, although, the principal effect of TMD was to increase the probability of CM in the initial intermediate at the expense of the final step of the process. This suggests that the presence of TMD enhanced flickering pore formation, leading to an increase in CM early in the process. Supported by NIGMS grant 32707 to BRL.

Ethanol and Methanol: Adsorption Rates and Rates of Intermediate Formation at Pt Single Crystal Electrodes

not Available.

3-Hydroxy-3-methylglutaryl-CoA Synthase: Participation of Invariant Acidic Residues in Formation of the Acetyl-S-Enzyme Reaction Intermediate‡

Inactivation of HMG-CoA synthase by a carboxyl-directed reagent, 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC), in a concentration-dependent and substrate-protectable manner suggested that the active site contains reactive acidic amino acids. This observation prompted functional evaluation of 11 invariant acidic amino acids by site-directed mutagenesis. Characterization of the isolated synthase variants' ability to catalyze overall and partial reactions identified three mutant synthases (D99A, D159A, and D203A) that exhibit significant diminution of k(cat) for the overall reaction (10(2)-, 10(3)-, and 10(4)-fold decreases, respectively). D99A, D159A, and D203A form the acetyl-S-enzyme intermediate very slowly (0.0025, 0.0026, 0.0015 U/mg, respectively, measured at pH 7. 0 and 22 degrees C) as compared to the wild-type synthase (1.59 U/mg), where intermediate formation approaches rate-limiting status. Differences in substrate saturation do not account for impaired activities or rates of intermediate formation. The structural integrity of the purified mutants' active sites is demonstrated by their abilities to bind a spin-labeled acyl-CoA analogue (R.CoA) with affinities and stoichiometries comparable to values measured for wild-type synthase. The impact of three distinct amino acids on reaction intermediate formation supports a mechanism of acetyl-S-enzyme formation that probably requires formation and directed collapse of a tetrahedral adduct. (18)O-induced shift of the (13)C NMR signal of (13)C acetyl-S-enzyme demonstrates that an analogous tetrahedral species is produced upon solvent exchange with the acetyl-S-enzyme. Partial discrimination between the functions of D99, D159, and D203 becomes possible based on the observation that D159A and D203A synthases exhibit retarded kinetics of solvent (18)O exchange while D99A fails to support (18)O exchange.

Aromatic nucleophilic substitution reaction of 2,4-dinitrohalogenobenzene and ethyl α-cyano-phenylacetate benzyl anion

The aromatic nucleophilic substitution reaction of 2,4-dinitrohalogenobenzene (DNHB; halogeno =F, Cl, Br) with ethyl α-cyanophenylacetate (ECPA) benzyl anion has been studied by means of UV and IR spectrophotometry as well as gas chromatography. The rates of intermediate formation and product formation were determined respectively. In contrast with common S N Ar reactions, the formation rate constant of the intermediates, substituted cyclohexadienates, was in the order of halogen =F>Br>Cl, whereas the rate constant of product formation was in the order F>Cl>Br, which revealed that the departure of the halogen atom was the rate-limiting step for the overall reaction in the presence of a sterically hindered nucleophile. These reactions could be catalyzed by the solvated electrons. The corresponding reaction intermediates, radical species, were detected by ESR and the reactions were proved to proceed mainly by a solvated-electron induced radical chain reaction mechanism.

A microcompartmentation analysis of intermediate leakage response to substrate excess in a membrane‐bound bifunctional enzyme: Local control of hydroxyprogesterone channeling efficiency during cytochrome P450XVII‐catalysed androgen biosynthesis

Evidence is presented for the first time that the cytochrome P450XVII-catalysed androgen formation from progesterone (P) in rat testicular microsomal membranes represents a metabolic sequence that exhibits the ability of intrinsic regulation of intermediate transfer and product formation efficiency. Exposure of this system, which catalyses a hydroxylation and oxidative cleavage reaction sequence, to increasing P concentration results in a decreased specific retention of the putative intermediate, 17 alpha-hydroxyprogesterone (HP) in the membrane compartment, and in a decreased HP conversion to androgens in favour of increasing HP transfer into the extramembrane space. This behaviour results in a decreased ratio of product vs. intermediate formation rates, which is interpreted as a partial "uncoupling" of the normal hydroxylation and cleavage reaction sequence catalysed by P450XVII. A similar pattern can likewise be observed in isolated testicular Leydig cells after exposure to increasing P concentrations under more physiological continuous-flow conditions. Further calculations indirectly indicate that the specific retention of HP in the membrane compartment can partially be attributed to its specific association with the P450XVII during catalysis. The results strongly suggest the existence of a local "channel" that becomes more leaky and therefore less effective if loaded with high influx rates. This pattern may be related to significant but incomplete competition of exogenously entering P and endogenously formed and transiently bound HP for oxygen attack at the P450XVII active site.

Detection and accumulation of tetrahedral intermediates in elastase catalysis.

Tetrahedral intermediates in the reaction of elastase with specific di- and tripeptide p-nitroanilide substrates have been detected, accumulated, and stabilized at high pH by using subzero temperatures and fluid aqueous/organic cryosolvents. The tetrahedral adducts are characterized by spectra with lambda max of 359 +/- 2 nm, compared with thata of 380 nm for p-nitroaniline and 315-320 nm for the substrates. The maximal concentration of intermediate that could be accumulated varied with the different substrates from 40 to 100% of the active enzyme present. The pH dependence of the reactions indicated that formation of the tetrahedral intermediates was rate-limiting at low pH (pK* = 7.0 at -39 degrees C) and that collapse to the acylenzymes was rate-determining at high pH. When corrected for the effect of temperature and cosolvent, the rate of intermediate formation was in good agreement with that measured at 25 degrees C in aqueous solution by stopped-flow techniques.