Common inhibitors

Polymerization inhibitors are key additives which prevent premature gelation of the adhesive. The foimulator must carefully balance shelf stability and the required cure on demand. Due to its high propagation rate, MMA is difficult to inhibit. Some comments on specific inhibitors follow. The most common inhibitor to be found in component monomers is 4-methoxyphenol, which is also called the methyl ether of hydroquinone. This inhibitor is effective only in the presence of oxygen. A mechanism has been proposed, and is illustrated in Scheme 13 [128]. 

The most common inhibitor in field gas situations, where the inhibitor will not be recovered and reused, is methanol. It is a relatively inexpensive inhibitor. Methanol is soluble in liquid hydrocarbons, about 0.5% by weight. If there is condensate in the stream, additional methanol is required because some of that methanol will dissolve in the condensate. Also, some of the methanol vaporizes and goes into the gas state. 

Common inhibitors include stable radicals (Section 5.3.1), oxygen (5.3.2), certain monomers (5.3.3), phenols (5.3.4), quinones (5.3.5), phenothiazine (5.3.6), nitro and nitroso-compounds (5.3.7) and certain transition metal salts (5.3.8). Some inhibition constants (kjkp) are provided in Table 5.6. Absolute rate constants (kj) for the reactions of these species with simple carbon-centered radicals arc summarized in Tabic 5.7. 

Tabic 5.7 Absolute Rate Constants k2) for the Reaction of Carbon-Centered Radicals with Some Common Inhibitors... 

The absolute rate constants for attack of carbon-centered radicals on p-benzoquinone (38) and other quinones have been determined to be in the range I0M08 M 1 s 1.1 -04 This rate shows a strong dependence on the electrophilicity of the attacking radical and there is some correlation between the efficiency of various quinones as inhibitors of polymerization and the redox potential of the quinone. The complexity of the mechanism means that the stoichiometry of inhibition by these compounds is often not straightforward. Measurements of moles of inhibitor consumed for each chain terminated for common inhibitors of this class give values in the range 0.05-2.0.176... 

Enzyme Substrate Common Inducers Common Inhibitors... 

Comparisons of affinity among different inhibitors for a common enzyme, or among different enzymes for a common inhibitor, are best done in terms of the relative dissociation constants or the related Gibbs free energy of binding. 

Reactive metabolites of xenobiotics may differ in reactivity, and therefore have varying impact on enzymatic activities in terms of proximity to their origin. For example, some intermediates are highly reactive and directly inhibit the enzyme that leads to their formation. These substances are commonly referred to as suicide inhibitors, for obvious reasons. Some suicide inhibitors, such as piperonyl butoxide (PBO), a pesticide synergist) are common inhibitors of certain CYP isozymes. PBO amplifies the toxicity of certain insecticides by inhibiting the insect s CYP enzymes that are involved in its degradation. It is metabolized to a highly reactive carbene, which forms an inhibitor complex with the heme iron of CYP, as shown in Scheme 3.6. 

This chapter will look at some of the most useful early cooling water treatments and later discuss many of the common inhibitor materials used plus various more modem programs, including their formulations and application. 

Figure 9.6 Some common inhibitors of xenobiotic-metabolizing enzymes.

Cyt 1A2 metabolizes clozapine common substrates - amitriptyline, clomipramine, propranolol, theophylline, warfarin, caffeine. Common inhibitors - fluvoxamine, paroxetine. 

Cyt 2D6 metabolizes haloperidol, risperidone, thioridazine, sertindole, olanzapine and clozapine common substrates - fluoxetine, paroxetine, sertraline, venlafaxine, amitriptyline, clomipramine, desipramine, imipramine, nortriptyline, propranolol, metoprolol, timolol, codeine, encainide, flecanide. Common inhibitors - paroxetine, sertraline, fluoxetine. 

Cyt 3A3/4 metabolizes clozapine, sertindole, quetiapine common substrates -tricyclic antidepressants, nefazodone, sertraline, carbamazepine, ethosuximide, terfenadine, benzodiazepines, diltiazem, nifedipine, verapamil, erythromycin, cyclosporine, lidocaine, quinidine, cisapride, paracetamol. Common inhibitors -nefazodone, fluvoxamine, fluoxetine, ketoconazole. 

