Kinetic Evidence (kinetic + evidence)

Distribution by Scientific Domains


Selected Abstracts


Kinetic Evidence for the Influence of Subsurface Oxygen on Palladium Surfaces Towards CO Oxidation at High Temperatures

CHEMISTRY - AN ASIAN JOURNAL, Issue 1 2009
Chinnakonda
Abstract Transient state kinetics of the catalytic oxidation of CO with O2 on Pd-surfaces has been measured under isothermal conditions by using a molecular beam approach. Systematic studies were carried out as a function of reaction temperature and CO+O2 composition. With sufficient kinetic evidence, we have demonstrated the positive influence of subsurface oxygen towards CO-adsorption and oxidation to CO2 at high temperatures (600,900,K) on Pd-surfaces, and the likely electronic nature of the surface changes with oxygen in the subsurface. These studies also provide a direct proof for CO-adsorption with a significantly reactive sticking coefficient at high temperatures on Pd-surfaces exhibiting a significant subsurface O-coverage. [source]


Kinetic evidence for the copper peroxide intermediate with two copper ions in proximity

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 7 2006
S. Selvarani
The decomposition of caroate (peroxomonosulfate, PMS) is catalyzed by Cu(II) ions even at 5 10,5 M in aqueous alkaline solution. The rate is second order in copper(II) ions concentrations and first order in [PMS]. The rate constant values are found to decrease with increase in hydroxide ion concentrations. The turnover number for the reaction is estimated as >1000. The experimental results suggest that the formation of peroxide type intermediate with two copper(II) ions is the rate-determining step. This peroxide intermediate reacts with another molecule of PMS to give the products oxygen, SO and copper ions. The overall entropy of activation is positive with a value of ,20 cals/mol/K. The very high turnover number suggests that Cu(II) ion is one of the best catalysts for the decomposition of caroate ions in alkaline medium. The reaction also represents a system in which metal ion catalyzed decomposition of caroate does not involve radical intermediates. 2006 Wiley Periodicals, Inc. Int J Chem Kinet 38: 439,443, 2006 [source]


Kinetic evidence for the occurrence of kinetically detectable intermediates in the cleavage of N -ethoxycarbonylphthalimide under N -methylhydroxylamine buffers

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 2 2002
M. Niyaz Khan
The kinetics of the aqueous cleavage of N -ethoxycarbonylphthalimide (NCPH) in CH3NHOH buffers of different pH reveals that the cleavage follows the general irreversible consecutive reaction path NCPH ENMBC AB, where ENMBC, A, and B represent ethyl N -[o -(N -methyl- N -hydroxycarbamoyl)benzoyl]carbamate, N -hydroxyl group cyclized product of ENMBC, and o -(N -methyl- N -hydroxycarbamoyl)benzoic acid, respectively. The rate constant k1 obs at a constant pH, obeys the relationship k1 obs = kw + knapp [Am]T + kb[Am]T2, where [Am]T is the total concentration of CH3NHOH buffer and kw is first-order rate constant for pH-independent hydrolysis of NCPH. Buffer-dependent rate constant kb shows the presence of both general base and general acid catalysis. Both the rate constants k2 obs and k3 obs are independent of [Am]T (within the [Am]T range of present study) at a constant pH and increase linearly with the increase in aOH with definite intercepts. 2001 John Wiley & Sons, Inc. Int J Chem Kinet 34: 95,103, 2002 [source]


Kinetic evidence of an on-pathway intermediate in the folding of lysozyme

PROTEIN SCIENCE, Issue 1 2000
Yawen Bai
Abstract By means of a kinetic test, it was demonstrated that one of the folding intermediates (I,) of hen lysozyme with ,-domain folded and ,-domain unfolded is on the folding pathway under the classical definition. l, folds to the native (N) state directly (unfolded (U) , I, , N) without having to unfold to U and then refold to N through alternative folding pathways as in I, , U , N. [source]


Supercomplex organization of the mitochondrial respiratory chain and the role of the Coenzyme Q pool: Pathophysiological implications

BIOFACTORS, Issue 1-4 2005
Maria Luisa Genova
Abstract In this review we examine early and recent evidence for an aggregated organization of the mitochondrial respiratory chain. Blue Native Electrophoresis suggests that in several types of mitochondria Complexes I, III and IV are aggregated as fixed supramolecular units having stoichiometric proportions of each individual complex. Kinetic evidence by flux control analysis agrees with this view, however the presence of Complex IV in bovine mitochondria cannot be demonstrated, presumably due to high levels of free Complex. Since most Coenzyme Q appears to be largely free in the lipid bilayer of the inner membrane, binding of Coenzyme Q molecules to the Complex I-III aggregate is forced by its dissociation equilibrium; furthermore free Coenzyme Q is required for succinate-supported respiration and reverse electron transfer. The advantage of the supercomplex organization is in a more efficient electron transfer by channelling of the redox intermediates and in the requirement of a supramolecular structure for the correct assembly of the individual complexes. Preliminary evidence suggests that dilution of the membrane proteins with extra phospholipids and lipid peroxidation may disrupt the supercomplex organization. This finding has pathophysiological implications, in view of the role of oxidative stress in the pathogenesis of many diseases. [source]


Kinetic evidences for facilitation of peptide channelling by the proteasome activator PA28

FEBS JOURNAL, Issue 20 2000
Ralf Stohwasser
The activation kinetics of constitutive and IFN,-stimulated 20S proteasomes obtained with homomeric (recPA28,, recPA28,) and heteromeric (recPA28,,) forms of recombinant 11S regulator PA28 was analysed by means of kinetic modelling. The activation curves obtained with increasing concentrations of the individual PA28 subunits (RecP28,/RecP28,/RecP28,+ RecP28,) exhibit biphasic characteristics which can be attributed to a low-level activation by PA28 monomers and full proteasome activation by assembled activator complexes. The dissociation constants do not reveal significant differences between the constitutive and the immunoproteasome. Intriguingly, the affinity of the proteasome towards the recPA28,, complex is about two orders of magnitude higher than towards the homomeric PA28, and PA28, complexes. Striking similarities can been revealed in the way how PA28 mediates the kinetics of latent proteasomes with respect to three different fluorogenic peptides probing the chymotrypsin-like, trypsin-like and peptidylglutamyl-peptide hydrolyzing like activity: (a) positive cooperativity disappears as indicated by a lack of sigmoid initial parts of the kinetic curves, (b) substrate affinity is increased, whereby (c), the maximal activity remains virtually constant. As these kinetic features are independent of the peptide substrates, we conclude that PA28 exerts its activating influence on the proteasome by enhancing the uptake (and release) of shorter peptides. [source]


Kinetic Evidence for the Influence of Subsurface Oxygen on Palladium Surfaces Towards CO Oxidation at High Temperatures

CHEMISTRY - AN ASIAN JOURNAL, Issue 1 2009
Chinnakonda
Abstract Transient state kinetics of the catalytic oxidation of CO with O2 on Pd-surfaces has been measured under isothermal conditions by using a molecular beam approach. Systematic studies were carried out as a function of reaction temperature and CO+O2 composition. With sufficient kinetic evidence, we have demonstrated the positive influence of subsurface oxygen towards CO-adsorption and oxidation to CO2 at high temperatures (600,900,K) on Pd-surfaces, and the likely electronic nature of the surface changes with oxygen in the subsurface. These studies also provide a direct proof for CO-adsorption with a significantly reactive sticking coefficient at high temperatures on Pd-surfaces exhibiting a significant subsurface O-coverage. [source]