Activation deactivation

Disulfides. As shown in Figure 4, the and h-chains of insulin are connected by two disulfide bridges and there is an intrachain cycHc disulfide link on the -chain (see Insulin and other antidiabetic drugs). Vasopressin [9034-50-8] and oxytocin [50-56-6] also contain disulfide links (48). Oxidation of thiols to disulfides and reduction of the latter back to thiols are quite common and important in biological systems, eg, cysteine to cystine or reduced Hpoic acid to oxidized Hpoic acid. Many enzymes depend on free SH groups for activation—deactivation reactions. The oxidation—reduction of glutathione (Glu-Cys-Gly) depends on the sulfhydryl group from cysteine. 

J. B. Butt and E. E. Petersen, Activation, Deactivation, and Poisoning of Catalysts, Academic Press, San Diego, Calif., 1988. 

It can also be noted that reversible chain transfer, in RAFT and similar polymerizations, and reversible activation-deactivation, in NMP and ATRP,... 

We can distinguish several sub-classes of activation-deactivation processes according to their mechanism. These are shown in Scheme 9.1 -Scheme 9.3. 

The reversible chain transfer process (c) is different in that ideally radicals are neither destroyed nor formed in the activation-deactivation equilibrium. This is simply a process for equilibrating living and dormant species. Radicals to maintain the process must be generated by an added initiator. 

A wide range of nitroxidcs and derived alkoxyamincs has now been explored for application in NMP. Experimental work and theoretical studies have been carried out to establish structure-property correlations and provide further understanding of the kinetics and mechanism. Important parameters are the value of the activation-deactivation equilibrium constant K and the values of kaa and (Scheme 9.17), the combination disproportionation ratio for the reaction of the nilroxide with Ihe propagating radical (Section 9.3.6.3) and the intrinsic stability of the nitroxide and the alkoxyamine under the polymerization conditions (Section 9.3.6.4). The values of K, k3Cl and ktieact are influenced by several factors.11-1 "7-"9 ... 

The activity of initiators in ATRP is often judged qualitatively from the dispersity of the polymer product, the precision of molecular weight control and the observed rates of polymerization. Rates of initiator consumption are dependent on the value of the activation-deactivation equilibrium constant (A") and not simply on the activation rate constant ( acl). Rate constants and activation parameters are becoming available and some valuable trends for the dependence of these on initiator structure have been established.292"297... 

Optimal conditions for ATRP depend strongly on the particular monomer(s) to be polymerized. This is mainly due to the strong dependence of the activation-deactivation equilibrium constant (A ), and hence the rate of initiation, on the type of propagating radical (Section 9.4.1.3). When using monomers of different types, polymer isolation and changes in the catalyst are frequently necessary before making the second block... 

Thus, it seems that the concept of anomeric electronic activation-deactivation at the anomeric center taking precedence over armed-disarmed in the remainder of the pyranose ring might have reasonable validity, but, in many instances, the difference in reactivity of the p-nitrophenyl thioglycoside versus the p-acetamidophenyl thioglycoside is not enough to make this work (Scheme 1l).94... 

An alternative could be steric activation-deactivation at the anomeric centers of the donor and acceptor. An example using this concept is shown in Scheme 12. In the electronic activation-deactivation concept, deactivated p-nitrophenyl thioglycosides can, after functioning as glycosyl acceptors, be transformed into... 

This redefinition establishes the effective activation/deactivation equilibrium constant K=k k 2 = k lk2. (Note that in the cellular automata models, the rate constants k are expressed as transition probabilities per iteration Pi) Using the above redefinition, the mechanism of Eq. (9.1) becomes a set of first-order reactions... 

Liquid catalyst Low thermal stability Soluble in water Low activity per weight Small pore size Low activity Deactivates in water, but not in organic phase Medium activity... 

This same [e] experimental protocol leads to a graphical overlay plot that yields valuable kinetic information if the two experiments described in Table 50.1 are plotted together as reaction rate vs. [2], the two curves will fall on top of one another ( overlay ) over the range of [2] common to both only if the rate is not significantly influenced by changes in the overall catalyst concentration within the cycle, including catalyst activation, deactivation or product inhibition. Overlay in same excess plots, therefore, may be used to confirm catalyst robustness or identify problems such as catalyst deactivation or product inhibition. 

Equation (89) shows that the allowance for the variation of the charge of the adsorbed atom in the activation-deactivation process in the Anderson model leads to the appearance of a new parameter 2EJ U in the theory. If U — 2Er, the dependence of amn on AFnm becomes very weak as compared to that for the basic model [see Eq. (79)]. In the first papers on chemisorption theory, a U value of 13eV was usually accepted for the process of hydrogen adsorption on tungsten. However, a more refined theory gave values of 6 eV.57 For the adsorption of hydrogen from solution we may expect even smaller values for this quantity due to screening by the dielectric medium. 

Figure 4.4. (A) Stale diagram showing the loss of excitation energy via radiationless decay through the d-d state. (B) Temperature dependence of the lifetime of Ru(bpy)ji+ in a micellar media. The solid line is the best fit using a thermally activated deactivation via the d-d state. (Reprinted from Ref. 15 with permission. Copyright 1986 American Chemical Society.)...

In conclusion, nucleophilic substitution by H20, Cl, low- and high-molecular-weight thiols, and other nucleophiles plays a major role in the metabolism of platinum complexes. These reactions direct the activation, deactivation, toxification, detoxification, distribution, and excretion of platinum anticancer drugs. Given the large differences in reactivity, and the multiplicity... 

A. Knell, P. Barnickel, A. Baiker, and A. Wokaun, Co Oxidation over Au/Zr02 catalysts— Activity, deactivation behavior, and reaction-mechanism, J. Catal. 137(2), 306-321 (1992). 

In the case of hydrogenase, the substrate hydrous are always present and so at reducing potentials the enzyme will generate hydrogen. In solutions of H2, the hydrogen-oxidizing activity can also be observed. Because measurements can be made over timescales of milliseconds to hours, it was possible to observe both the extremely rapid reaction of the enzyme with hydrogen, and the slow activation/deactivation processes at more positive potentials. 

Catalysis Monitoring of preparation, activation, deactivation, study under working conditions Weckhuysen 2004" Jentoft 2009 "... 

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