Chain release mechanisms

Similar domains are known from NRPS systems in which reductase domains are sometimes used as chain release mechanisms, which release an aldehyde or primary alcohol. In the case of MOS and citrinin biosynthesis the reductive release mechanism makes good sense as this provides the products with C-1 at the correct oxidation state (Fig. 2c). 

Transfer to A1 was reported to be operative with several non-metallocene catalysts. It is the only chain-release mechanism operative with the diamido complexes MCl2 ArN(CH2) NAr catalysts, as well as with the mono-and tris(benzamidinate) catalysts, since no olefinic resonances were observed in the H or 13C NMR spectra of these polymers.275 276 This chain-release reaction is also dominant with bis(phenoxy-imine)zirconium cat-... 

The modeling of the bimolecular /3-methyl transfer to a coordinated propene molecule was attempted by Cavallo and others. DFT calculations on the CEZr-(isobutyl) system and combined QM/MM calculations on the Me2C(3-f-Bu-l-Ind)2Zr(isobutyl)+ sys-tem229 gave extremely high activation barriers, which ruled it out as a viable chain release mechanism. However, experimental evidence is building up that it can also occur via associative displacement with the monomer. 

Chain release after a 2,1 insertion has been already described in section III.F. Recent studies on chain release mechanisms have shown that, when even low amounts of secondary insertions occur, 2-butenyl end groups become relevant, and often more frequent than the vinylidene end group. This has been observed. for example, in C2-symmetric ansa-zir-conocenes such as rac-C2H4(l-Ind)2ZrCl2, rac-Czl A-... 

SCHEME 1.5 Hydride transfer chain release mechanisms (A) fl-hydride elimination (transfer to metal) (B) fi-hydride transfer to monomer (P = growing polymer chain). (Adapted with permission from Resconi, L. Cavallo, L. Fait, A. Piemontesi, F. Chem. Rev. 2000,100, 1253-1345. Copyright 2000 American Chemical Society.)... 

One very important feature of metallocene-catalyzed propylene polymerization is that more than one polymer chain is produced by each metal site. This is possible through mechanisms of chain release which follow monomer insertion, both 1,2-insertion and 2,1-insertion. Scheme 1.5 depicts the two most common chain release mechanisms f)-hydride transfer to the metaP (with or without associative displacement by incoming monomer) and -hydride transfer to the monomer. Both processes result in identical chain endgroups (vinylidene and n-propyl), but can be studied by examining the rate law for chain release. 

Another chain release mechanism is -methyl transfer from the polymer to the metal center (Scheme 1.6a). This process occurs when highly substituted Cp ligands are present" and... 

Agronomic Properties and Nutrient Release Mechanism. The conversion of UF reaction products to plant available nitrogen is a multistep process, involving dissolution and decomposition. Materials are slow to enter the soil solution by virtue of their low solubiUty. Longer polymer chain products are less soluble than shorter chains and take longer to become available to the plants. 

Once an enzyme-catalysed reaction has occurred the product is released and its engagement with the next enzyme in the sequence is a somewhat random event. Only rarely is the product from one reaction passed directly onto the next enzyme in the sequence. In such cases, enzymes which catalyse consecutive reactions, are physically associated or aggregated with each other to form what is called a multi enzyme complex (MEC). An example of this arrangement is evident in the biosynthesis of saturated fatty acids (described in Section 6.30). Another example of an organized arrangement is one in which the individual enzyme proteins are bound to membrane, as for example with the ATP-generating mitochondrial electron transfer chain (ETC) mechanism. Intermediate substrates (or electrons in the case of the ETC) are passed directly from one immobilized protein to the next in sequence. 

