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Concentration active intermediates

Rate laws of this form u.sually involve a number of elementary reactions and at least one active intermediate. An at live intermediate is a high-energy molecule that reacts virtually as fast as it is fomied. As a result, it is present in very small concentrations. Active intermediates (e.g., A 1 can be formed by collision or interaction with other molecules,... [Pg.378]

It has been concluded from an estimate of Ki and K2 that the uncharged amino alcohol as well as the dipolar structure are present in small concentrations (24). The decomposition is strongly retarded as the pH is lowered and this phenomenon has been explained by assuming the zwitterions to be the active intermediates [Eq. (10)]. [Pg.110]

FIGURE 18.12 The use of inhibitors to reveal the sequence of reactions in a metabolic pathway, (a) Control Under normal conditions, the steady-state concentrations of a series of intermediates will be determined by the relative activities of the enzymes in the pathway, (b) Plus inhibitor In the presence of an inhibitor (in this case, an inhibitor of enzyme 4), intermediates upstream of the metabolic block (B, C, and D) accumulate, revealing themselves as intermediates in the pathway. The concentration of intermediates lying downstream (E and F) will fall. [Pg.579]

Figure 6. A hypothetical scheme for the control of the number of active crossbridges in smooth muscle. Following the activation of a smooth muscle by an agonist, the concentrations of intermediates along the main route begins to build up transiently. This is shown by the thickened arrows. Also, cAMP is generated which is universally an inhibitor in smooth muscle. Cyclic AMP in turn combines with protein kinase A, which accounts for most of its action. The downstream mechanisms, however, are not well worked out and at least three possibilities are likely in different circumstances. First, protein kinase A is known to catalyze the phosphorylation of MLCK, once phosphorylated MLCK has a relatively lower affinity for Ca-calmodulin so that for a given concentration of Ca-calmodulin, the activation downstream is reduced. The law of mass action predicts that this inhibition should be reversed at high calcium concentrations. Other cAMP inhibitory mechanisms for which there is evidence include interference with the SR Ca storage system, and activation of a MLC phosphatase. Figure 6. A hypothetical scheme for the control of the number of active crossbridges in smooth muscle. Following the activation of a smooth muscle by an agonist, the concentrations of intermediates along the main route begins to build up transiently. This is shown by the thickened arrows. Also, cAMP is generated which is universally an inhibitor in smooth muscle. Cyclic AMP in turn combines with protein kinase A, which accounts for most of its action. The downstream mechanisms, however, are not well worked out and at least three possibilities are likely in different circumstances. First, protein kinase A is known to catalyze the phosphorylation of MLCK, once phosphorylated MLCK has a relatively lower affinity for Ca-calmodulin so that for a given concentration of Ca-calmodulin, the activation downstream is reduced. The law of mass action predicts that this inhibition should be reversed at high calcium concentrations. Other cAMP inhibitory mechanisms for which there is evidence include interference with the SR Ca storage system, and activation of a MLC phosphatase.
If the enzyme concentration is fixed at a value well below the substrate concentration, and the concentration of substrate is then titrated, one finds that the initial velocity of the reaction varies as illustrated in Figure 2.7. At the lower substrate concentrations, the initial velocity tracks linearly with substrate concentration. At intermediate values of [5], the initial velocity appear to be a curvilinear function of [5], while at higher substrate concentrations, the initial velocity appears to reach a maximum level, as if the active site of all the enzyme molecules are saturated with... [Pg.35]

Monomeric additions result in the first and second rate expressions. Polymeric molecules react with the activated intermediate at a rate that is proportional to their cumulative molar concentration P q. ... [Pg.278]

Mata, J. F., et al. Role of the human concentrative nucleoside transporter (hCNTl) in the cytotoxic action of 5[Prime]-deoxy-5-fluorouridine, an active intermediate metabolite of capecitabine, a novel oral anticancer drug. Mol. Pharmacol. 2001, 59, 1542— 1548. [Pg.274]

