Aliphatic carboxylic acid degradation

Stable metabolic associations generally between pairs of anaerobic bacteria have been termed syntrophs, and these are effective in degrading a number of aliphatic carboxylic acids or benzoate under anaerobic conditions. These reactions have been discussed in reviews (Schink 1991, 1997 Lowe et al. 1993) that provide lucid accounts of the role of syntrophs in the degradation of complex organic matter. Two examples are given here to illustrate the experimental intricacy of the problems besetting the study of syntrophic metabolism under anaerobic conditions ... 

Again the close correspondence between the measured radical and carbon dioxide yields for 7-radiolysis of the N-acetyl amino acids in the solid state suggests that the mechanisms for radical production and carbon dioxide formation are closely related, as they were for the aliphatic carboxylic acids. The following mechanism has been proposed (5.) in order to account for the major degradation products and observed radical intermediates. 

In the poly carboxylic acids, carbon dioxide is the major product of radiolysis, but the carbon monoxide yields are greater than they are for the aliphatic carboxylic acids. However, the radical yields are not greater than expected on the basis of the model compounds, which suggests that excited states play an important role in the degradation of these poly acids. 

Decarboxylation designates the loss of a carboxyl group, as a result of enzymatic microbial activity. For example, the carboxyl group of aliphatic carboxylic acids differentially degrades the molecule (Fig. 15.4). In a subsurface environment characterized by extensive microbial activity, catalytic decarboxylations for both naturally occurring and exogenous organic compounds may occur. 

The pyrolysis oil collected between 200 to 275 C in the second step was 18.14 % by wt. on an anhydrous wood basis and corresponded to the hemicelluloses degradation . In general, hemicelluloses decompose to yield furans and its derivatives as well as a series of aliphatic carboxylic acids. Hardwoods are composed of pentosan-based hemicelluloses, while hexosan-based hemicelluloses are found in softwoods. Hardwood produces more low molecular weight carboxylic acids and furans than softwood upon decomposition . In contrast, hardwood contains a lower quantity of fatty acids than softwood. Cl - C6 carboxylic acids, fatty acids (e.g. Cia), fiiran and derivatives were identified in this fraction. In the second pyrolysis step, a little rearrangement of the lignin polymer occurred which was based on the formation of small amount of phenolic compounds 0,395 % by wt., anhydrous wood basis). The main components of this fraction are listed in Table 2. Betulin and lupeol were abundant in this fraction. Furthermore, levoglucosan which has been reported earlier as a major compound in the cellulose-derived pyrolysis oils, was identified in the second... 

Barbier-Wieland degradation. Stepwise carboxylic acid degradation of aliphatic acids (particularly in sterol side chains) to the next lower homo log. The ester is converted to a tertiary alcohol that is dehydrated with acetic anhydride, and the olefin oxidized with chromic acid to a lower homologous carboxylic acid. 

Unhindered aliphatic carboxylic acids such as those found in steroidal side-chains can be degraded by three carbons by a four-step procedure (Scheme 11), via dihydro-oxazole formation, dehydrogenation, and ozonolysis,which should be applicable to a good range of (saturated) substrates. 

In the Barbier-Wieland degradation, the alpha-methylene group in an aliphatic carboxylic acid is removed in a sequence of reaction steps, effectively a chainshortening. The inverse procedure is the Amdt-Eistert synthesis, where an acid is converted into acyl halide and reacts with diazomethane to give the highest homolog. 

Figure 18.12 Effect of fluorescent device exposure on carboxylic acid production in Spectar copolymer as determined by SF4 treatment 1816cm-1, aromatic acid peak 1841 cm1, aliphatic acid peak [11]. Reprinted from Polymer, 41, Grossetete, T., Rivaton, A., Gardette, J.-L., Hoyle, C. E., Ziemer, M., Fagerburg, D. R. and Clauberg, H., Photochemical degradation of poly(ethylene terephtha-late)-modified copolymer, 3541-3554, Copyright (2000), with permission from Elsevier Science...

The preferred route for reducing the molecular weight of PVA involves chain scission at the 1,3-diketone site (see Fig. 6). As the diketone element is chemically not very stable, a spontaneous degradation of oxidised PVA was also discussed [80]. Nevertheless, the preferred degradation pathway is most likely the biochemical process because enzymes were identified that showed high activity with diketone substrates [81], especially with oxidised PVA. The p-diketone hydrolase (BDH EC 3.7.1.7) hydrolyses aliphatic p-diketones to form methyl ketones and carboxylic acids in equimolar amounts [82]. The enzymatic cleavage of C-C bonds in p-diketones is not well studied [83]. BDH enzymes could be isolated from different PVA-degrading strains, purified, characterised and cloned [84]. 

The tendency for PVC to degrade by the unzipping type of dehydrochlorination is reduced by the presence of heat stabilizers, such as barium and cadmium salts of high-molecular-weight carboxylic acids and epoxidized unsaturated aliphatic esters. The general reaction for cadmium salts is as follows ... 

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