Two-step Reaction Scheme Research Articles

Pre-steady State of Reaction of Nucleoside Diphosphate Kinase with Anti-HIV Nucleotides

The pre-steady-state reaction of Dictyostelium nucleoside diphosphate (NDP) kinase with dideoxynucleotide triphosphates (ddNTP) and AZT triphosphate was studied by quenching of protein fluorescence after manual mixing or by stopped flow. The fluorescence signal, which is correlated with the phosphorylation state of the catalytic histidine in the enzyme active site, decreases upon ddNTP addition according to a monoexponential time course. The pseudo-first order rate constant was determined for different concentrations of the various ddNTPs and was found to be saturable. The data are compatible with a two-step reaction scheme, where fast association of the enzyme with the dideoxynucleotide is followed by a rate-limiting phosphorylation step. The rate constants and dissociation equilibrium constants determined for each dideoxynucleotide were correlated with the steady-state kinetic parameters measured in the enzymatic assay in the presence of the two substrates. It is shown that ddNTPs and AZT triphosphate are poor substrates for NDP kinase with a rate of phosphate transfer of 0.02 to 3.5 s-1 and a KS of 1-5 mM. The equilibrium dissociation constants for ADP, GDP, ddADP, and ddGDP were also determined by fluorescence titration of a mutant F64W NDP kinase, where the introduction of a tryptophan at the nucleotide binding site provides a direct spectroscopic probe. The lack of the 3'-OH in ddNTP causes a 10-fold increase in KD. Contrary to "natural" NTPs, NDP kinase discriminates between various ddNTPs, with ddGTP the more efficient and ddCTP the least efficient substrate within a range of 100 in kcat values.

Continuous synthesis and characterization of silicon carbide nanorods

A two-step reaction scheme has been employed for the synthesis of SiC nanorods at 1400°C. SiO vapour was generated via the silicon reduction of silica, and then this SiO vapor reacted with carbon nanotubes to form SiC nanorods. The morphology and structure of the nanorods were characterized by XRD, TEM, IR and Raman spectroscopy. The nanorods are single crystalline β-SiC with the diameters ranging from 3 to 40 nm. A broad photoluminescence peak located around 430 nm under 260 nm UV fluorescent light excitation at room temperature is observed. A growth model of SiC nanorods is proposed.

Potassium removal from water-methanol-polyol mixtures by ion exchange on Amberlite 252

Abstract Equilibrium isotherms for the uptake of potassium from polyol-water-methanol mixtures on Amberlite 252, a strong acid ion exchange resin, have been obtained in order to assess the possibility of using such a system to purify industrial polyether-polyols. The selectivity was very high. A two-step reaction scheme was proposed and equilibrium data were correlated by an equation derived according to the mass action law. Effective intraparticle diffusivities were obtained using a homogeneous model and the effects of the viscosity of the bulk solution and resin swelling on the exchange rate are discussed. A simple empirical equation correlating diffusivity with viscosity and resin swelling is suggested. Breakthrough curves were obtained under a wide range of conditions, being well correlated by the expression for breakthrough curves with rectangular isotherm. The proposed purification procedure is generally applicable to different polyol types, being economically and technically feasible.

Characterization of partially saturated poly(propylene fumarate) for orthopaedic application

A partially saturated linear polyester based on poly(propylene fumarate) (PPF) was synthesized for potential application in filling skeletal defects. The synthesis was carried out according to a two-step reaction scheme. Propylene glycol and fumaryl chloride were first combined to form an intermediate fumaric diester. The intermediate was then subjected to a transesterification to form the PPF-based polymer. This method allowed for production of a polymer with a number average molecular weight up to 1500 and a polydispersity index of 2.8 and below. The polymeric backbone structure was investigated through the use of FTIR and NMR. Kinetic studies of the transesterification allowed mapping of the molecular weight increase with reaction time. The final product was also characterized by thermal and solubility analysis.