The kinetic analysis proves that formation of very active radical from intermediate product can increase the reaction rate not more than twice. However, the formation of inactive radical can principally stop the chain reaction [77], Besides the rate, the change of composition of chain propagating radicals can influence the rate of formation and decay of intermediates in the oxidized hydrocarbon. In its turn, the concentrations of intermediates (alcohols, ketones, aldehydes, etc.) influence autoinitiation and the rate of autoxidation of the hydrocarbon (see Chapter 4). [Pg.236]

Oscillations of concentrations of intermediates can occur when one substance in a sequence of reactions is either an activator or an inhibitor for a reaction step that occurs earlier in the sequence. Such activation or inhibition can control intermediate forming steps and will be responsible for producing oscillation in the concentration of the intermediate. The important conditions required to generate oscillations in a chemical system are ... [Pg.121]

This reaction proceeds via the transition state illustrated in Fig. 10.2. An Sn2 reaction (second order nucleophilic substitution) in the rate limiting step involves the attack of the nucleophilic reagent on the rear of the (usually carbon) atom to which the leaving group is attached. The rate is thus proportional to both the concentration of nucleophile and substrate and is therefore second order. On the other hand, in an SnI reaction the rate limiting step ordinarily involves the first order formation of an active intermediate (a carbonium ion or partial carbonium ion, for example,) followed by a much more rapid conversion to product. A sampling of a and 3 2° deuterium isotope effects on some SnI and Sn2 solvolysis reactions (i.e. a reaction between the substrate and the solvent medium) is shown in Table 10.2. The... [Pg.320]

E = Faraday constant). The equilibrium potential E is dependent on the temperature and on the concentrations (activities) of the oxidized and reduced species of the reactants according to the Nemst equation (see Chapter 1). In practice, electroorganic conversions mostly are not simple reversible reactions. Often, they will include, for example, energy-rich intermediates, complicated reaction mechanisms, and irreversible steps. In this case, it is difficult to define E and it has only poor practical relevance. Then, a suitable value of the redox potential is used as a base for the design of an electroorganic synthesis. It can be estimated from measurements of the peak potential in cyclovoltammetry or of the half-wave potential in polarography (see Chapter 1). Usually, a common RE such as the calomel electrode is applied (see Sect. 2.5.1.6.1). Numerous literature data are available, for example, in [5b, 8, 9]. [Pg.32]

As described elsewhere in this chapter, alterations in the activity of a number of lung enzymes have been described after acute and chronic ozone exposure. With the possible exceptions of the sulfhydryl-containing enzyme succinic dehydrogenase and the cytochrome P-4 en me benzopyrene hydroxylase, it is difficult to determine whether these findings are due to a direct oxidative effect of ozone or are secondary to changes in protein synthesis, concentrations of intermediates, or destruction of cells or organelles. [Pg.351]

Figure 12.16 Multiple sites of hormone effects on the same biochemical process. The hormone binds to its receptor activating the effector system which increases the activity of two separate reactions in the same biochemical pathway (process) to increase flux through the pathway. This means that the flux can change without large changes in the concentrations of intermediates in the pathway, i.e. activation of E and E4 ensures increased flux from S to the product P with little change in the concentrab ons of A, B or C. Figure 12.16 Multiple sites of hormone effects on the same biochemical process. The hormone binds to its receptor activating the effector system which increases the activity of two separate reactions in the same biochemical pathway (process) to increase flux through the pathway. This means that the flux can change without large changes in the concentrations of intermediates in the pathway, i.e. activation of E and E4 ensures increased flux from S to the product P with little change in the concentrab ons of A, B or C.
Reaetion (31) suggests acrolein as a key intermediate in SCR of NO by propylene. The formation of nitro species by this reaetion was already diseussed in the literature and evideneed by a significant reduction in the surface concentration of propylene adspecies in the presenee of a NO2 + O2 mixture[133, 134]. Note that the role of organie nitro species as active intermediate in the SCR of NO over Cu-ZSM-5 was already diseussed by Hayes et al.[135]. In addition, in TPR experiments, we observe Cu forming predominantly on the surfaee of Cu-Al-MCM-41 after exposure to CsHg, and a redox of Cu and Cu during the reaetion. It is therefore possible to postulate that the divalent eopper ion is reduced to monovalent in the conditions of the reaetion between adsorbed propylene and NO2 species. [Pg.67]

