Depolymerization pathways

The degradation products of GOS were 1,3-dimethyl pyrogallol (HI), 2-(2 ,6 dimethoxy phenoxy)-2-propenal (Vni), 2-(2, 6 -dimethoxy phenoxy)-3-hydroxypropanal (XII), and GOS-Dimer. These products show that the reaction includes oxidative polymerization and the cleavage of -0-4 ether linkage following the alkyl-phenyl cleavage. This depolymerization pathway of GOS is also similar to that of SOS (Table I). 

These apparent contradictions can be rationalized in terms of a model which incorporates plasma-induced polymerization along with depolymerization. PBS has long been known to exhibit a marked temperature-dependent etch rate in a variety of plasmas. This is clearly seen in the previously published Arrhenius plots (3,7) for two different plasma conditions (Figure 1). This dependence is characteristic of an etch rate that is dominated by an activated material loss as would occur with polymer depolymerization. The latter also greatly accelerates the rate of material loss from the film. Bowmer et al. (10-13) have shown in fact that poly(butene-l sulfone) is thermally unstable and degrades by a depolymerization pathway. A similar mechanism had been proposed by Bowden and Thompson (1) to explain dry-development (also called vapor-development) under electron-beam irradiation. 

Zigmond, 1988). The ATP-hydrolysis that accompanies actin polymerization, ATP —> ADP + Pj, and the subsequent release of the cleaved phosphate (Pj) are believed to act as a clock (Pollard et ah, 1992 Allen et ah, 1996), altering in a time-dependent manner the mechanical properties of the filament and its propensity to depolymerize. Molecular dynamics simulations suggested a so-called back door mechanism for the hydrolysis reaction ATP ADP - - Pj in which ATP enters the actin from one side, ADP leaves from the same side, but Pj leaves from the opposite side, the back door (Wriggers and Schulten, 1997b). This hypothesis can explain the effect of the toxin phalloidin which blocks the exit of the putative back door pathway and, thereby, delays Pi release as observed experimentally (Dancker and Hess, 1990). 

LMWHs are formed by various methods of fractionation or depolymerization of polymeric heparin (Table 7.1). Although they are enzymatically derived from UFH, they have a different site of achon and can be administered subcutaneously. LMWHs exert their anhcoagulant effect by inhibiting factor Xa and augmenting tissue-factor-pathway inhibitor, but minimally affect thrombin or factor Ila (Figs. 7.1 and 7.2)." Thus, the PTT, a measure of antithrombin (anh-factor Ila) achvity, is not used to measure the achvity of LMWHs. 

Hirshman CA, Zhu D, Panettieri RA, Fmala CW. Actin depolymerization via the beta-adrenoceptor in airway smooth muscle cells a novel PKA-independent pathway. Am J Physiol Cell Physiol 2001 281(5) C1468-1476. 

An enzyme, acid, or oxidative "breaker" is added to effect a controlled depolymerization and thus a programmed loss of fluid viscosity. This depolymerization is timed to occur when the sandladen fluid is opposite the productive formation. The sand then drops out of suspension and is packed against the formation. The sand creates a high permeability fluid pathway from the formation... 

A probable pathway for the degradation of hemicelluloses via free-radical intermediates has been proposed by Fengel and Wegener (1989) and is shown in Figure 5.1. Hemicellulose polymers are depolymerized to form oligosaccharides and monosaccharides, which are dehydrated to form furfural (pentoses) and hydroxymethyl furfural (hexoses). 

At higher temperatures (>250°C), endothermic depolymerization reactions dominate and generate volatile anhydrosugars and related monomeric compounds, see pathway... 

Based on the rate data, it is possible to rank the relative importance of these reactions. Therefore, it follows that reactions 1 and 3 are of key importance, followed by reaction 2 and possibly reactions 4, 5 and 6. Unfortunately, little or no data exists for the latter set of reactions under the conditions employed for poly(amic acid) preparation. However, if isoimide formation were to occur to any significant degree, then reactions 5 and 6 are possible with reaction 6 being the more dominant pathway, since amine concentration due to end-groups or depolymerization would be quite low [21, 22],... 

The key concept consists of PHA biosynthesis and subsequent depolymerization of produced PHA in cells therefore, it canbe called an in vivo PHA depolymerization process (15). RHAs produced by the depolymerization of PHAs can be excreted into the medium when the further intracellular metabolism of RHAs is inhibited. Because all the PHA-producing bacteria also possess PHA degradation pathways, in vivo depolymerization of PHA can be achieved by natural PHA-producing bacteria when the proper conditions are met. 

We have recently reported the development of a process for the in vivo depolymerization of PHA in several natural PHA producing bacteria including A. latus, R. eutropha, and pseudomonads (15). Metabolic pathways involved in the synthesis and degradation of PHB are shown in Fig. 1. 

The decomposition of aromatic BMIs follows another pathway involving succinimide ring cleavage followed by carbon monoxide evolution. Moreover it must be pointed out that at temperature as low as 260 °C a depolymerization process can occur probably depending on the chain mobility as was evidenced for model compounds [81]. 

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