Mathematical Modeling of a 2.4 MW Swirling Pulverized Coal Flame

Predictions and measurements of a swirling unstaged, high NOx, pulverized coal flame are presented. The predictions have been obtained by means of relatively simple models for turbulence, turbulent combustion and turbulent nitric oxide chemistry. Turbulence is modeled using the standard k-epsilon model. The turbulent combustion model incorporates a two-step reaction scheme together with an eddy break-up model. In the NO- chemistry model, thermal-NO and fuel-NO chemical reaction rates are statistically averaged over the fluctuating temperature using the Beta probability density function. Radiation is computed by means of the Discrete Transfer Radiation model. A constant linear absorption coefficient is assumed while scattering is not taken into account. The paper contains a unique, comprehensive comparison between predictions and measurements of a pulverized coal flame issued from an industrial-scale burner. The coal fired was Gottelborn hvBb coal. Attention is paid to velocities, turbulence quantities, temperatures, coal burnout levels and species concentrations of oxygen, carbon monoxide, carbon dioxide, nitrogen oxides precursors and nitrogen oxides. Radiative heat fluxes near the furnace refractory wall and the heat extraction of the cooling loops are addressed also.

Two-step refractive index changes by photoisomerization and photobleaching processes in the films of non-linear optical polyurethanes and a urethane–urea copolymer

Photoinduced refractive index and absorption spectrum changes in the films of polyurethanes and a urethane-urea copolymer with large second-order non-linear coefficient d have been studied. These changes were explained by a two-step reaction scheme including a trans–cis photoisomerization followed by a photobleaching reaction. The former process was reversible and the latter was irreversible, so that a thermally stable refractive index was obtained. The changes were accelerated by heating. A channel waveguide was fabricated by the processes and light propagation through the waveguide was observed.

Mathematical Modeling of a 2. 25 MWtSwirling Natural Gas Flame. Part 1: Eddy Break-up Concept for Turbulent Combustion; Probability Density Function Approach for Nitric Oxide Formation

ABSTRACT Predictions and measurements of a swirling unstaged, high NOx, natural gas flame are presented. The predictions have been obtained by means of relatively simple models for turbulence, turbulent combustion, radiation and turbulent nitric oxide chemistry. Turbulence is modeled by means of the standard K-epsilon model. The turbulent combustion model incorporates a two-step reaction scheme together with an eddy break-up model. Radiation is computed by means of a four-flux method and the Exponential Wide Band Model is used to calculate local radiative properties. In the NO-chemistry model, thermal-NO and prompt-NO chemical reaction rates are statistically averaged using the Beta probability density function. The paper contains a unique comparison between measurements and predictions of a natural gas flame issued from an industrial-scale burner. Special attention is given to establishing the importance of thermal boundary conditions on both the energy balance and calculated NO emissions. The accuracy o...

Deflagration to detonation transition in combustible gas mixtures

This paper presents the results of a computational investigation of the process of deflagration to detonation transition in a combustible gas mixture. The type of combustion (i.e., deflagration or detonation) supported by a two-step reaction scheme is studied as a function of the activation energies. It is shown that both a deflagration to detonation transition and a deflagration wave that lags behind a leading shock are possible. Two types of deflagration to detonation transitions are found theoretically: initiation of detonation from the flame zone and initiation of detonation along a contact discontinuity in the compressed gas near the primary shock wave.

Continuous synthesis of silicon carbide whiskers

A two-step reaction scheme has been employed for the synthesis of SiC whiskers at 1450 °C under an argon or hydrogen flow. First, SiO vapour was generated via the carbothermal reduction of silica in a controlled manner. Second, the generated SiO vapour was reacted with carbon-carrying vapours such as CO and CH4, which resulted in the growth of SiC whiskers on a substrate away from the batch. A higher growth rate was observed in the hydrogen atmosphere due to the formation of CH4 which provides a more favourable reaction route. By the use of thermodynamic calculations, the preferred reaction routes have been selected for an efficient synthesis of SiC whiskers, and a continuous reactor has been designed. The system consists of a boat-train loaded with the silica-carbon mixture and iron-coated graphite substrate above it in an alumina-tube reactor. By pushing the boat-train into the hot zone at a fixed speed, SiO vapour is constantly generated. High-quality SiC whiskers have been grown on the substrate with diameters of 1–3 μm. The yield was about 30% based on the silicon input as SiO2 and silicon output as SiC whiskers. This demonstrates the feasibility of continuous production of high-quality SiC whiskers which does not require additional processes such as purification and classification.

