Nicotinic acid derivatives, metal

Naphthalimide, methylation of, 255 Naphthols, tautomerism of, 5 Naphtho-r,2, 4,5-selenazoles, 350 Nicotinic acid derivatives, metal catalysts, action on, 183, 186 Nitrones, 84, 88, 92, 99... 

The indirect determination of certain organic substances can be made by complexometric titration methods. Such methods depend on the formation of an insoluble product between the organic material and a metal then, either the excess metal in solution is determined by a suitable titration with EDTA, or the metal-containing precipitate is decomposed and the liberated metal ions titrated. Thus, for example, narcotine, papaverine, codeine, strychnine and brucine have been determined by formation of iodobis-muthate complexes, chlorpromazine and quinine" as cadmium iodide complexes, purines and nicotinic acid derivatives by precipitation with mercury and barbiturates by precipitation with zinc. ... 

In the first family, the metal is coordinated by one molecule of the pterin cofactor, while in the second, it is coordinated to two pterin molecules (both in the guanine dinucleotide form, with the two dinucleotides extending from the active site in opposite directions). Some enzymes also contain FejSj clusters (one or more), which do not seem to be directly linked to the Mo centers. The molybdenum hydroxylases invariably possess redox-active sites in addition to the molybdenum center and are found with two basic types of polypeptide architecture. The enzymes metabolizing quinoline-related compounds, and derivatives of nicotinic acid form a separate groups, in which each of the redox active centers are found in separate subunits. Those enzymes possessing flavin subunits are organized as a2jS2A2, with a pair of 2Fe-2S centers in the (3 subunit, the flavin in the (3 subunit, and the molybdenum in the y subunit. 

Pyridine is a tertiary amine its aqueous solution shows an alkaline reaction and precipitates the hydroxides of metals, some of which are soluble in an excess of the amine. Salts of pyridine like those of other amines form characteristic double salts with metallic halides. The ferrocyanide of pyridine and the addition-product of pyridine and mercuric chloride are difficultly soluble in water these compounds are used in the purification of the base. Pyridine is a very stable compound it can be heated with nitric acid or chromic acid without undergoing change but at 330° it is converted by a mixture of nitric acid and fuming sulphuric acid into nitropyridine, a colorless compound that melts at 41° and boils at 216°. At a high temperature pyridine is converted into a sulphonic acid by sulphuric acid. Chlorine and bromine form addition-products, e.g., C5H5N.CI2, at the ordinary temperature when these are heated to above 200°, substitution-products are formed. The hydroxyl derivative of pyridine is made by fusing the sulphonic acid with sodium hydroxide it resembles phenol in chemical properties. The three possible carboxyl derivatives of pyridine are known. The a-acid is called picolinic acid, the jS-acid nicotinic acid (664), and the 7-acid isonicotinic acid. 

Enzymes show stereospecificity towards their substrates, and often require cofactors to assist and/or participate in their catalytic activity. Cofactors include divalent metal ions, e.g. Mg +, Zn + and many low molecular weight molecules derived in part from growth factors (vitamins), e.g. nicotinic acid and biotin (see Table 17.2). 

Its nitrogen readily becomes pentavalent, forming quaternary ammonium salts. Some derivatives of known properties, useful for qualitative identification are the hydrochloride, white crystals melting at 272° with decomposition the nitrate, m.p. 184-185° the picrate, yellow rhombic prisms (from absolute alcohol) melting at 221-222° to a red oiB the />-toluidine, m.p. 150° and the anilide , m.p. 85°. Alkyl iodides readily react with it to form water-soluble compounds. The carboxylic group of nicotinic acid behaves typically, forming salts with alkalies, alkaline earth hydroxides or heavy metals the latter salts are quite insoluble and their preparation is useful to separate nicotinic acid from mixtures. 

With ammonia, the gas combines to form ammonium sulphide or ammonium hydrosulphide, according to the relative quantities of the reagents.4 The alkaloids, which may be regarded as organic derivatives of ammonia, also combine with hydrogen sulphide, forming crystalline salts such salts of cinchonine, quinine, strychnine, brucine and nicotine have been known for many years.5 When heated in the gas, the alkali metals yield the acid sulphides ... 

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