Depolymerization of dimeric

R. P. Bell and E. C. Baughan [J. Chem. Soc., 1937, (1947)] investigated the generalized acid-base catalysis of the depolymerization of dimeric dihydroxyacetone. In terms of the general formulation of the first-order rate constant... 

The depolymerization of dimeric dihydroxyacetone and glycollalde-hyde ° has been studied in aqueous solution, and again shows general acid-base catalysis. Qualitatively similar catalytic behaviour has been observed in the mutarotation of optically active a-keto-esters in alcoholic solution,and in exchange reactions between alcohols and esters, both of which presumably take place by the reversible addition of alcohol to the carbonyl group. 

Swinholide A, isolated from the marine sponge Theonella swinhoei, sequesters actin dimers and induces their formation. One molecule of swinholide A binds to one dimer. In addition, swinholide A can sever F-actin by binding to the neighbouring protomers. Increased depolymerization of F-actin has also been reported. 

Intrigued by the finding that Eca PLs exhibit notable differences in their kinetics, HPAEC analyses were carried out to examine the products from the depolymerization of PGA and 31% esterified pectin. After 18 h of incubation with PGA, PL1 and PL2 had produced mainly di- and trimers. Similariy, main products of PL3 action were trimers, followed by dimers. Moreover, it was the only enzyme found to produce monomers from unesterified substrates with a degree of polymerization >3. Using 31% esterified pectin as a substrate, similar end products were released by the PLs as from PGA. In addition to the products described, traces of tetra- up to octamers were detectable. While PL1 and PL2 released di- and trimers at almost... 

Recently, another class of MAPs has been identified that lead to depolymerization of MT, stathmin and katanin [11], Stathmin destabilizes MT either by reducing available tubulin dimer or by altering the frequency of catastrophe at MT plus ends [18]. In contrast, katanin is an ATP-ase that severs MT [19]. In neurons, one important role of katanin is to release the MT from the MTOC and allow transport of the assembled MT into axons and dendrites. 

Colchicine (6) is used in the treatment of a broad range of diseases including acute gout and Mediterranean fever [28] and induces depolymerization of tubulin. This compound (6) distorts the tubulin/microtubule equilibrium by binding to the tubulin dimer and halting mitosis in the metaphase. The reason this approach is such a successful target in cancer therapy is that... 

Microtubules have a key role in mitosis and cell-proliferation. They are dynamic assemblies of heterodimers of a and f3 tubullin. In the cell-reproduction cascade tubulin polymerizes fast and subsequently depolymerizes. Tubulin dimers are unusual guanyl nucleotide binding (G) proteins, which bind GTP reversibly at a site in the (3-tubulin. GTP irreversibly hydrolyzes to GDP during polymerization. 

Figure 3.4 HPLC trace showing the formation of 5 % PET/PBT cyclic oligomers from the depolymerization of 5% PET/PBT. Cyclic dimers are indicated at 3.0 and 4.0 min, with trimers appearing at 5.0 and 6.0min, etc. the small peaks represent mixed co-cyclics...

A possible reductive role for veratryl alcohol oxidase is proposed in Figure 5. Laccases from C. versicolor can produce both polymerization and depolymerization of lignin (29). In phenolic lignin model dimers, laccase can perform the same electron abstraction and subsequent bond cleavage as found for lignin peroxidase (30). The phenolic radical is however likely to polymerize unless the quinoid-type intermediates can be removed, for example by reduction back to the phenol. Veratryl alcohol oxidase, in... 

The best known drugs acting as antimitotics are the vinca alkaloids, vincristine (7.90) and vinblastine (7.91). They are very complex indole derivatives that nevertheless have been synthesized. Both are quite effective in various leukemias and in Hodgkin s lymphoma, but show considerable neurotoxicity. Vinblastine and vincristine bind specifically to the microtubular protein tubulin in dimeric form, precipitating depolymerization of the microtubules and functionally acting as a mitotic poison. Vinorelbine (7.92) is a semisynthetic vinca alkaloid functionally identical to vinblastine. 

If, as a criterion of value to the study of lignin depolymerization by alkaline hydrolysis, the maximum yield of oxygen-bearing, phenylpropanoid derivatives is chosen, then the conditions of such a study have been optimized here at a treatment severity corresponding to a reaction temperature of 300°C for 1 hour. Under these conditions, 20% of the lignin is recovered as ether-solubles of which 55% is identifiable as monomeric derivatives. The rest of this material probably consists of dimeric-type compounds not identified by capillary gas chromatography. 

Thioacidolysis allows the distinction between products derived from lignin and products derived from />coumaric and ferulic acids, and the distinction between products derived from cinnamaldehydes and cinnamyl alcohols. Recent improvements have made it possible to estimate the fraction of free phenolic groups in uncondensed lignin (see Section 1.3.1), and to depolymerize the dimers, so that they can be included in the analysis of the lignin composition. 

The structures of complexes of drugs with tubulin dimers, microtubules and, to some extent, with zinc-sheets of tubulin need to be further explored to reveal the structural basis of polymerization and depolymerization of tubulin. Still, there is another level of insight, namely which one of these structures (or further ones yet to be discovered) represents optimally the conformation responsible for the clinical effects of the drug [78],... 

Obolentsev (264) has published researches on the conversions of individual hydrocarbons over silica-alumina. Depolymerization of triisobutylene as the temperature was raised from 200 to 365° resulted in a decrease of the liquid products from 45 to 60 % progressively to 24 to 26 % with a corresponding increase in the yield of the gaseous products the dimer was also depolymerized, although other workers reported it to be stable. A mechanism was postulated which involved the intermediate formation of an alkylated cyclobutane (264). Isopropylbenzene was dealkylated with simultaneous disproportionation to diisopropylbenzene at 350-450° (266). Disproportionation may become the principal reaction at the higher temperature (267). 

Examples for the treatment of other ketenes are few. The ordinary ketene lamp has been modified for the depolymerization of dimethylketene dimer (86%). Ethylcarbethoxyketene can be obtained from its dimer in 80-90% yields by heating at 180-200° under a pressure of 15 mm. ... 

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