Synthesis, surface, and cell‐adhesion properties of polyurethanes containing covalently grafted RGD‐peptides

In an attempt to improve endothelial cell adhesion and growth on a polyurethane copolymer, cell adhesive RGD-containing peptides were grafted to the polymer backbone. Two peptide grafting reaction schemes, including one-step and two-step approaches, were developed. FTIR and amino acid analysis confirmed that coupling of the peptide to the polyurethane backbone was achieved by both the one-step and two-step methods. However, the two-step approach showed a higher peptide coupling efficiency and resulted in better control of the orientation of the grafted peptide. The two-step reaction scheme was used to prepare Gly-Arg-Gly-Asp-Ser-Tyr (GRGDSY), Gly-Arg-Gly-Asp-Val-Tyr (GRGDVY), and Gly-Arg-Gly-Glu-Ser-Tyr (GRGESY) peptide-grafted polyurethanes with two different peptide densities (100 and 250 mumol/g polymer). Dynamic contact angle measurements indicated that the surfaces of the peptide-grafted polyurethanes were more hydrophilic than the starting and carboxylated versions of the precursor polyurethane. In addition, the surface hydrophilicity of the peptide-grafted polymers increased with increasing bulk peptide density. Electron spectroscopy for chemical analysis suggested that the grafted peptide was present at the polymer-air interface, in vacuo, for the peptide-grafted polyurethanes. The surface peptide density appeared to correlate with the incorporated peptide density in the bulk. In vitro endothelial cell adhesion experiments showed that, without the presence of serum in culture medium, the GRGDSY- and GRGDVY-grafted polyurethanes dramatically enhanced cell attachment and spreading compared with the starting, carboxylated, and GRGESY-grafted polymers. Increasing the peptide density from 100 to 250 mumol/g polymer for the GRGDSY- and GRGDVY-grafted polyurethanes resulted in an increase in cell attachment. With approximately the same peptide density (100 or 250 mumol/g polymer), the GRGDVY-grafted polymers supported more adherent cells than did the GRGDSY-grafted polymers. Similar trends were observed in the in vitro endothelial cell growth studies using culture medium containing serum and endothelial cell growth supplement. The GRGDSY- and GRGDVY-grafted polyurethanes promoted more cell growth than did the starting polyurethane. However, the presence of adhesive serum proteins and growth factor diminished the differences between the cell-adhesive peptide grafted polymers and the GRGESY-grafted polymers.

Synthesis, Surface and Cell-Adhesion Properties of Polyurethanes Containing Covalently Grafted RGD-Peptides

AbstractIn an attempt to improve endothelial cell adhesion and growth on a polyurethane copolymer, cell adhesive RGD-containing peptides were grafted to the polymer backbone. Two peptide grafting reaction schemes, including one-step and two-step approaches, were developed. Amino acid analysis confirmed that the two-step approach had a higher peptide coupling efficiency. The two-step reaction scheme was utilized to prepare GRGDSY, GRGDVY and GRGESY (inactive control) peptide grafted polyurethanes with two different peptide densities (100 and 250 μmol/g polymer). Dynamic contact angle measurements indicated that the surfaces of the peptide grafted polyurethanes were more hydrophilic than the starting and carboxylated versions of the precursor polyurethane. In-vitro endothelial cell adhesion experiments showed that, without the presence of serum in culture medium, the GRGDSY- and GRGDVY-grafted polyurethanes dramatically enhanced cell attachment and spreading. Increasing the peptide density from 100 to 250 μmol/g polymer for the GRGDSYand GRGDVY-grafted polyurethanes resulted in an increase in cell attachment. With approximately the same peptide density (100 or 250 μmol/g polymer), the GRGDVY-grafted polymers supported more adherent cells than the GRGDSY-grafted polymers. Similar trends were observed in the in-vitro endothelial cell growth studies using culture medium containing serum and endothelial cell growth supplement. These RGD-peptide grafted polyurethanes may be useful in providing an easily prepared cell-adhesive substrate for various implantable devices and hybrid organs.