For molecules at a degree of polymerization n or larger, the mathematical model incorporates branch formation reactions which include a free radical of size j and a polymeric specie of degree of polymerization m n. The consequence is the formation of a free radical of molecular size j + m. Furthermore, due to the relatively high concentration initially of the 1,2-polybutadiene constituent at j = n, the derivation assumes that all polymeric species of size j n are unsaturated and are capable of branch and/or crosslink formation. Polymeric species are denoted by Pj free radical intermediates are described by Aj. Therefore, the first activated intermediate capable of formation by branching reactions is Ajj via Aq + Pj, -> Ajj. Conservation laws yield... [Pg.323]

This type of recurring formula represents the molar concentration of free radicals up to a degree of pol3mierization j=2n-l. At molecular weights twice that of the initial 1,2-polybutadiene, j=2n, the initial substitution of the expression for Ajj in the rate of formation due to branching occurs and results in a second major change in the overall functionality of the descriptive relationship for the concentration of activated intermediates. Consider the conservation laws at this degree of polymerization... [Pg.324]

This polarimetric method was made even more general by utilizing chiral HPLC techniques. The L-UNCAwas dissolved in the solvent at a concentration of 0.33 M at 20 °C. The tertiary amine (1.5 equiv) was added at time zero. The solution was allowed to stand for an experimentally determined delay time, during which the only process that can occur was epimerization, since there is no nucleophile present. The delay time was determined after carrying out several experiments with different delay times and chosen so as to fall within or just after the first half-life for racemization. At the end of the delay period, benzylamine was added. Benzylamine is a very powerful nucleophile that reacts virtually instantly (regardless of the type of activation) with the resulting mixture of l- and d-UNCAs to form the benzyl amides and quench the epimerization process. Thus, a snapshot of the ratio of l/d activated intermediates at the instant of benzylamine addition was obtained by measurement of the l/d ratio of the benzyl amide products. [Pg.665]

Different properties of Equation (1), particularly a number of independent parameters Kj (see Equation (2)) and relationships between them, the properties of apparent kinetic order (i.e. 51ni /51nc ) and apparent activation energy (i.e. dlnR/di—l/RT)) in terms of concentrations of intermediates and parameters of detailed mechanism have been found (see the monograph by Yablonskii et al., 1991). [Pg.54]

In the Favorski reaction [8], etbyne is coupled with a carbonyl compound in the presence of powdered alkali hydroxide suspended in an organic solvent, in which the acetylene has good solubility. Some acetylenic carbinols, derived from ketones, can be obtained in high yields by introducing acetylene at atmospheric pressure. The active intermediate possibly is a metal acetylide formed in low concentration. [Pg.80]

The compensation relationships mentioned here for the decomposition of formic acid on metals (Table III, K-R and Figs. 6 and 7) cannot be regarded as established, meaningful kinetic descriptions of the reactions concerned, since the magnitudes of the calculated values of B and e depend on the selection of data to be included in the calculation. While there is evidence of several sympathetic interrelationships between log A and E, the data currently available do not accurately locate a specific line and do not define values of B and e characteristic of each system, or for all such systems taken as a group. The pattern of observations is, however, qualitatively attributable to the existence of a common temperature range within which the adsorbed formate ion becomes unstable. The formation of this active intermediate, metal salt, or surface formate, provides a mechanistic explanation of the observed kinetic behavior, since the temperature dependence of concentration of such a participant may vary with the prevailing reaction conditions. [Pg.293]


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Activated intermediate

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