Catalytic trigger

A major experimental issue to be addressed is the rate and means by which particles are hydrolyzed and solubilized to provide substrates for heterotrophic bacteria, and the role of free enzymes in this process. Burns (1982) reviewed the possible locations and origin of enzyme activities in soils, and particularly underscored the potential importance of enzyme-humic complexes in microbial catalysis of substrates. As Burns (1982) discussed, enzymes associated with soil particles or humic substances are not subject to the same biochemical and physical restraints as are enzymes newly produced by microbial cells. Soil-held (or sediment-held) enzymes may therefore play a catalytic trigger role in substrate degradation, providing critical signals about substrate availability to the local microbial community. The conceptual model presented by Vetter et al. (1998) suggested that release of free enzymes into the environment may in fact represent... 

With fixed [CO], and increasing [02] the steady-state reaction rate W is initially zero (the overall surface is covered with CO) and it can then jump to the value k22[CO]212kt [02]. With further increase in [02], the reaction rate varies inversely with [02]. In turn, with constant [02] the reaction rate rises quadratically with increasing [CO] and then "jumps down to zero values. This example indicates that rather simple but non-linear schemes can be characterized by complex dynamic behaviour. In radiophysical terms, scheme (119) can be called a simple catalytic trigger since, in this case, there exist two stable steady states. 

Hence we have managed to obtain the simplest detailed mechanisms providing a possibility to achieve several (in this case three) steady-states of the catalyst surface. These are three-step adsorption mechanisms (7) whose parameters satisfy definite inequalities. Using the radiophysical terminology, these mechanisms can be called the simplest catalytic triggers. It is these mechanisms that must be used to interpret experiments in which critical effects are observed. 

When the steps in mechanism (32) are reversible (k x, k 2 > 0), it can easily be shown that the qualitative character of Wfa) and W(k2) [Fig. 17(a), (b)] is preserved. Unlike the above catalytic trigger, in this case the multiplicity of steady states can also be observed at k3- co ([A] - oo), i.e. the region of parameters in which there are three steady states is infinite. 

Thus the mechanism formed by steps (l)-(4) can be called the simplest catalytic oscillator. [Detailed parametric analysis of model (35) was recently provided by Khibnik et al. [234]. The two-parametric plane (k2, k 4/k4) was divided into 23 regions which correspond to various types of phase portraits.] Its structure consists of the simplest catalytic trigger (8) and linear "buffer , step (4). The latter permits us to obtain in the three-dimensional phase space oscillations between two stable branches of the S-shaped kinetic characteristics z(q) for the adsorption mechanism (l)-(3). The reversible reaction (4) can be interpreted as a slow reversible poisoning (blocking) of... 

Similar base-catalysed isomerization of both a- and P-alkynylallylic alcohols and related allenic systems to give highly substituted furans have been extensively studied by Marshall and co-workers <93JOC3435> arguably, a culmination of these studies is the identification of 10% silver nitrate on silica as an electrophilic catalytic trigger <95JOC5966 9805263>. 

It is not impossible, for example in weather modification (rain making, hail prevention and fog dissipation), but almost through catalytic triggering. 

Electropolymerization of conducting polymers differs markedly from other polymerization reactions. In the normal electrochemically induced polymerization reactions, the electrode catalytically triggers chain growth and, consequently, the process requires htde electricity [18]. By contrast, the anodic oxidation leading... 

The reduced set of equations for the time derivatives of 8az and 8bz, Eqs. (7.123) and (7.124), with not three but two variables, in which 9(bz) is regarded as a parameter, is a catalytic trigger. For this system, a unique steady state is stable. If there are three steady states, the two outer steady states are stable and the middle steady state is unstable. 

Binding of metabolites and second messengers to sites distinct from the catalytic site of enzymes triggers conformational changes that alter or the... 

Guo, H.C. Ma, J.A. (2006) Catalytic Asymmetric Tandem Transformations Triggered by Conjugate Additions. Angewandte Chemie International Edition, 45, 354-366. 