Photocatalytic decomposition of 2,4-dichlorophenol in aqueous TiO 2 suspensions

2,4-Dichlorophenol was decomposed in the presence of TiO 2 suspensions by illumination with a low pressure u.v. light. The experiments were carried out under various pH conditions, reaction times and TiO 2 loadings. The decomposition rate followed Langmuir-Hinshelwood kinetics. The decomposition and demineralization of 2,4-dichlorophenol by the photocatalytic process can be described fairly well by a simplified two-step consecutive reaction scheme.

Analysis of the kinetic barriers for ligand binding to sperm whale myoglobin using site-directed mutagenesis and laser photolysis techniques.

Time courses for NO, O2, CO, methyl and ethyl isocyanide rebinding to native and mutant sperm whale myoglobins were measured at 20 degrees C following 17-ns and 35-ps laser excitation pulses. His64 (E7) was replaced with Gly, Val, Leu, Phe, and Gln, and Val68 (E11) was replaced with Ala, Ile, and Phe. For both NO and O2, the effective picosecond quantum yield of unliganded geminate intermediates was roughly 0.2 and independent of the amino acids at positions 64 and 68. Geminate recombination of NO was very rapid; 90% rebinding occurred within 0.5-1.0 ns for all of the myoglobins examined; and except for the Gly64 and Ile68 mutants, the fitted recombination rate parameters were little influenced by the size and polarity of the amino acid at position 64 and the size of the residue at position 68. The rates of NO recombination and ligand movement away from the iron atom in the Gly64 mutant increased 3-4-fold relative to native myoglobin. For Ile68 myoglobin, the first geminate rate constant for NO rebinding decreased approximately 6-fold, from 2.3 x 10(10) s-1 for native myoglobin to 3.8 x 10(9) s-1 for the mutant. No picosecond rebinding processes were observed for O2, CO, and isocyanide rebinding to native and mutant myoglobins; all of the observed geminate rate constants were less than or equal to 3 x 10(8) s-1. The rebinding time courses for these ligands were analyzed in terms of a two-step consecutive reaction scheme, with an outer kinetic barrier representing ligand movement into and out of the protein and an inner barrier representing binding to the heme iron atom by ligand occupying the distal portion of the heme pocket. Substitution of apolar amino acids for His64 decreased the absolute free energies of the outer and inner kinetic barriers and the well for non-covalently bound O2 and CO by 1 to 1.5 kcal/mol, regardless of size. In contrast, the His64 to Gln mutation caused little change in the barrier heights for all ligands, showing that the polar nature of His64 inhibits both the bimolecular rate of ligand entry into myoglobin and the unimolecular rate of binding to the iron atom from within the protein. Increasing the size of the position 68(E11) residue in the series Ala to Val (native) to Ile caused little change in the rate of O2 migration into myoglobin or the equilibrium constant for noncovalent binding but did decrease the unimolecular rate for iron-O2 bond formation.(ABSTRACT TRUNCATED AT 400 WORDS)

Numerical study of turbulent reacting flows in solid-propellant ducted rocket combustors

A theoretical model has been developed to investigate turbulent mixing and combustion processes in the main combustion chamber of a solid-propellant ducted rocket. The formulation is based on Favre-averaged conservation equations with a two-step chemical reaction scheme and is solved by a semi-implicit finite-difference method. Turbulence closure is achieved using a well-known k-e two-equation model. Calculated flow structures show good agreement with preliminary experimental results obtained from the schlieren flow-visualization study. The influences of various parameters, including dome height and inlet flow angle, on the propulsive performance of the system are investigated in detail.

The use of holography to investigate complex photochemical reactions

The growth of a hologram can be used to follow the temporal course of a photochemical reaction. In this paper the application of this technique to reactions involving more than one photochemical step is considered. A theoretical framework is developed by which the hologram growth curves can be predicted provided one knows the appropriate kinetic equations. Three different kinetic schemes are explicitly considered: a one-step reaction, two parallel reactions, and a two-step consecutive reaction scheme. The calculations are compared with experimental results obtained for the reaction of benzophenone in polymethylmethacrylate.

