Oxoacidity function

Tremillon introduced the common oxoacidity function, ft, as a measure of the relative acidity (basicity) of solutions of various substances in molten media. This parameter is similar to the well-known protic acidity functions, H0 (Hammett function) and pA (Izmailov s acidity function) considered in Part 1. In terms of the primary medium effects, the function ft may be represented as the sum of the primary medium effect of O2- for the given medium (—logy0O2 ) and the instrumentally measured pO (pOl) in this medium ... 

According to this proposition, the common oxoacidity function, Cl, may be represented by the formula... 

The above-mentioned papers [169, 170] report the determination of the upper limit of basicity which can be achieved in alkali metal chlorides and the melts containing polyvalent metal chlorides as constituent parts of the melt. Owing to their small ionic radii and high charge, these cations demonstrate elevated acidic properties. Increase of the initial molality of oxide ions finally results in the precipitation of the solid oxide, formed by such acidic cations, from the saturated solution. The oxoacidity function corresponding to this limit may be described by the following equation ... 

The oxobasicity index allows us to connect the instrumental values of pO obtained by performing a variety of experiments, with the values of the common oxoacidity function, ll, by a rather simple relationship... 

The effect of ionic melts on the acid-base processes occurring in their background is shown in this Part to be described successfully by the common oxoacidity function, 12. 

Fig. 3.7.21. Potentiometric titration curves of Mg2+ in chloride melts at 700 °C expressed via the Cl common oxoacidity function (1) KCl-NaCl, (2) SrCl2-KCl-NaCl, (3) BaCl2-KCl-NaCl, (4) KCl-LiCl, (5) CaCl2-KCl.

R. Combes, B. Tremillon, F. De Andrade, M. Lopes and H. Ferreira, Definition and Use of an Oxoacidity Function O for the Comparison of Acidity Levels at 1000 K of Some Chloride Melts Involved in Electrometallurgical Processes, Anal. Lett. 15A (1982) 1585-1600. 

There are several experimental methods, which permit estimation of the oxobasicity indices, pIl, and the oxoacidity function, Q They are used for calculation of these indices on the basis of an experimental parameter (equilibrium constant of a preliminary chosen acid-base process, solubility product of metal oxide or simply emf of a certain solution) with the nse of a definite non-thermodynamic assumptioa... 

The oxoacid is then metabolised in the usual way (see below). High activities of the oxidase are found in Uver and kidney where its function is to remove rapidly (i.e. detoxify) the D-amino acids. Failure to metabolise these amino acids could lead to their accumulation in cells with the danger of osmotic effects or interference in the metabolism of L-amino acids. 

Thiamine diphosphate (TPP, 3), in cooperation with enzymes, is able to activate aldehydes or ketones as hydroxyalkyl groups and then to pass them on to other molecules. This type of transfer is important in the transketo-lase reaction, for example (see p. 152). Hydroxyalkyl residues also arise in the decarboxylation of 0x0 acids. In this case, they are released as aldehydes or transferred to lipoamide residues of 2-oxoacid dehydrogenases (see p. 134). The functional component of TPP is the sulfur- and nitrogen-containing thiazole ring. 

Other minor systems are also in use. Some are traditional, and some are very restricted in their application. These include acid nomenclature (inorganic, for oxoacids and derivatives), replacement nomenclature (mainly organic, to denote replacement of skeletal atoms in a parent rather than replacement of hydrogen atoms — oxa-aza replacement is one variant), functional class nomenclature (this is again principally organic and involves the use of type names such as alcohol, acid and ether) and subtractive nomenclatures (such as organic-deoxy and inorganic-debor). These will all be referred to briefly as appropriate. 

Oxalocrotonate tautomerase. This bacterial enzyme, which functions in the degradation of toluene (Chapter 25), is actually an isomerase. It catalyzes rapid interconversion of an unconjugated unsaturated a-oxoacid such as 4-oxalocrotonate with an intermediate enol (which may leave the enzyme) and the isomeric conjugated oxoacid (Eq. 13-31).168-170 A related 5-carboxymethyl-2-hydroxymuconate isomerase... 

While Tetrahymena must have lipoic acid in its diet, we humans can make our own, and it is not considered a vitamin. Lipoic acid is present in tissues in extraordinarily small amounts. Its major function is to participate in the oxidative decarboxylation of a-oxoacids but it also plays an essential role in glycine catabolism in the human body as well as in plants.295 296 The structure is simple, and the functional group is clearly the cyclic disulfide which swings on the end of a long arm. Like biotin, which is also present in tissues in very small amounts, lipoic acid is bound in covalent amide linkage to lysine side chains in active sites of enzymes 2963... 

The unique function of lipoic acid is in the oxidation of the thiamin-bound active aldehyde (Fig. 15-15) in such a way that when the complex with thiamin breaks up, the acyl group formed by the oxidative decarboxylation of the oxoacid is attached to the... 

The degradation of nicotinic acid by Clostridium barkeri involves the cleavage of the intermediate 2,3-dimethylmalate 132 from which propionic and pyruvic acids are formed by a specific lyase (EC 4.1.3.32). In the reverse direction, the enzyme must have the unusual capacity to deprotonate propionic acid at the a-carbon instead of the carboxylic acid function, or next to an anionic car-boxylate. Purified dimethylmalic acid aldolase has been used to catalyze the stereospecific addition of 133 to the oxoacid acceptor, yielding the (2R,3S) configurated dimethylmalic acid 132 at the multi-gram scale [381]. The substrate tolerance of this enzyme has not yet been determined. 

The absence in halobacteria of the oxoacid dehydrogenase complexes creates another puzzle. In most known systems, the role of the enzyme lipoamide dehydrogenase is to reoxidize the lipoic acid that is involved in the oxidation of the oxoacids in the oxoacid dehydrogenase complexes. This enzyme was nonetheless found in H. halobium and purified to homogeneity by Danson et al. (1986). What, then, is its function It is likely that lipoamide dehydrogenase assumes a different role in halobacteria. Another reducing system unique to... 

Constructing a substitutive name generally involves the replacement of hydrogen atoms in a parent structure with other atoms or atom groups. Related operations, often considered to be part of substitutive nomenclature, are skeletal replacement (Section IR-6.2.4.1) and functional replacement in oxoacid parents (Section IR-8.6). Note that some operations in parent hydride-based nomenclature are not substitutive operations (e.g. formation of cations and anions by addition of H+ and H, respectively, cf. Sections IR-6.4.1 and IR-6.4.5). Names formed by the modifications of parent hydride names described in those sections are still considered part of substitutive nomenclature. 

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