Instabilities chemical

Chemical instability can involve one or more of a variety of chemical reactions including  

In the deamidation reaction, the side-chain amide linkage in a glutamine (Gin) or asparagine (Asn) residue is hydrolysed to form a free carboxylic acid the Asn peptides being more susceptible to deamidation than the Gin peptides. 

The hydrolysis reactions are completely reversible, i.e. the hnal products of deamidation can interconvert through the cyclic imide 

Amino acid or sequence Mechanism of degradation Formulation strategy 

Cysteine-cysteine Aggregation Addition of surfactants, polyalcohols and other excipients 

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Figure A3.14.15. The difTerential flow-induced chemical instability (DIFICI) in the BZ reaction. (Reprinted with pennission from [44], C5 The American Physical Society.)...

If tlie diffusion coefficients of tlie chemical species are sufficiently different, new types of chemical instability arise which can lead to tlie fonnation of chemical patterns and ultimately to spatio-temporal chaotic behaviour. 

Because the facilitated transport process employs a specific reactive carrier species, very high membrane selectivities can be achieved. These selectivities are often far higher than those achieved by other membrane processes. This one fact has maintained interest in facilitated transport since the 1970s, but the problems of the physical instability of the liquid membrane and the chemical instability of the carrier agent are yet to be overcome. 

Both 2- and 3-nitrothiophenes are reduced by tin and hydrochloric acid to the corresponding aminothiophenes. In contrast to anilines, the free bases are very unstable their salts and acyl derivatives, however, are stable. 2-Aminothiophene can be diazotized and the resulting diazonium salt coupled with /3- naphthol. The chemical instability of aminothiophenes compared with aniline is illustrated by the ring opening of 2-amino-3-ethoxycar-bonylthiophenes (157) with ethanolic sodium ethoxide to give cyanothiolenones (158) <75JPR861). 

Fhosphoric acid does not have all the properties of an ideal fuel cell electrolyte. Because it is chemically stable, relatively nonvolatile at temperatures above 200 C, and rejects carbon dioxide, it is useful in electric utility fuel cell power plants that use fuel cell waste heat to raise steam for reforming natural gas and liquid fuels. Although phosphoric acid is the only common acid combining the above properties, it does exhibit a deleterious effect on air electrode kinetics when compared with other electrolytes ( ) including such materials as sulfuric and perchloric acids, whose chemical instability at T > 120 C render them unsuitable for utility fuel cell use. In the second part of this paper, we will review progress towards the development of new acid electrolytes for fuel cells. 

For processes under development, the most cost-effective means of avoiding potential risk is to eliminate those materials that are inherently unsafe that is, those materials whose physical or physico-chemical properties lead to them being highly reactive or unstable. This is somewhat difficult to achieve for several reasons. First, without a full battery of tests to determine, for example, flammability, upper/lower explosivity limits and their variation with scale, minimum ignition temperatures, and so on, it is almost impossible to tell how a particular chemical will behave in a given process. Second, chemical instability may make a compound attractive to use because its inherent reactivity ensures a reaction proceeds to completion at a rapid enough rate to be useful that is, the reaction is kinetically and thermodynamically favoured. 

Today it has become clear that the effect of trace elements in living systems, in food, and in the environment depends on the chemical form in which the element enters the system and the final form in which it is present. The form, or species, clearly governs its biochemical and geochemical behaviour. lUPAC (the International Union for Pure and Applied Chemistry) has recently set guidelines for terms related to chemical speciation of trace elements (Templeton et al. 2000). Speciation, or the analytical activity of measuring the chemical species, is a relatively new scientific field. The procedures usually consist of two consecutive steps (i) the separation of the species, and (2) their measurement An evident handicap in speciation analysis is that the concentration of the individual species is far lower than the total elemental concentration so that an enrichment step is indispensable in many cases. Such a proliferation of steps in analytical procedure not only increases the danger of losses due to incomplete recovery, chemical instability of the species and adsorption to laboratory ware, but may also enhance the risk of contamination from reagents and equipment. 

Disruption of the native structure of a protein can also contribute to chemical instability by accelerating the rates of a variety of degradation routes, including deamidation, hydrolysis, oxidation, disulfide exchange, /1-elimination, and racemization. 

Dmg-induced damage to GI tissue Chemical instability in the GI tract Extensive first-pass metabolism... 

Although the reduction of ferricyanide has distinct advantages over the copper deposition reaction, in terms of obtaining a well-defined plateau, its usefulness in free-convection measurements is limited by a combination of Effects (3b) and (3c), as well as its chemical instability (see Section III,B). 

Undoubtedly because of its chemical instability, examples of a secondary CF2 group bound to amino nitrogen are rare, with a chemical shift being reported only for PhCF2N(CH3)2 (-72 ppm). 

Pfizer s initial formulation of amlodipine relied on a maleate salt, but two problems were encountered "(1) the chemical instability of... 

If it is assumed that electrolysis product may be cyanamide (as the simplest substance in terms of IR spectroscopic data for the melt), then the chemical instability of NH2CN and the possibility of the reversible reaction ... 

Also termed bivalent gap and ethylenic linkage. The type of bond in which two valency bonds link two atoms in a molecule. It is typical of compounds showing unsaturation, such as ethylene. A double bond does not indicate extra strength of the bond but rather chemical instability and reactivity. 

KEYWORDS Acid mine drainage (AMD),chemical instability, ochre-precipitates, schwertmannite, goethite. 

Flicker, M. Ross, J. (1974). Mechanism of chemical instability for periodic precipitation phenomena. J. Chem. Phys., 60, 3458-65. 

Ortoleva, P. (1984). The self-organization of Liesegang Bands and other precipitate patterns. In Chemical Instabilities Applications in Chemistry, Engineering, Geology and Material Science, ed. G. Nicolis F. Baras, pp. 289-97. Dordrecht Reidel. 

R. Lefever and G. Nicolis, Chemical instabilities and sustained oscillations. J. Theor. Biol. 30... 

Intermolecular reactions also play an important role in the stability of car-bapenems. Thienamycin (5.45), the first carbapenem discovered, is destroyed very rapidly in aqueous solution its chemical instability is due to intermolecular aminolysis of the /3-lactam induced by the NH2 group in the C(2) side... 

Chemical modifications have been carried out in the carbapenem series with a view to reduce the extent of metabolic degradation. The first compound in this series was thienamycin (5.45), which was found to undergo extensive metabolism in rodents and monkeys [121][156] the degree of metabolic degradation, together with its chemical instability, have prevented marketing of this compound. 

A medicinal example is provided by klerval (Fig. 6.18). The aspartic acid residue in this tripeptide analogue is also a site of chemical instability. At pH 1, cleavage of the Asp-Xaa bond (Fig. 6.18, Reaction b) was second in importance after C-terminal deamidation (see Sect. 6.3.2.1), and cleavage of the Xaa-Asp bond (Fig. 6.18, Reaction c) was third. At pH 4, cleavage of the Asp-Xaa bond was the major reaction and was accompanied by the formation of the succinimide and the iso-aspartyl peptide cleavage of the Xaa-Asp bond was minor. At pH 7, the major products were the L-iso-Asp and D-iso-Asp peptides, together with minor amounts of the D-Asp peptide. 

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