Magnesium ion inner sphere complex in the anticodon loop of phenylalanine transfer ribonucleic acid.

The binding of Ca2+ and Mg2+ to tRNAPhe is analyzed by equilibrium titrations and temperature-jump measurements using the Wye base fluorescence as a label. Titration experiments starting with the folded structure of the tRNA (high salt and low temperature) show that Ca2+ and Mg2+ binding detected by Wye base fluorescence changes is associated with equilibrium constants between 1 x 10(3) and 3 x 10(3) M-1. The binding of Ca2+ leads to an increase of the relaxation time associated with a conformation change of the anticodon loop and to a decrease of the corresponding amplitude. These data are represented quantitatively by a two-step reaction scheme with a preferential binding of Ca2+ to one of the anticodon conformations. When Mg2+ is added, an extra relaxation process is observed with time constants around 1 ms. This process demonstrates the formation of a Mg2+ inner sphere complex. Relaxation time constants and amplitudes are represented quantitatively by a three-step reaction scheme. Mg2+ binds preferentially to one of the anticodon conformations. In the absence of Mg2+, these conformations are populated almost equally with a transition rate constant around 5 x 10(3) s-1. The Mg2+ inner sphere complex is formed with a relatively low rate constant of (1--2) x 10(3) s-1, indicating a conformational barrier. These data strongly suggest that the Mg2+ site analyzed in the present investigation corresponds to the anticodon site with a distorted octahedral coordination characterized by X-ray analysis. The results are discussed in terms of the anticodon function and also with respect to their implications upon Mg2+ binding to nucleic acids in general.

A chemical and physical characterization of alumina-supported Rh 6(CO) 16

The decarbonylation and carbonylation reactions of the metal cluster carbonyl Rh 6(CO) 16 supported on Al 2O 3 were investigated using volumetric gas adsorption measurements, ir spectroscopy, isotopic measurements, transmission electron microscopy, and EPR spectroscopy. Complete decarbonylation by O 2 and subsequent recarbonylation to Rh 6(CO) 16 could be achieved at room temperature provided sufficient physically adsorbed H 2O was present. A two-step reaction scheme is presented which accounts for the experimental data; it is suggested that the Rh 6 cluster remained intact during these reactions. When heated, in vacuo, above 250 °C, the catalyst lost its ability to undergo the reversible carbonylation/decarbonylation cycle, and the resulting material resembled a highly dispersed conventional Rh catalyst. TEM data showed that no larger crystallites (>10 Å) of Rh are formed even when heated in vacuo to 450 °C.

Analytical and experimental study of turbulent methane-fired backmixed combustion

Abstract : Numerical predictions of the backmixed flowfield of a methane-fired laboratory combustor are compared to experimental observations. The goal is to clarify the mechanisms responsible for pollutant formation in continuous backmixed combustion and to improve predictive methods. The numerical description adopts a simplified effective viscosity model and a two-step kinetic reaction scheme for the methane oxidation. A reaction mechanism to account for the production of nitric oxide is also considered. Laser Doppler velocimetry is used to exerimentally measure velocity profiles in the flowfield. Conventional gas sampling techniques are to measure species concentration profiles.

The kinetics of decarboxylation of uronic acids, polysaccharides, and oxycelluloses

Measurements of the rate of carbon dioxide evolution from sulphuric acid solutions of galacturonic acid have revealed that it follows first-order kinetics, but some carbon dioxide is also formed in a side reaction at a constant rate. Glucuronolactone, polyglucuronic acid, and oxycelluloses decarboxylate in essentially homogeneous sulphuric acid solutions according to a two-step reaction scheme, both steps being kinetically first order. This main reaction is accompanied by side reactions in which carbon dioxide is evolved at an approximately constant rate. Mathematical expressions have been derived and applied to compute the constants of the rate equation for the decarboxylation of materials containing uronic acid.