Stoichiometric factor of inhibition

In the above calculations the m value has often the meaning of a parameter, which specifies the molecular structure of the inhibitor (see [4]). For example, it is taken equal to double the number of OH-groups in the moleeules of phenols. However this is not an universal interpretation because actually, as objectively mentioned in [11,30], m represents a kinetic quantity. The stoichiometric factor of inhibition being a derivative of the inhibitor s molecular structure, is self-expressed through a set of reactions including the moleeule of the inhibitor and its conversion products ... 

The stoichiometric factors of inhibition and the rate constants of the ter-penephenols (TP) with isobornyl and isocamphyl substituents were determined by the reaction with peroxy radicals of ethylbenzene. The reactivity was found to decrease for o-alkoxy compared with o-alkyl substituent caused by the intramolecular hydrogen bond formation that is conformed by FTIR-spectroscopy. The inhibitory activity for mixtures of terpene-phenols with 2,6-di-ferf-butyl phenols in the initiated oxidation of ethylbenzene was also studied. In spite of the similar antiradical activities of terpenephenols with isobornyl and isocamphyl sunstituents, the reactivity of phenoxyl radicals formed from them are substantially different that is resulted from the kinetic data for mixtures of terpenephenols with steri-cally hindered phenols. 

In more quantitative terms, the analysis developed in Section 5.3.2 may be applied here. It is, however, necessary to take into account inhibition by the substrate as depicted in Scheme 5.3. At low substrate concentration, however, inhibition can be neglected. When complete control by substrate diffusion prevails, the current-potential response obeys the conditions of total catalysis, being given by equation (5.25) (dotted line in Figure 5.24), as discussed in Section 5.3.2, introducing a stoichiometric factor of 2, while the peak potential is given by equation (5.26). 

Desulfurization using cell-free extracts The first report of desulfurization by cell-free extract of R. erythropolis was by Ohshiro et al. [180], This report showed stoichiometric desulfurization of DBT by a cell-free system and identified NADH as a necessary co-factor for desulfurization. Subsequently, the enzyme activity of cell-free extracts of the strain R. erythropolis D-l was found to be inhibited by a 2-HBP, and its analog 2,2 -dihydroxybiphenyl (DBHP). Sulfate did not inhibit enzyme activity [90], further proving that its role is not in controlling enzyme activity directly but via a genetic repression mechanism as indicated above. 

It was discovered that the addition of 1,3-cyclohexadiene to the Rh -catalyzed reactions increased the rate of butadiene polymerization by a factor of over 20 [20]. Considering the reducing properties of 1,3-cyclohexadiene, this effect could be due to the reduction of Rh to Rh and stabilization of this low oxidation state by the diene ligands. With neat 1,3-cyclohexadiene, Rh is reduced to the metallic state. These emulsion polymerizations are sensitive to the presence of Lewis basic functional groups. A stoichiometric amount of amine (based on Rh) is sufficient to inhibit polymerization completely. It was also discovered that styrene could be polymerized using the Rh catalyst. However, the atactic nature of the polymer, along with the kinetic behavior of the reaction, indicated that a free-radical process, rather than a coordination-insertion mechanism, was operative. 

Consequently, the retarder may be consumed slowly while oxygen uptake is only reduced slightly, but the effect occurs well past the time at which two peroxyl radicals have been generated from the initiator for every molecule of retarder. Under these conditions, a retarder may appear to react with more than two peroxyl radicals. This situation is quite often observed and causes misinterpretation of results concerning inhibition efficiency, unless a reliable method is used to determine the stoichiometric factor and antioxidant activity (See Section II.A.)... 

Several sulfur analogs (XI, XII) were also synthesized and their reactivities measured during inhibition of styrene autoxidation. The stoichiometric factors, n, were less than 2 for these compounds, so their antioxidant activities were reported as n x values. Compounds XII are compared with a-Toc and hydroxychromans in Table 4. It is seen that in all cases the activities of the sulfur analogs are lower than the vitamin E class. 

Yamamura and coworkers used an oxygen absorption method to study the effects of a series of 46 dihydric phenols on inhibition of azo-initiated oxidation of tetralin . They reported activities in terms of the stoichiometric factor, n, and the rate of oxygen absorption, during induction periods. The 13 catechols studied all showed higher n factors (n = 2.0-2.3) and lower values than any other of the diols. Unfortunately, they were not able to obtain values. 

The rate of initiation is generally measured by an induction period method using an antioxidant (AH) that has a known stoichiometric factor n, defined as the number of radicals trapped by each molecule of antioxidant. Because a-tocopherol is known to have an n value of 2, a known concentration is used to determine the induction period, T, during which the oxidation is inhibited. 

An alternative pathway for activating the cascade has recently been demonstrated in which factor XII is absent from the reaction mixture [42-45]. Two different groups have isolated two different proteins, each of which seems to activate the HK-prekallikrein complex. One is heat-shock protein 90 [46] and the other is a prolylcarboxypeptidase [47]. Neither protein is a direct prekallikrein activator as is factor Xlla or factor Xllf because each activator requires HK to be complexed to the prekallikrein. In addition, the reaction is stoichiometric, thus the amount of prekallikrein converted to kallikrein equals the molar input of heat-shock protein 90 (or prolylcarboxypeptidase). These proteins can be shown to contribute to factor Xll-independent prekallikrein activation and antisera to each protein have been shown to inhibit the process. When whole endothelial cells are incubated with normal plasma or factor Xll-deficient plasma, the rate of activation of the deficient plasma is very much slower than that of the normal plasma, the latter being factor Xll-dependent [45]. Under normal circumstances (with factor XII present), formation of any kallikrein will lead to factor Xlla formation even if the process were initiated by one of these cell-derived factors. 

Antithrombin, already mentioned in the context of heparin, is the most abundantly occurring natural inhibitor of coagulation. It is a single-chain 432 amino acid glycoprotein displaying four oligosaccharide side chains and an approximate molecular mass of 58 kDa. It is present in plasma at concentrations of 150 pig ml 1 and is a potent inhibitor of thrombin (factor Ha), as well as of factors IXa and Xa. It inhibits thrombin by binding directly to it in a 1 1 stoichiometric complex. 

HQ inhibition of the oxidation of acrylic acid and methyl methacrylate by 2 in the presence of initiator azobisisobutyronitrile has been compared with that for oxidation of acrylic acid by 4-methoxyphenol under the similar conditions. Reaction between the semiquinone radical and oxygen decreases the stoichiometric inhibition factor and the efficiency of HQ as the inhibiting agent. ... 

There are numerous examples of enantioselective reductions employing the CBS catalyst from both academic and industrial laboratories that attest to the generality and robustness of the process [114, 120]. Corey showcased its use in an enantioselective synthesis of ginkgolide B (201), a potent antagonist of the platelet activating factor (Scheme 2.24) [135]. Treatment of 199 with catalyst 187 and BH3-THF as the stoichiometric reductant furnishes alcohol 200 in 88% yield and 93% ee. A recent application of its use drawn from the pharmaceutical industry is the synthesis of ezetimibe (204), the first drug approved for inhibition of intestinal cholesterol absorption (Scheme 2.25) [136]. 

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