Kinetic Aspects (kinetic + aspect)

Distribution by Scientific Domains

Selected Abstracts

From monosilane to crystalline silicon, part II: Kinetic considerations on thermal decomposition of pressurized monosilane

J. O. Odden
Kinetic aspects of the thermal decomposition of monosilane at 690,830 K and initial pressures of 0.1,3.7 MPa in a free-space reactor are considered. Neglecting the preparatory initiation period for the reaction (which is difficult to evaluate under the present dynamic conditions), the onset temperature for the decomposition is stipulated to some 700,710 K, independent of the initial monosilane pressure. The overall reaction appears to be of first order throughout the progressing decomposition process. We observe considerably lower reaction rates under the high-pressure conditions than existing models in the literature suggest. A modified model is proposed that simulates the observed reaction rates within 1% and moreover predicts credible concentrations of the involved gaseous species. A key feature of the modified model is incorporation of two third-body assisted surface reactions, which generate monosilane from disilane and disilane from trisilane. 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 309,321, 2006 [source]

A comparative kinetic study on the singlet molecular oxygen-mediated photoxidation of ,- and ,-chymotrypsins

M.A. Biasutti
Abstract: Kinetic aspects of the sensitized photooxidation of ,- and ,-chymotrypsins have been studied at pH 6 and 8. The sensitization, employing classical O2(1,g)-photogenerators, such as xanthene dyes, is a kinetically intricate process because of the presence of ground state dye,protein associations and to the simultaneous participation of superoxide ion and singlet molecular oxygen [O2(1,g)]. Both proteins, that possess the same distribution pattern of photooxidizable amino acids, suffer a pure O2(1,g)-mediated photodynamic attack, using the carbonylic sensitizer Perinaphthenone. Overall and reactive rate constants for the O2(1,g)-quenching (in the order of 108 and 107/M/s, respectively), and rates of oxygen consumption determined by time-resolved, spectroscopic and polarographic methods indicate that ,- and ,-chymotrypsins are less photooxidizable at pH 6, as a result of an enhancement of the O2(1,g)-physical quenching component. In general terms, ,-chymotrypsin exhibits the greater overall proclivity to interact with O2(1,g), whereas structural factors, possibly evidenced by a higher exposure of the reactive tryptophan residues, impart an increased photooxidation degree to the proteins at pH 8, specially to the ,-chymotrypsin. [source]

Capillary electrophoretic investigation of the enantioselective metabolism of propafenone by human cytochrome P-450 SUPERSOMES: Evidence for atypical kinetics by CYP2D6 and CYP3A4

Minoo Afshar
Abstract An enantioselective CE method was used to identify the ability of CYP450 enzymes and their stereoselectivity in catalyzing the transformation of propafenone (PPF) to 5-hydroxy-propafenone (5OH-PPF) and N -despropyl-propafenone (NOR-PPF). Using in,vitro incubations with single CYP450 enzymes (SUPERSOMES), 5OH-PPF is shown to be selectively produced by CYP2D6 and N -dealkylation is demonstrated to be mediated by CYP2D6, CYP3A4, CYP1A2, and CYP1A1. For the elucidation of kinetic aspects of the metabolism with CYP2D6 and CYP3A4, incubations with individual PPF enantiomers and racemic PPF were investigated. With the exception of the dealkylation in presence of R -PPF only, which can be described by the Michaelis,Menten model, all CYP2D6-induced reactions were found to follow autoactivation kinetics. For CYP3A4, all NOR-PPF enantiomer formation rates as function of PPF enantiomer concentration were determined to follow substrate inhibition kinetics. The formation of NOR-PPF by the different enzymes is stereoselective and is reduced significantly when racemic PPF is incubated. Clearance values obtained for CYP3A4 dealkylation are stereoselective whereas those of CYP2D6 hydroxylation are not. This paper reports the first investigation of the PPF hydroxylation and dealkylation kinetics by the CYP2D6 enzyme and represents the first report in which enantioselective CE data provide the complete in,vitro kinetics of metabolic steps of a drug. [source]

Nitration of nitrobenzene at high-concentrations of sulfuric acid: Mass transfer and kinetic aspects

AICHE JOURNAL, Issue 3 2010
M. Rahaman
Abstract This article reports studies on mass transfer and kinetics of nitration of nitrobenzene at high concentrations of sulfuric acid in a batch reactor at different temperatures. The effects of concentration of sulfuric acid, speed of stirring, and temperature on mass transfer coefficient were investigated. The kinetics of nitration under homogenized conditions was studied at different sulfuric acid concentrations at these temperatures. The reaction rate constants were determined. The variation of rate constant with sulfuric acid concentration was explained by the Mc function. The activation energies of the reactions were determined from the Arrhenius plots. The regimes of the reactions were determined using the values of the mass transfer coefficients and the reaction rate constants. A model was developed for simultaneous mass transfer and chemical reaction in the aqueous phase. The yields of the three isomers of dinitrobenzene were determined, and the variation of isomer distribution with sulfuric acid concentration and temperature was analyzed. This work demonstrates that more than 90% conversion of nitrobenzene is possible at high-sulfuric acid concentrations resulting in high yield of the product even at moderate temperatures and at low speeds of stirring. 2009 American Institute of Chemical Engineers AIChE J, 2010 [source]

