Chain inhibition

Elsewhere in this chapter we shall see that other reactions-notably, chain transfer and chain inhibition-also need to be considered to give a more fully developed picture of chain-growth polymerization, but we shall omit these for the time being. Much of the argumentation of this chapter is based on the kinetics of these three mechanistic steps. We shall describe the rates of the three general kinds of reactions by the notation Rj, Rp, and R for initiation, propagation, and termination, respectively. 

The use of the two isomeric acids yields an irregular chain inhibiting crystallisation. This has two consequences ... 

Figure 10.1. Schematic diagram showing inhibition of synthesis of amino acids a) single chain inhibition occurs when enzyme controlling committed step (S ) is inhibited by increasing concentrations of product AAj b) branched chain inhibition of by increased concentration of AA2 occurs at a post-branching step (sj), while permitting continued production of product of other branch (AAj). In general, each step is controlled by a single enzyme.

Structurally, the prevention of /9-oxidation was achieved by two means (a) side-chain inhibition and (b) ring inhibition. 

Mashino T, Usui N, Okuda K, Hirota T, Mochizuki M (2003) Respiratory chain inhibition by fullerene derivatives Hydrogen peroxide production caused by fullerene derivatives and a respiratory chain system. Bioorg. Med. Chem. 11 1433-1438. 

This type of effect can occur in all tissues and is caused by a metabolic inhibitor such as azide or cyanide, which inhibits the electron transport chain. Inhibition of one or more of the enzymes of the tricarboxylic acid cycle such as that caused by fluoroacetate (Fig. 6.7) also results in inhibition of cellular respiration (for more details of cyanide and fluoroacetate see chap. 7). 

It is evident from the foregoing condensations that a branched chain inhibits the formation of a cyclic dichloro-tellurium derivative containing the bivalent diketone radical. Hence j8-diketones with terminal branched chains (RR CH.CO. and RR CH.CH2.CO.) cannot be employed in the production of yellow cyclic tellurium derivatives such as are obtained from w-butyrylacetone. 

Other types of step can occur, such as inhibition, but these are not essential for the characterization of a reaction as a chain. Inhibition occurs when a chain carrier reacts with a product in the reverse of a propagation step, or with an added substance, and as a consequence the overall rate is reduced. 

Blockade of fibrinogen y-chain Inhibits fibrinogen link to their platelets receptors... 

Meixlsperger, S., F. Kohler, T. Wossning, M. Reppel, M. Muschen, and H. Jumaa. 2007. Conventional light chains inhibit the autonomous signaling capacity of the B cell receptor. Immunity 26 323-333. 

With regard to high-molecular-weight sulfur compounds, Maxted (36) demonstrated that at low temperatures the specific toxicity of sulfur increases with the size of the molecule. The molecule is adsorbed by means of sulfur atom anchorage, around which the free rotation of the carbon chain inhibits the adsorption of reactants on all of the adjacent surface. 

Above the -relaxation process, the 2,4-TDI/PTMO polymer displayed a short rubbery plateau at a storage modulus of about 5 MPa while 2,6-TDI/PTMO was capable of crystallization, as evidenced by the ac-loss process. This difference in dynamic mechanical properties demonstrates the effect of a symmetric diisocyanate structure upon soft-segment properties. As previously discussed, single urethane links can sometimes be incorporated into the soft-segment phase. The introduction of only one of these diisocyanate molecules between two long PTMO chains inhibits crystallization if the diisocyanate is asymmetric. In the case of a symmetric diisocyanate, soft-segment crystallization above Tg can readily occur. The crystals formed were found to melt about 30°C below the reported melting point for PTMO homopolymer, 37°-43°C (19), possibly because of disruption of the crystal structure by the bulky diisocyanate units. 

Ahnert-Hilger G, Weller U, Dauzenroth ME, ef al. (1989a) The tetanus toxin light chain inhibits exocytosis. FEBS Left. 242 245—8. 

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