Tricarboxylic acid moiety

The AAL-toxins are hydroxylated, long-chain alkylamines with one esterified pro-pane-1,2,3-tricarboxylic acid moiety. The difference between TA and TB is that TB lacks a hydroxyl group at R4 (Fig. 1). TA and TB are equally active against susceptible tomatoes (56, 58). However, removal of the hydroxyl group at R3, which produces TC, renders the compound much less active (300 ng vs. 10 ng per plant to cause visible symptoms) (28). TD and TE are acetylated forms of TB and TC, respectively. They represent up to 40% of the AAL-toxins produced by fermentation in liquid medium. TD and TE are essentially inactive in phytotoxicity testing and are probably metabolic products from detoxification of the active metabolites (28). 

The cycle starts with reaction between the acetyl moiety of acetyl-CoA and the four-carbon dicarboxylic acid ox-aloacetate, forming a six-carbon tricarboxylic acid, citrate. In the subsequent reactions, two molecules of COj are released and oxaloacetate is regenerated (Figure 16-1). Only a small quantity of oxaloacetate is needed for the oxidation of a large quantity of acetyl-CoA oxaloacetate may be considered to play a catalytic role. 

Glutarimides may be regarded as oxidized piperidines, and many drugs containing this moiety are sedatives and anticonvulsants. A spiro derivative, alonimid (105) is such a sedative-hypnotic agent. It can be prepared by K t-butoxide catalyzed biscyano-ethylation of phenylacetonitrile, leading to 101. Alkaline hydrolysis produces tricarboxylic acid 102 which is smoothly Converted to the glutaric acid anhydride (103) with acetic anhydride. Friedel-Crafts... 

Qjiinoproteins are a class of enzymes with cofactors possessing a quinone moiety, which participates directly in catalysis. Several dehydrogenases have been characterized which possess the dissociable cofactor 4,5-dihyro-4,5-dioxo-l-H-pyrrolo[2,3-f quinoline-2,7,9-tricarboxylic acid, which is now commonly called pyrroloquinoline quinone or PQCi. ° Earlier, several other enzymes were believed to possess covalently attached PQCJ as a cofactor. This is now known to be false and an unfortunate consequence of erroneous analytical techniques. Although there are no examples of enzymes possessing covalently bound PQft, covalent quinoproteins with other protein-derived quinones have been identified. 

To obtain a dianhydride containing a diphenyl fragment, the reaction in a melt of naphthalene-l,4,5-tricarboxylic acid with 4,4 -diacetoxydiphenyl was employed [121], Some characteristics of a series of bisfnaphthalic anhydrides) with ester moieties are given in Table 6. 

Beryllium poisoning is treated with large doses of sodium salicylate, or smaller doses of aurine tricarboxylic acid. The latter, which has three salicylate moieties linked to a central carbon atom, does not assist the excretion of this metal, as salicylate does, but keeps it, in the tissues, as a very insoluble complex. 

Another group of toxins called the AAL-toxins was fotmd to have a structural relationship to the fumonisins, since they have only one tricarboxylic acid (TCA) moiety. AAL-toxin TAi (265) and TA2 (266) (Fig. 5.2) are produced by the fungus Alternaria alternata f. sp. lycopersici and can lead to phytotoxic effects on several crops such as tomatoes and weeds (200). Due to the toxic effects of long alkyl-chain pyridinium compounds, this new class of fumonisins is of high interest (201). 

Strassman and Weinhouse 180) pointed out that the distribution of the radioactivity in carbons 3, 4, 5, and 6 of lysine was very close to that expected in the succinate moiety of a-ketoglutarate arising from acetate by way of the tricarboxylic acid (TCA) cycle. The observed labeling in lysine is compatible with a condensation of a four-carbon and an acetyl moiety. Succinate itself cannot be the four-carbon unit, because of the unsym-metrical labeling in positions 6 and 2 of lysine. 

The details of carbon metabolism in the citric acid cycle are beyond the scope of this article. In brief, pyruvate is first oxidatively decarboxylated to yield CO2, NADH, and an acetyl group attached in an ester linkage to a thiol on a large molecule, known as coenzyme A, or CoA. (See Fig. 2.) Acetyl CoA condenses with a four-carbon dicar-boxy lie acid to form the tricarboxylic acid citrate. Free CoA is also a product (Fig. 6). A total of four oxidation-reduction reactions, two of which are oxidative decarboxylations, take place, which results in the generation of the three remaining NADH molecules and one molecule of FADH2. The citric acid cycle is a true cycle. For each two-carbon acetyl moiety oxidized in the cycle, two CO2 molecules are produced and the four-carbon dicarboxylic acid with which acetyl CoA condenses is regenerated. 

Although H NMR spectra of aryllead triacetates can be recorded in CDCI3 solution in the presence of tetra-methylsilane, cleavage of the methyl-silicon bond takes place when tetramethylsilane is treated with an aryllead(rv) tricarboxylate in trifluoroacetic acid.164 When the aryl moiety is substituted with an electron-withdrawing group, quantitative conversion of TMS was observed to give aryl(methyl)lead dicarboxylates 111, (Equation (136)) that were even isolated in two instances, although in modest to moderate yields 55% yield for the 4-fluorophenyl compound and 23% yield for the 4-trifluoroacetamidophenyl compound. 

The prevalence of the carboxylate moiety in both biogenic and man-made molecules of interest makes this functional group a popular target for anion host chemistry. Needless to say, carboxylates are a major constituent of proteins, peptides and amino acids, and the expansion of proteomics begets increasing requirements for means of specific detection of such biomolecules. Other relevant examples of carboxylates include fatty acids, while many small molecule di- and tricarboxylates are implicated in key metaboUc pathways such as the citric acid cycle (e.g. citrate, succinate, fumarate and malonate). Carboxylated anthropogenic molecules include trichloroacetic acids, anionic surfactants and S-lactam antibiotics. 

Rai and Mehrotra later showed that the reaction of Al(OBu >3 with carboxylic acids yields the tricarboxylates more readily, as in this case the intermediate product A1(OBu )(OOCR)2 is monomeric and continues to react in a facile manner with the third mole of carboxylic acids. In fact the reactions of metal alkoxides with carboxylic acids are facilitated thermodynamically since the carboxylate moiety tends to bind the metal in a bidentate mode, thus ... 

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