A molecular probe with dual output signals offers two detection modes allowing use of the same probe in different environments. We have demonstrated how an AB2 self-immolative dendron with double quinone methide release mechanism can be applied to create a molecular probe with UV-Vis and fluorescence modes for the detection of a specific catalytic activity.15 The molecular probe is illustrated in Fig. 5.36. The central unit of the probe (the molecular adaptor) is linked to an enzymatic substrate that acts as a trigger and to two different reporter molecules. Cleavage of the enzymatic substrate triggers the release of the two reporters and a consequent activation of their signals. 

Iridium as an electrode material has received considerable attention in the last decade not only because of its excellent catalytic properties but also in relation to the electrochromic effect observed for anodic iridium oxide films (AIROF). Electrochromism of iridium was thought to be of technical relevance for display applications and triggered several studies of the electrochemical and optical properties of AlROFs [67, 85-88],... 

In a similar way as described for the hydroformylation, the rhodium-catalyzed silaformylation can also be used in a domino process. The elementary step is the formation of an alkenyl-rhodium species by insertion of an alkyne into a Rh-Si bond (silylrhodation), which provides the trigger for a carbocyclization, followed by an insertion of CO. Thus, when Matsuda and coworkers [216] treated a solution of the 1,6-enyne 6/2-87 in benzene with the dimethylphenylsilane under CO pressure (36 kg cm"2) in the presence of catalytic amounts of Rh4(CO)12, the cyclopentane derivative 6/2-88 was obtained in 85 % yield. The procedure is not restricted to the formation of carbocycles rather, heterocycles can also be synthesized using 1,6-enynes as 6/2-89 and 6/2-90 with a heteroatom in the tether (Scheme 6/2.19). Interestingly, 6/2-91 did not lead to the domino product neither could 1,7-enynes be used as substrates, while the Thorpe-Ingold effect (geminal substitution) seems important in achieving good yields. 

The binding of the correct substrate triggers a change in the structure of the enzyme that brings catalytic groups into exactly the right position to facilitate the reaction. 

Table IV and Fig. 3 provide a comparison of the effectiveness of some of the activators. These comparisons are based on the amounts of C6 dienes produced within a given period of time after the addition of the organic chloride to a totally inactive Rh1 complex. As can be seen from Fig. 3, the very best activator triggers the catalytic reaction with almost unnoticeable induction period, while a substantial induction period is apparent with the least efficient activator.

As shown in Section 2.2.7, chemical reactions may be triggered by electrons or holes from an electrode as illustrated by SrnI substitutions (Section 2.5.6). Instead of involving the electrode directly, the reaction may be induced indirectly by means of redox catalysis, as illustrated in Scheme 2.15 for an SrnI reaction. An example is given in Figure 2.30, in which cyclic voltammetry allows one to follow the succession of events involved in this redox catalysis of an electrocatalytic process. In the absence of substrate (RX) and of nucleophile (Nu-), the redox catalysis, P, gives rise to a reversible response. A typical catalytic transformation of this wave is observed upon addition of RX, as discussed in Sections 2.2.6 and 2.3.1. The direct reduction wave of RX appears at more negative potentials, followed by the reversible wave of RH, which is the reduction product of RX (see Scheme 2.21). Upon addition of the nucleophile, the radical R is transformed into the anion radical of the substituted product, RNu -. RNu -... 

Catalysis by various low-valent metalloporphyrins of the type already depicted in Section 3.7.2 (see reference lb for a precise list) is represented in Figures 4.3 and 4.4 for several cyclic and acyclic 1,2-dibromides. A striking example of the contrast between redox and chemical catalyses is shown in Figure 4.3a, with fluorenone anion radical on the one hand and iron(I) octaethylporphyrin on the other. Starting with the oxidized, inactive form of the catalyst, in each case—the active form is produced at a reversible wave. Addition of the same amount of 1,2-dibromocyclohexane triggers a catalytic increase in the current that is considerably less in the first... 

In the converse case where substrate diffusion in the film is fast, the system passes from zone R to zone E+R and then to zone ER. This is again a purely kinetic situation, but it now involves the catalytic reaction and electron diffusion rather than substrate diffusion in the preceding case. At this point, interference of substrate diffusion triggers the passage from zone ER to zone ER+S and ultimately to zone S, where the response is controlled entirely by substrate diffusion. 

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