Know the common inhibitors of protein synthesis and how they work. 

The common inhibitor for all exopolyphosphatases was heparin, which suppressed the activities of both sub-cellular preparations and purified enzymes as a competitive inhibitor (Table 6.4.) (Andreeva et al, 1994). 

Some reagents react with the initiating radical to give unreactive substances, a process known as inhibition. A common inhibitor for vinyl polymerisations is hydroquinone, which reacts by the transfer of two hydrogen radicals to the initiator radicals (Fig. 2.4). This gives quinone and unreactive initiator and has the net effect of causing a lag time in the polymerisation and a decrease in the initiator concentration. Monomers are often stored in the presence of inhibitor in order to prevent polymerisation. The amount and type of inhibitor may vary depending on the monomer batch and the manufacturer. For inter-laboratory comparisons of materials to be possible, it is therefore important to remove the inhibitor and purify the monomers prior to use [13]. 

Inhibitors of translation - A number of the common inhibitors of prokaryotic translation are also effective in eukaryotic cells. They include pactamycin, tetracycline, and puromycin. Inhibitors that are effective only in eukaryotes include cycloheximide and diphtheria toxin. Cycloheximide inhibits the peptidyltransferase activity of the eukaryotic ribosome. Diphtheria toxin is an enzyme, coded for by a bacteriophage that is lysogenic in the bacterium Corynebacterium diphtheriae. It catalyzes a reaction in which NAD+ adds an ADP ribose group to a specially modified histidine in the translocation factor eEF2, the eukaryotic equivalent of EF-G (Figure 28.36). Because the toxin is a catalyst, minute amounts can irreversibly block a cell s protein synthetic machinery. As a result, pure diphtheria toxin is one of the most deadly substances known. 

Recall What are the two most common inhibitors of steps of the citric acid cycle and the reaction catalyzed by pyruvate dehydrogenase ... 

Cellobiose must be cleaved into its constituent monosaccharides in order to be metabolized by . coli. There are two main ways in which this reaction can occur, hydrolysis and phosphorolysis. P-Glucosidase and cellobiose phosphorylase from Saccharophagus degradans were expressed in E. coli. The results showed that phosphorolysis cells tolerate common inhibitors (sodium acetate) more effectively and produce recombinant proteins more effectively than hydrolysis cells. However, hydrolysis cells utilize xylose more effectively in combination with cellobiose [192]. 

Recently, it has been postulated that even using the common inhibitor aspirin it should nonetheless be possible to interfere more or less selectively with the thromboxane biosynthesis by platelets. The inhibition of the vessel wall cyclo-oxygenase was found to be of shorter duration than that in platelets [401,402] this is expected since the nucleated endothelial cells can rapidly synthesize new enzyme. Furthermore, it has been demonstrated that the platelet cyclo-oxygenase is much more sensitive to aspirin, requiring both lower concentration and shorter exposure to the drug for complete inhibition than does the vascular enzyme [403]. 

Selecting a suitable inhibitor for a corroding system can be a daunting exercise. There are no hard and fast rules, and equations or theories to guide the choice are very limited. Hence, the choice of an inhibitor to be used in the field is a major challenge because synergism is often present between different inhibitors and the environment to be controlled. However, mixtures of inhibitors are the usual choice in commercial formulations. Here, we have described some corrosive systems and the most common inhibitors used to protect them. Efforts are still being carried on by corrosionists to formulate and compound more effective inhibitors for varied environments. 

These inhibitors are film-forming componnds that have a general action over the metal surface and therefore interfere with both anodes and cathodes indirectly. The most common inhibitors of this class are the silicates and phosphates (French et al. 1989). 

Cationic polymerizations are not affected by common inhibitors of free-radical polymerizations. They can, however, be greatly influenced by impurities that can act as ion scavengers. These... 

Chapter 4 A section on free-radical inhibitors was added to show students how some of the common inhibitors break the free-radical chain reaction, and their importance in chemistry and biochemistry. 

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