Multiple chain termination

Cyclic chain termination by antioxidants. Oxidation of some substances, such as alcohols or aliphatic amines, gives rise to peroxyl radicals of multiple (oxidative and reductive) activity (see Chapters 7 and 9). In the systems containing such substances, antioxidants are regenerated in the reactions of chain termination. In other words, chain termination occurs as a catalytic cyclic process. The number of chain termination events depends on the proportion between the rates of inhibitor consumption and regeneration reactions. Multiple chain termination may take place, for instance, in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and variable-valence metal compounds. 

The oxidation of some classes of substances (alcohols, aliphatic amines) gives peroxyl radicals, which possess both oxidative and reductive actions. In these systems, a several inhibitors terminate chains and are regenerated again in acts of chain termination catalytic chain termination takes place. The number of chain terminations depends on the ratio of the rates of inhibitor regeneration to its irreversible consumption. In several cases, multiple chain termination is observed in polymers. Inhibitors of multiple chain termination are aromatic amines, nitroxyl radicals, and compounds of variable-valence metals. 

Template binding RNA polymerase (RNAP) binds to DNA and locates a promoter (P) melts the two DNA strands to form a preinitiation complex (PIQ. (2) Chain initiation RNAP holoenzyme (core + one of multiple sigma factors) catalyzes the coupling of the first base (usually ATP or GTP) to a second ribonucleoside triphosphate to form a dinucleotide. (3) Chain elongation Successive residues are added to the 3 -OH terminus of the nascent RNA molecule. (4) Chain termination and release The completed RNA chain and RNAP are released from the template. The RNAP holoenzyme re-forms, finds a promoter, and the cycle is repeated. 

C domains that oatalyzed the formation of multiple amide bonds Transglutaminases Chain Termination Strategies Thioesterase-catalyzed chain release Alternative chain release through reduction Condensation domains as chain termination catalysts Diketopiperazine formation Oxidative ohain termination... 

Modification experiments and primer extension analyses were performed on RNA from all four plant species. Representative gels are shown in Fig. 4. A large number of stops occur in the native RNA without DMS treatment. This phenomenon has been observed reproducibly in multiple RNA preparations using a variety of polymerization temperatures, and it may be due to a strong secondary structure within the rRNA that cannot be sequenced by reverse transcriptase. Also, there are many naturally occurring modified base residues within the rRNA sequences that can impede reverse transcriptase.23 Another possibility is that a portion of the RNA in each preparation is degraded at specific sensitive sites, and thus chain terminations result. Similar patterns of termination have been ob-... 

In terms of potential chemical interactions, the effects of NO on ROS-induced injury are multiple, and some effects can be classified as prooxidant and others may be classified as antioxidant still others can be classified as both. In terms of the metal-catalyzed Haber-Weiss reaction, there are two primary effects of -NO. The binding of NO to metal ions will prevent the Fenton reaction and thus results in an antioxidant action. Another important antioxidant action of NO (and its oxidized product N02) is its reaction with hpid radicals, thus resulting in radical chain termination. ... 

There are several ways in which block copolymers can be made. The three main methods are (1) sequential addition of monomers, (2) the preparation of a functionalized polymer followed by the use of the functionalized polymer as a macroinitiator or chain-stopper for initiation or termination of polymerization of the second monomer, and (3) use of a multiple-headed initiator. The purity of the block copolymers produced in these processes is dependent upon the livingness (lack of side reactions that lead to termination) of the chemistry used to make them. If the integrity of the chain-ends is maintained throughout the polymerization because all possible termination mechanisms are absent or eliminated, then pure block copolymers can be produced. If, however, impurities get into the process or if there are side reactions that lead to chain termination, the resulting block copolymers are contaminated with some homopolymer. Depending upon the application, some contamination of homopolymer in the block copolymer may be acceptable. 

H2 addition is different from other methods of chain termination. For example, when the reaction temperature is raised, the termination rate is multiplied by some factor, which is determined by the activation energy. If the responses of all the sites are approximately the same, the MW distribution is shifted intact to lower MW, without distortion. (To understand why, note that the GPC curve is plotted on a logarithmic MW scale, and the multiplication of the various termination rates by a constant factor amounts to an additive shift across the MW distribution.) However, when H2 is added to the reactor, a new termination pathway is opened, adding to the overall termination rate. If each site is affected equally, then a constant increment is added to the termination rate at each site, which means that the MW distribution should be distorted. High-MW sites should respond proportionately more than the others. 

Instability typically arises from the interaction of two phenomena with different dependences on a reaction parameter In a nonisothermal reaction, the dependence on temperature is exponential for heat generation by the reaction, but linear for heat loss to the cooling coil or environment in a reaction with chain branching, the dependence on radical population is exponential for acceleration by branching, but quadratic for chain termination. A reaction is unstable if acceleration outruns retardation. This can cause an explosion or, in a CSTR, lead to multiple steady states. Feinberg s network theory can help to assess whether an isothermal reaction admits multiple steady states in a CSTR. 

Furthermore, the 3 -azido functional moiety [—= N = N] predominantly prevents the formation of a 5, 3 -phosphodiester bond and, therefore, AZT gives rise to DNA-chain termination effectively, thereby producing an incomplete proviral DNA substantially. Besides, AZT-monophosphate also executes competitive inhibition of the specific cellular thymidylate kinase, thereby lowering the intracellular levels of thymidine triphosphate. It has been reported that the point mutations at multiple sites prevailing in the reverse transcriptase invariably causes resistance that may ultimately lead to a rather lower degree of affinity for AZT. ... 

The hypothesis of multiple build-in where a chain can interact with a chain Cj leads one to reflect on the possibility of a chain termination by combination. If reactions were occurring in which termination could occur by simple desorption and also by combination, two peaks would be observed. The second maxima would have a center at approximately twice the value of the flrst, as doubling of the most prevalent adsorbed chain lengths is likely (25). Furthermore, secondary events such as those discussed above or chain transfer could cause the distributions of the two peaks to be different from one another. Thus the fact that secondary reactions during Fischer-Tropsch synthesis occur and that multiple build-in and termination by combination are viable propositions help rationalize distributions that do not follow the Schulz-Flory law and appear with more than a single maximum. 

In conclusion, it appears that there are multiple factors responsible for chain termination in plant lipids, particularly at the Cm and Cm levels. The speciflcity of a number of enzymes, and these include stearoyl-ACP desat-urase, oleoyl-ACP hydrolase, elongases employing either malonyl-ACP or malonyl-CoA, and reductases, as well as membrane barriers which permit the flow of a specific free acid from one site to another reactive site, is proba-... 

The proposed kinetic model of the postpolymerization describes the multiple experimental data well and is in good agreement with all the characteristics of the postpolymerization kinetics listed above. However, the introduction of two types of radicals sharply differing by characteristic life times into a kinetic scheme is an inevitable simplification of a real set of characteristic life times of active radicals. Fhrthermore, it cannot be indirectly re-passed on the kinetics of monofunctional monomer postpolymerization which, the same as stationary kinetics, can be characterized by differences from the kinetics of bifunctional monomer postpolymerization. The term hionomolecular chain termination , introduced in Refs. [ 55, 56] as an active center of the radical self-burial act in the act of chain propagation, did not have a theoretical basis via the relation of kx with k. ... 

Fraser, N. W., Sehgal, P. B., and Darnell, J. E., 1979, Multiple discrete sites for premature RNA chain termination in adenovirus-2 infection Enhancement by 5,6-dichloro-l-p-ribofuranosylbenzimidazole, Proc. Natl. Acad. Sci. USA 76 2571-2575. 

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