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Rate Limiting (rate + limiting)
Terms modified by Rate Limiting Selected AbstractsNucleophilic heteroaromatic substitution: Kinetics of the reactions of nitropyridines with aliphatic amines in dipolar aprotic solventsINTERNATIONAL JOURNAL OF CHEMICAL KINETICS, Issue 3 2008Chukwuemeka Isanbor Rate data are reported for the reactions of 2-chloro-5-nitropyridine 2a, 2-chloro-3-nitropyridine 2b, and the corresponding 2-phenoxy derivatives 2c with n -butylamine, pyrrolidine and piperidine and 2d with n -butylamine and pyrrolidine in dimethyl sulfoxide (DMSO) as solvent. The same reactions in acetonitrile had been reported earlier (Crampton et al., Eur J Org Chem 2007, 1378,1383). Values in these solvents are compared with those of 2,4-dinitrochlorobenzene 3a, 2,6-dinitrochlorobenzene 3b, and the corresponding nitroactivated diphenyl ethers 3c and 3d. Reactions with n -butylamine in both solvents gave values of kobs, which increase linearly with amine concentration indicating that nucleophilic attack is rate limiting. The only exception is the reactions in acetonitrile with 2c where base catalysis was observed. Values of k1, the rate constant for the nucleophilic attack, decrease in the order pyrrolidine > piperidine > n -butylamine. In acetonitrile, kinetic data show that k/k ratios are more than unity while the inverse is the case in DMSO. With the phenoxy derivatives, substitution was the only process observed. Base catalysis detected in the reactions of the 1-phenoxy derivatives is attributed to rate-limiting deprotonation of the initially formed zwitterionic intermediate. Our results shed more light on fundamental aspects of activation, hydrogen bonding, and steric effects associated with an aza or a nitro group in the molecules investigated as it affects the nucleophilic aromatic substitution (SNAr) reaction pathways. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 40: 125,135, 2008 [source] Hydrolysis and microbial community analyses in two-stage anaerobic digestion of energy cropsJOURNAL OF APPLIED MICROBIOLOGY, Issue 3 2007D.G. Cirne Abstract Aims:, The roles of the diverse populations of micro-organisms responsible for biodegradation of organic matter to form methane and carbon dioxide are rudimentarily understood. To expand the knowledge on links between microbial communities and the rate limiting, hydrolytic stage of two-stage biogas production from energy crops, this study was performed. Methods and Results:, The process performance and microbial communities (as determined by fluorescence in situ hybridization) in two separate two-stage batch digestions of sugar beets and grass/clover were studied. The microbial populations developed in the hydrolytic stage of anaerobic digestion of beets and grass/clover showed very few similarities, despite that the hydrolysis dynamics were similar. In both substrates, the solubilization of organic material was rapid for the first 10 days and accompanied by a build-up of volatile fatty acids (VFAs) and lactate. Between days 10 and 15, VFA and lactate concentrations decreased, as did the solubilization rates. For both substrates, Archaea started to appear in the hydrolytic stage between days 10 and 15, and the fraction of Bacteria decreased. The major bacterial group detected in the leachate fraction for beets was Alphaproteobacteria, whereas for grass/clover it was Firmicutes. The number of cells that bound to probes specifically targeting bacteria with cellulolytic activity was higher in the digestion of grass than in the digestion of beet. Conclusions:, This study allowed the identification of the general bacterial groups involved, and the identification of a clear shift in the microbial population when hydrolysis rate became limiting for each of the substrates investigated. Significance and Impact of the Study:, The findings from this study could be considered as a first step towards the development of strategies to stimulate hydrolysis further and ultimately increasing the methane production rates and yields from reactor-based digestion of these substrates. [source] Insights into the mechanisms of flavoprotein oxidases from kinetic isotope effects,JOURNAL OF LABELLED COMPOUNDS AND RADIOPHARMACEUTICALS, Issue 11-12 2007Paul F. Fitzpatrick Abstract Deuterium, solvent, and 15N kinetic isotope effects have been used to probe the mechanisms by which flavoproteins oxidize carbon,oxygen and carbon,nitrogen bonds in amines, hydroxy acids, and alcohols. For the amine oxidases D -amino acid oxidase, N -methyltryptophan oxidase, and tryptophan monooxygenase, D -serine, sarcosine, and alanine are slow substrates for which CH bond cleavage is fully rate limiting. Inverse isotope effects