Chain reactions suppression

The principal characteristic of induced reactions of this type which have not been stressed so far, is that the extent of the induced change greatly decreases and in most cases reaction even ceases in the presence of chain-breaking substances. The induced reaction can be suppressed by any substances reacting with chain carriers at a higher rate than does the acceptor, and the product of the reaction of the suppressor can easily react with the inductor. Since the concentration of the chain carriers is generally low, the supressors of induced chain reactions exert considerable effect even in small quantity. The effect is particularly pronounced when the suppressor reacts reversibly. 

As strong antioxidants and scavengers of superoxide, hydroxyl and peroxyl radicals, tea flavonoids can suppress radical chain reactions and terminate lipid peroxidation (Kumamoto and Sonda, 1998, Yang and Wang, 1993). 

Syntheses are limited to mercuric salts of weak acids (2,110). Generally, increasing the length of the straight alkyl chain decreases the extent of decarboxylation (e.g., Ref. 133). Electron-withdrawing substituents suppress decarboxylation. For example, mercurials are not formed with Me02C, Cl, and Me(CH2)nO substituents on the a carbon (137,148,149), but some decarboxylation occurs with these on the j8 carbon (135-137). Chain decarboxylation predominated in reactions in benzene, butyric acid [R = Me(CH2)2] (150), or acetic acid (R = Me) (124). The chain reaction was also observed for R = Me(CH2)2 in the absence of solvent and in ethylacetate or heptane solution, but in these media the radical displacement reaction was dominant (2,150). When benzene was used as solvent... 

A number of phagemid display systems that fuse Ab chains to either full-length or N-terminally deleted gp3 fusions have been described, along with the polymerase chain reaction (PCR) primer sets, restriction enzymes, and host strains required for the display of scFvs and Fabs (3—7). Some systems are commercially available. There is an important difference in whether the Ab-gp3 fusions are with the whole of the protein or with just the C-terminal domain. Fusions to the whole of gp3 require that the expression of the fusion is suppressed until after it is infected with the helper phage because of the resistance... 

In aqueous solution the photolysis of H202 may lead to a chain reaction. However, at relatively high intensities and low peroxide concentrations, the chain reaction is suppressed. In the vapor phase, the concentration of H202 is necessarily low, ca. 1 mm., and in the above study the intensity was relatively high so that photolysis could be studied in the presence of some dark reaction. These conditions are comparable with those used in the nonchain region for aqueous solutions, and the results are similar after allowance is made for the effect of solvent on the efficiency of the primary process (96). 

This phenomenon was explained by the existence of two distinct mechanisms of H2O2 photodegradation a chain mechanism at low incident photon flow and a non-chain mechanism at higher photon flows. The suppression of high quantum yields at high incident Op is possibly due to high concentrations of initial hydroxyl and hydroperoxyl radicals, which effectively terminate the radical chain reaction of the Haher-Weiss cycle (see Fig. 6-8). 

J. H. S. Green et al., J. Chem. Phys., 21, 178 (1953). These authors claimed to suppress chain reactions in EtBr, w-PrBr, w-BuBr, and f-BuBr by use of cyclohexene. It is not clear whether or not such suppression is completely effective or whether heterogeneous reaction on the glass surface persists. See also J. Chem. Soc., 2455 (1955). 

Reactions of organic compounds, especially hydrocarbons, with oxygen in the gas or liquid phase at moderate temperatures (below 150° C), are important both as industrial processes and as natural decomposition phenomena that are to be suppressed if possible. They are chain reactions, but differ from thermal cracking in that they usually requires initiation. An initiator may have been added intentionally or be present as an impurity or early minor product, possibly a hydroperoxide that had accumulated upon prolonged standing in contact with air. 

Silver vessels behave quite differently from either uncoated quartz or Pyrex, or salt coated vessels. Here the slow reaction is almost completely suppressed [41]. Added gases have little influence, and the introduction of a silica rod into the vessel fails to initiate any observable chain reaction. The explosion limits are also displaced to higher temperatures. 

Solid extinguishers such as sand or clay are also used to cover the oil or grease under a fire. They also suppress fire by blanketing. They are suitable for metal fires. Sodium and potassium bicarbonate are also used as solid extinguishers for liquid fuel. They act as chain reaction inhibitors. At high temperatures, they decompose to give carbon dioxide that itself is an extinguisher that suppresses fire. 

Low molecular weight antioxidants react with ROS in cell compartments which for some reasons are lack of antioxidant enzymes. Thus, suppression of bifurcate chain reactions of lipid peroxidation in hydrophobic core of cell membrane is mostly effectively performed by vitamin E (a-tocopherol). Interaction of lipid molecules with hydroxyl radical in the absence of vitamin E results in bifurcation of oxidative processes and formation of peroxyl and alcoxyl radicals. They are quickly accumulated in the restricted volume of the membrane and reaction began to be uncontrolled. a-Tocopherol interacts with peroxyl radicals with high affinity, reduces them and is then oxidized itself into relatively nonactive phenoxyl radical [8]. The latter can be accumulated within the bilayer until it will be returned to initial state by reduction by ascorbate [9]. Pair Vitamin E - Vitamin C is a good example of a mutual interaction between hydrophobic and hydrophilic low molecular weight antioxidants. Recently, tight relations were demonstrated for several natural antioxidants which interaction balances the red/ox state of the cell [3.5.10-12]. Figure 4 demonstrates such interaction between some of them. 

H. Oberacher, W. Parson, G. Holzl, P.J. Oefner, C.G. Huber, Optimized suppression of adducts in polymerase chain reaction products for semi-quantitative SNP genotyping byLC-MS, J. Am. Soc. Mass Spectrom., 15 (2004) 1897. 

Itransfer cyclization Free-radical chain reactions are usually terminated by abstraction of a hydrogen atom. In contrast, standard radical cyclization of the 6-iodohexyne (1) induces isomerization to the (iodomethylene)cyclopentane (2). The formation of minor amounts of the reduced product 3 can be suppressed by use of Bu3Sn—SnBu3 and irradiation (95% yield of 2). 

Allylic bromination is usually done with A-bromosuccinimide (NBS), which keeps the concentration of bromine low by reacting with the HBr formed in the first propagation step to produce the bromine needed for the second propagation step. This low bromine concentration suppresses the addition chain reaction, discussed next, by allowing time for the addition step to reverse before a bromine molecule is encountered. 

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