When the microencapsulated liposomes are left untreated the lipid bilayer provides a barrier to diffusion through which the entrapped protein does not pass until the liposomes gradually become leaky, primarily due to oxidation of the phospholipid side chains. This mechanism results in a delayed release. Triton or sonic treatment of the microencapsulated liposomes provide pulsed re ease. Since both detergent and sonication disrupt lipid bi ayers, the mechanism by which pulsed release is achieved may be that these stimuli initially disrupt the liposomes and then the lipid reforms around some of the protein solution inside the capsule, possibly in an altered lamellar form alternatively, the treatment could disrupt only the more susceptible liposomes, leading to two phases of release, first from the freed protein and later from protein that remained liposome-entrapped. 

Effect of Fiber Degradation on the Corrosion Solution. Hydrolysis and oxidation of protein and cellulose have been described in the literature primarily with the focus on degradation in industrial processing conditions. In alkaline conditions, amino acids are released from silk in a chain unzipping mechanism in acidic conditions, the scissions are random (8,9). As the polymer deteriorates, free carboxyl and amine end groups are formed. Tyrosine oxidizes to a quinone this reaction gives aged silk its yellow coloration. Amorphous areas of the fiber are attacked first. 

For 100 < P < 1000, the measured diffusion coefficients for N = P no longer follow the N 2 reptation prediction. In the same range of N values, D remains proportional to N 2 if P N, i.e. if the motion of the chains surrounding the test chain are frozen down during the diffusion time of the test chain. The comparison of the data obtained with N = P and with N P clearly puts into evidence the acceleration of the dynamics associated with the matrix chains, similarly to what has yet been observed with other polymers [11, 12, 42 to 44] or in solutions [10]. This acceleration, by a factor close to three, can be attributed to the constraint release mechanism [7, 8, 13], the effects of fluctuations of the test chain inside its tube [9] being a priori the same in the two situations P = N and P N. 

Figure 3.27 Depiction of the constraint-release mechanism of relaxation. In (a), the topological constraint imposed on chain A by chain C is released, as the end of chain C crosses under chain A. Even if C eventually re-entangles with A, chain A has been given a chance to change its orientation, as illustrated in the two-dimensional depiction in (b) (From Doi and Edwards, copyright 1986 by Oxford University Press, Inc. Used by permission of Oxford University Press, Inc.)...

There are in fact two additional processes for relaxation which are quite natural to consider. One is the renewal of conformation by release of constraints which confine each chain, arising from diffusion of the surrounding chains which supply the con-straints This constraint release mechanism would operate in liquids, where the obstacles are themselves parts of reptating chains, but not in a network. 

Free radical trap theories. Fire-retardant chemicals release free radical inhibitors at pyrolytic temperatures that interrupt the chain propagation mechanism of flammability. 

Chain release by /3-CH3 transfer to the metal is unimolecular (Scheme 16)32,207,259 ancj obviously limited to the presence of propylene or other 2-methyl-substituted a-olefins, such as isobutene,260 and 2-methyl-l,5-hexadiene.261 No /3-alkyl transfer has been reported for higher alkyls, except for /3-trimethylsilyl transfer262 and cases in which a strained ring is formed.263-265 /3-Methyl transfer is an important and sometimes prevalent cause of molecular mass depression in the case of propylene polymerization with sterically hindered metallocenes262,266,267 or high polymerization temperatures combined with low propylene concentrations.268 Eisen reported this mechanism to be the... 

Constraint release mechanism when chain B reptates away, it releases the constraint on chain A. Later, this constraint is replaced by chain C. which confines chain A in a displaced tube. 

Although we have said that translation of the codons could continue to the end of the m-RNA this does not usually happen. Instead translation is terminated by codons that do not bind to amino acyl t-RNA that is, they do not correspond to a particular amino acid, the full-stop codons referred to above. Release of the polypeptide is probably also mediated by a peptidyl transferase, but one which uses water rather than another amino acid as the peptidyl acceptor. GTP and yet another protein factor are also required for release, After chain release m-RNA and t-RNA remain bound to the ribosomes, but we know that the ribosomes can be used several times, so there must be an as yet undiscovered mechanism for releasing the RNA and regenerating free ribosomes. 

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