for each of 0.992,0.996 are consistent with a common mechanism involving hydride transfer from the uncharged amine. Computational analyses of possible mechanisms support this conclusion. Deuterium and solvent isotope effects with wild-type and mutant variants of the lactate dehydrogenase flavocytochrome b2 show that OH and CH bond cleavage are not concerted, but become so in the Y254F enzyme. This is consistent with a highly asynchronous reaction in which OH bond cleavage precedes hydride transfer. The results of Hammett analyses and solvent and deuterium isotope effects support a similar mechanism for alcohol oxidase. Copyright © 2007 John Wiley & Sons, Ltd. [source] Effects of non-steady-state iron limitation on nitrogen assimilatory enzymes in the marine diatom thalassiosira weissflogii (BACILLARIOPHYCEAE)JOURNAL OF PHYCOLOGY, Issue 1 2000Allen J. Milligan Since the recognition of iron-limited high nitrate (or nutrient) low chlorophyll (HNLC) regions of the ocean, low iron availability has been hypothesized to limit the assimilation of nitrate by diatoms. To determine the influence of non-steady-state iron availability on nitrogen assimilatory enzymes, cultures of Thalassiosira weissflogii (Grunow) Fryxell et Hasle were grown under iron-limited and iron-replete conditions using artificial seawater medium. Iron-limited cultures suffered from decreased efficiency of PSII as indicated by the DCMU-induced variable fluorescence signal (Fv/Fm). Under iron-replete conditions, in vitro nitrate reductase (NR) activity was rate limiting to nitrogen assimilation and in vitro nitrite reductase (NiR) activity was 50-fold higher. Under iron limitation, cultures excreted up to 100 fmol NO2,·cell,1·d,1 (about 10% of incorporated N) and NiR activities declined by 50-fold while internal NO2, pools remained relatively constant. Activities of both NR and NiR remained in excess of nitrogen incorporation rates throughout iron-limited growth. One possible explanation is that the supply of photosynthetically derived reductant to NiR may be responsible for the limitation of nitrogen assimilation at the NO2, reduction step. Urease activity showed no response to iron limitation. Carbon:nitrogen ratios were equivalent in both iron conditions, indicating that, relative to carbon, nitrogen was assimilated at similar rates whether iron was limiting growth or not. We hypothesize that, diatoms in HNLC regions are not deficient in their ability to assimilate nitrate when they are iron limited. Rather, it appears that diatoms are limited in their ability to process photons within the photosynthetic electron transport chain which results in nitrite reduction becoming the rate-limiting step in nitrogenassimilation. [source] Devitrification Kinetics and Mechanism of K2O,CaO,SrO,BaO,B2O3,SiO2 Glass-CeramicJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 6 2001Jau-Ho Jean The devitrification kinetics and mechanism of a low-dielectric, low-temperature, cofirable K2O,CaO,SrO,BaO,B2O3,SiO2 glass-ceramic have been investigated. Crystalline phases including cristobalite (SiO2) and pseudowollastonite ((Ca,Ba,Sr) SiO3) are formed during firing. Activation energy analysis shows that the nucleation of the crystalline phases is controlled by phase separation of the glass. The crystallization kinetics of both cristobalite and pseudowollastonite obey Avrami-like behavior, and the results show an apparent activation energy close to that of the diffusion of alkaline and alkali ions in the glass, suggesting that diffusion is rate limiting. The above conclusion is further supported by analysis of measured growth rates. [source] Effect of hydrogen on the synthesis of carbon nanofibers by CO disproportionation on ultrafine Fe3O4ASIA-PACIFIC JOURNAL OF CHEMICAL ENGINEERING, Issue 5 2009Wenxin Lu Abstract Carbon nanofibers (CNFs) are grown by catalytic CO disproportionation over ultrafine Fe3O4 catalyst at a hydrogen concentration of 0,29.17%, and the time-depending rates of CNFs growth are continuously monitored and the morphologies of the as-synthesized CNFs are analyzed. Increasing H2 concentration will lower CO dissociation energy and assist catalyst reconstruction so as to shorten the induction period and increase the growth rate of CNFs, but it will also increase the rate of catalyst deactivation because carbon hydrogasification is not possible and carbon diffusion in the catalyst particle is rate limiting. As a result of H2 -induced catalyst reconstruction and carbon deposition, the morphology of the CNFs changes from twisty to helical and to straight and becomes less entangled when the H2 concentration is increased. Copyright © 2009 Curtin University of Technology and John Wiley & Sons, Ltd. [source] | |||||||||||||