Comparison of BrdU and cyclin A as markers of the S-phase in oral precancerous lesions

Richard J. Oliver
Abstract: A study comparing bromodeoxyuridine (BrdU) and cyclin A as markers of cells in the S-phase in oral precancerous lesions was performed. These were also compared with the growth fraction (GF) as assessed by Ki-67. Biopsies of 15 lesions were obtained, presenting clinically as leukoplakia or erythroplakia of the lateral tongue or floor of mouth. Half of each biopsy was incubated in BrdU and routinely fixed and processed. Sequential sections from each block were cut and stained immunohistochemically with antibodies against the following proteins: BrdU, Ki-67 and cyclin A. Stained sections were quantified and the labelling indices (LI) expressed per 100 of the total nucleated cell population (%) and per millimetre basement length (/mm). The mean LI% for BrdU was 11.24% (SD 2.83), for cyclin A it was 12.76% (SD 3.88) and the GF% was 29.25% (SD 11.88). The mean LI/mm for BrdU was 40.93/mm (SD 11.88), for cyclin A it was 47.59/mm (SD 18.28) and the GF/mm was 110.72/mm (SD 49.30). The BrdU and cyclin A indices were significantly correlated with each other. In the more dysplastic cases, the cyclin A LI was quantitatively much larger than that for BrdU, suggesting that the protein was being overexpressed. It was concluded that as a tool to study the kinetic aspects of the cell cycle in dysplastic lesions cyclin A was limited by the fact that it is overexpressed. In minimally dysplastic lesions and normal epithelia, however, cyclin A may be a viable alternative to BrdU for the study of the S-phase. [source]

On/off switching on polymer conformation

Kenichi Yoshikawa
Abstract The manner of folding transition from elongated coil to compact globule of single polymer chain is discussed. Based on theoretical consideration, it is argued the semi-flexible polymer chain exhibits large discrete transition on the level of individual single chains, whereas the transition looks continuous, or cooperative, on the ensemble of chains. As the experimental verification, in the present article thermodynamic and kinetic aspects of folding transition of single giant DNA molecules are described. It is shown that rich variety of nano-ordered structures are obtained from single DNA molecules through suitable setup of the experimental conditions. The stability of such nano-structures generated from single polymer chain is discussed in relation to the ordered compact structure with large number of chains in semi-dilute and concentrated polymer solutions. [source]

Can Electrophilicity Act as a Measure of the Redox Potential of First-Row Transition Metal Ions?

Jan Moens
Abstract Previous contributions concerning the computational approach to redox chemistry have made use of thermodynamic cycles and Car,Parrinello molecular dynamics simulations to obtain accurate redox potential values, whereas this article adopts a conceptual density functional theory (DFT) approach. Conceptual DFT descriptors have found widespread use in the study of thermodynamic and kinetic aspects of a variety of organic and inorganic reactions. However, redox reactions have not received much attention until now. In this contribution, we prove the usefulness of global and local electrophilicity descriptors for the prediction of the redox characteristics of first row transition metal ions (from Sc3+|Sc2+ to Cu3+|Cu2+) and introduce a scaled definition of the electrophilicity based on the number of electrons an electrophile ideally accepts. This scaled electrophilicity concept acts as a good quantitative estimate of the redox potential. We also identify the first solvation sphere together with the metal ion as the primary active region during the electron uptake process, whereas the second solvation sphere functions as a non-reactive continuum region. [source]

Entropy production in chiral symmetry breaking transitions,

CHIRALITY, Issue 3-4 2008
Dilip Kondepudi
Abstract It is now well known that nonequilibrium chemical systems may reach conditions that spontaneously generate chiral asymmetry. One can find a host of model reactions that exhibit such behavior in the literature. Among these, models based on one originally devised by Frank have been studied extensively. Though the kinetic aspects of such model reactions have been discussed in great detail, the behavior of entropy in such systems is rarely discussed. In this article, the rate of entropy production per unit volume, ,, in a modified Frank model is discussed. It is shown that the slope of , changes at the point at which the asymmetric states appear, behavior similar to that observed in second-order phase transitions. Chirality, 2008. 2007 Wiley-Liss, Inc. [source]