Apparent First-order Kinetics (apparent + first-order_kinetics)

Distribution by Scientific Domains


Selected Abstracts


First-order stochastic cellular automata simulations of the lindemann mechanism

INTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 4 2004
Chad A. Hollingsworth
The Lindemann mechanism explains how apparent unimolecular chemical reactions arise from bimolecular collisions. In this mechanism an ingredient M activates reactants A through collisions, and the resulting activated species A* can either decay to products P or be deactivated back to A, again via collisions with M. A first-order stochastic cellular automata model described previously [Seybold, Kier, and Cheng, J Chem Inf Comput Sci 1997, 37, 386] has been modified to simulate this mechanism. It is demonstrated that this model accurately reflects the salient features of the Lindemann mechanism, including the normal second-order kinetic behavior at low [M] and apparent first-order kinetics at high [M]. At low [M] the mechanism is equivalent to a rate-limited sequential process, whereas at high [M] it becomes a preequilibrium with leakage to products. The model also allows an examination of the validity of the steady-state approximation normally employed in a deterministic analysis of this mechanism, and it is seen that this approximation is not well justified under reasonable conditions. 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 230,237 2004 [source]


DEGRADATION KINETICS OF ANTHOCYANIN IN ETHANOLIC SOLUTIONS

JOURNAL OF FOOD PROCESSING AND PRESERVATION, Issue 5 2006
KUO-CHAN TSENG
ABSTRACT The objective of this study was to investigate the degradation kinetics of anthocyanin in ethanolic model solutions simulating wine and liqueur in aging or long-term storage. Malvidin-3-glucoside, as the predominant anthocyanin in many cultivars of grape, was chosen to represent anthocyanins. The results from high performance liquid chromatography analysis show that the disappearance of malvidin-3-glucoside follows apparent first-order kinetics, and accelerates with the increase in ethanol concentration. The Ea values were found to be 22.80, 24.45, 24.35 and 22.75 kcal/mole at 0, 10, 30 and 50% ethanol concentrations, respectively. We propose that the decreased stability of anthocyanin at an elevated ethanol concentration is a result of a decreased extent of self-association in the solution. [source]


Hydrolysis of the amyloid ,-peptide (A,) 1,40 between Asp23,Val24 produces non-aggregating fragments.

JOURNAL OF MASS SPECTROMETRY (INCORP BIOLOGICAL MASS SPECTROMETRY), Issue 2 2005
An electrospray mass spectrometric study
Abstract The aggregation of full-length (residues 1,40) amyloid ,-peptide (A,) and fragments corresponding to residues 1,23 and 24,40 was studied by electrospray mass spectrometry, using gramicidin as a non-aggregating reference. Following a lag period, A,(1,40) at 140 M concentration aggregates with apparent first-order kinetics. Under acidic conditions A,(1,40) undergoes spontaneous cleavage between Asp23,Val24 and to a lesser extent also at two other Asp,X motifs. Incubation in acidic H218O showed incorporation of 18O in fragment A,(1,23), confirming that the Asp23,Val24 peptide bond had been hydrolyzed. Incubation of synthetic A,(1,23) and A,(24,40) peptides with A,(1,40) showed that A,(24,40) remained in solution for several months, that A,(1,23) partly disappeared from solution, whereas A,(1,40) completely disappeared. Further, treatment of sedimentable aggregates formed after co-incubation of the three peptides with hexafluoro-2-propanol or formic acid recovered the intensity of A,(1,40). These data support previous studies showing that the region of A, encompassing residues 16,24 is necessary for aggregation into amyloid fibrils. Copyright 2005 John Wiley & Sons, Ltd. [source]


Insect chymotrypsins: chloromethyl ketone inactivation and substrate specificity relative to possible coevolutional adaptation of insects and plants,

ARCHIVES OF INSECT BIOCHEMISTRY AND PHYSIOLOGY (ELECTRONIC), Issue 3 2009
Adriana R. Lopes
Abstract Insect digestive chymotrypsins are present in a large variety of insect orders but their substrate specificity still remains unclear. Four insect chymotrypsins from 3 different insect orders (Dictyoptera, Coleoptera, and two Lepidoptera) were isolated using affinity chromatography. Enzymes presented molecular masses in the range of 20 to 31,kDa and pH optima in the range of 7.5 to 10.0. Kinetic characterization using different colorimetric and fluorescent substrates indicated that insect chymotrypsins differ from bovine chymotrypsin in their primary specificity toward small substrates (like N- benzoyl-L-Tyr p- nitroanilide) rather than on their preference for large substrates (exemplified by Succynil-Ala-Ala-Pro-Phe p- nitroanilide). Chloromethyl ketones (TPCK, N- ,-tosyl-L-Phe chloromethyl ketone and Z-GGF-CK, N- carbobenzoxy-Gly-Gly-Phe-CK) inactivated all chymotrypsins tested. Inactivation rates follow apparent first-order kinetics with variable second order rates (TPCK, 42 to 130,M,1,s,1; Z-GGF-CK, 150 to 450,M,1,s,1) that may be remarkably low for S. frugiperda chymotrypsin (TPCK, 6,M,1,s,1; Z-GGF-CK, 6.1,M,1,s,1). Homology modelling and sequence alignment showed that in lepidopteran chymotrypsins, differences in the amino acid residues in the neighborhood of the catalytic His 57 may affect its pKa value. This is proposed as the cause of the decrease in His 57 reactivity toward chloromethyl ketones. Such amino acid replacement in the active site is proposed to be an adaptation to the presence of dietary ketones. 2009 Wiley Periodicals, Inc. [source]