Elongation reactions, hydrocarbon

Vaz A. H., Blomquist G. J., Wakayama E. J. and Reitz R. C. (1987) Characterization of the fatty acyl elongation reactions involved in hydrocarbon biosynthesis in the housefly Musca domestica. Insect Biochem. 18, 177-184. 

Elongation Reactions Involved in Hydrocarbon Biosynthesis in Insects... 

To date, the elongation reactions involved in hydrocarbon biosynthesis have only been examined in vitro in the housefly (23,... 

Fig. 1. The n-3/n-6 metabolic pathways. Precursors of the n-3 (18 3n-3, linolenic acid) and n-6 (18 2n-6, (/.-linoleic acid) are converted by a series of desaturation and (adding double bonds) and elongation (adding carbon atoms to the hydrocarbon backbone) reactions. Note that the same enzymes catalyze n-3 and n-6 desaturation and elongation reactions. Major metabolites are indicated. PUFAs with 20-carbon backbones (20 4n-6, arachidonic acid, and 20 5n-3, eicosapentaenoic acid) are precursors to the eicosanoids (prostaglandins, leukotrienes, thromboxanes). Docosahexaenoic acid (22 6n-3) is also indicated. Note that only a limited part of the metabolic pathway is shown in this figure.

Under the conditions where the chain oxidation process occurs, this reaction results in chain termination. In the presence of ROOH with which the ions react to form radicals, this reaction is disguised. However, in the systems where hydroperoxide is absent and the initiating function of the catalyst is not manifested, the latter has a retarding effect on the process. It was often observed that the introduction of cobalt, manganese, or copper salts into the initial hydrocarbon did not accelerate the process but on the contrary, resulted in the induction period and elongated it [4-6]. The induction period is caused by chain termination in the reaction of R02 with Mn"+, and cessation of retardation is due to the formation of ROOH, which interacts with the catalyst and thus transforms it from the inhibitor into the component of the initiating system. 

With no sufficient hydrogen present, the molecules get stuck on the surface. Owing to purely statistical reasons (Scheme I), this is more probable in an elongated position. Such molecules may combine with each other to give high molecular weight polymers ( coke ). Metal-catalyzed polymerization has actually been observed with lower molecular weight hydrocarbons (61). Such reactions are responsible for more rapid deactivation of the catalyst by trans isomers (Table III). 

The pathway The first committed step in fatty acid biosynthesis is the carboxylation of acetyl CoA to form malonyl CoA which is catalyzed by the biotin-containing enzyme acetyl CoA carboxylase. Acetyl CoA and malonyl CoA are then converted into their ACP derivatives. The elongation cycle in fatty acid synthesis involves four reactions condensation of acetyl-ACP and malonyl-ACP to form acetoacetyl-ACP releasing free ACP and C02, then reduction by NADPH to form D-3-hydroxybutyryl-ACP, followed by dehydration to crotonyl-ACP, and finally reduction by NADPH to form butyryl-ACP. Further rounds of elongation add more two-carbon units from malonyl-ACP on to the growing hydrocarbon chain, until the C16 palmitate is formed. Further elongation of fatty acids takes place on the cytosolic surface of the smooth endoplasmic reticulum (SER). 

The regulation of the chain length specificity to produce the specific blend of hydrocarbons often used in chemical communication appears to reside in the microsomal fatty acyl-CoA elongase reaction and not in the reductive conversion of fatty acyl-CoAs to hydrocarbon. The American cockroach produces three major hydrocarbons n-pentacosane, 3-methylpentacosane and (Z,Z)-9,12-heptacosadiene (Jackson, 1972). Studies with microsomes from integument tissue showed that stearyl-CoA was elongated up to a 26 carbon acyl-CoA that could serve as the precursor to n-pentacosane. In contrast, linoleoyl-CoA... 

In the elementary reactions of the pyrolysis, the atomic carbon is formed first. Then it transforms into the final product, whether it be soot, graphite, carbon nanofibers, or so forth. Why does the presence of catalysts make it possible to grow carbon nanofibers or nanotubes instead of soot In many cases, this is the so called carbide cycle that is characteristic of the catalytic process of hydrocarbon pyrolysis that is responsible for the growth of the elongated structures but not soot particles. The primary car bon atoms produced by pyrolytic decomposition of the hydrocarbon molecules are dissolved in the metal particle of the active catalyst compo nent to form a nonstoichiometric carbide (the carbon solution in the... 

Both turbulent burners and premix burners have been used for atomic fluorescence. The premix burner is usually round in shape (a modification of the Meker-type burner), since this provides better geometry for fluorescence than does a slot burner. For an optimum detection limit, the premix burner is also shielded that is, an inert gas such as argon or nitrogen is directed in a sheath around the flame. This elongates the interconal zone and lifts the secondary reaction zone above the burner, separating it from the lower part of the interconal zone where the excitation beam passes. The result is less background emission and less noise, particularly in hydrocarbon flames like air-acetylene or nitrous oxide-acetylene. The premix burner, especially when shielded, appears to offer increased sensitivity over the turbulent burner. 

Sinha and Keinan have reported a second synthesis of 100, which makes use not of building blocks, but of auxiliaries from the chiral C pool (AX as well as ent-AX). The synthesis begins with a reaction sequence that follows the Ci3 + C2 + Cj pattern to convert the C13 aldehyde into the hydrocarbon. This is then transformed by a threefold Sharpless asymmetric dihydrox-ylation (and partial acetalization) into the highly functionalized Cjg intermediate. The latter compound, after another chain elongation by means of the same C2 building block as used earlier, furnishes the Cig intermediate, and this then provides 100 by a three-step route (Scheme 49). 

Classification Thermoplastic polyamide Definition Polymeric amide formed by the reaction of adipic acid with hexylenediamine Empihcai (Ci2H22N202)x Formula [NH(CH2)6NHCO(CH2)4CO]n Properties Cryst. solid sol. in phenol, cresols, xylene, formic acid insol. in alcohol, esters, ketones, hydrocarbons dens. 1.090 m.p. 264 C tens. str. 80 MPa tens. mod. 3000 MPa elong. 

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