Deleterious effects, metal ions

The calcium form of EDTA instead of free EDTA is used in many food preparations to stabilize against such deleterious effects as rancidity, loss of ascorbic acid, loss of flavor, development of cloudiness, and discoloration. The causative metal ions are sequestered by displacing calcium from the chelate, and possible problems, such as depletion of body calcium from ingestion of any excess of the free chelant, had it been used, are avoided. 

The use of zinc sulfate to catalyze the potassium ferricyanide oxidation procedure251 is worthy of comment. It is possible that other metals would also catalyze this oxidation, but their presence in the system would have a deleterious effect on thfe fluorescence of the final product, while Zn++ ions have relatively little effect. For instance, Cu++ ions would be expected to catalyze the oxidation stage, but they would also have a strong quenching effect on the fluorescence of the final products.144 Some of the Zn++ ions will also presumably be removed from the solution as insoluble zinc ferro-cyanide. Anton and Sayre have recently questioned the value of zinc sulfate as a catalyst at low pH.252... 

Chelating agents can coordinate with metal ions at a minimum of two sites. Typically this bidentate coordination solubilizes or otherwise inactivates these metal ions. The effect is to counter any deleterious actions these metals might have on the system in which the chelates are used. The chelating agents form ring structures that incorporate the metal ion within the molecule250. 

Iron being a redox-active metal, it most hkely exerts its toxic effects through the generation of hydroxyl radical or by generation of ferryl ion. In iron loaded condition, there is generation of radical species leading to hpid peroxidation. Lipid peroxidation of cellular membrane would have deleterious effects on their function and hence on the function of the cell. There is also irreversible oxidation of ascorbic acid. Deficiency of ascorbic acid can lead to a reduction in the amount of iron available for erythropoiesis. 

The rate-determining step in an ion-exchange process is the diffusion of the adsorbable ions into the resin matrix. Retention times of 2-10 min are used in the uranium industry to attain full equilibrium. The metal ion to be recovered must almost completely occupy the resin fimctional sites to attain a very high degree of selectivity. Resins should be useful for at least two years if clean clarified leach liquors containing no poisonous ions are used. A drastic reduction in the usefulness of the resins is observed in the presence of such ions. Everest et al. (Ell) studied in detail the deleterious effects of thiocyanates, polythionates and sulfur, cobalt... 

Wang, S., Karbstein, K., Peracchi, A., Beigelman, L., Herschlag, D. Identification of the hammerhead ribozyme metal ion binding site responsible for rescue of the deleterious effect of a cleavage site phosphorothioate. Biochemistry 1999, 38(43), 14363-78. 

Other impurities can enter the slurry by being leached from the refractories themselves. Calcium and Magnesium are impurities that can have very deleterious effects on the stability of colloidal silica. Some refractories like Zircon are acidic in their reaction with colloidal silica and tend to reduce the pH over time. The stability of alkali stabilized colloidal silicas depend both on minimizing the presence of impurities (especially polyvalent metal ions) as... 

As has been indicated above, metal Ions are essential in every aspect of genetic Information transfer. Nevertheless, metal Ions can ilso cause deleterious effects In Information transfer either If they are present In the wrong kind or In the wrong concentration. Let us consider an example of each of these posslbllties first, that In which metals are present in the wrong kind. 

Now let us consider the case of essential metal Ions that, nevertheless, produce deleterious effects in the wrong concentration. Magnesium Ions are required for protein synthesis, yet Mg " " Ions in too high concentration lead to errors, as is illustrated by the studies of Szer and Ochoa (16) on the incorporation of phenylalanine and leucine in a ribosomal preparation using poly(U) as the messenger RNA. UUC as well as the UUU codon in poly(U) code for phenylalanine, so that incorporation of the latter represents correct translation. UUA and UUG code for leucine, so that leucine incorporation in this system is "incorrect." At low Mg2+ concentration only phenylalanine is in fact incorporated. Phenylalanine incorporation is maximal at lOmM Mg " " as the Mg " " concentration is increased, however, leucine also becomes incorporated and its maximal incorporation is at 20mM Mg2+ ( ). 

Metal Ions can produce a large variety of other effects on nucleic acids that could be deleterious if they occur during genetic information transfer. Metal ions can bring about the degradation of RNA (17, 18, 19), changes in the specificity of enzymes that act on DNA (20), changes in the conformation of polynucleotides and nucleic acid - protein complexes (21). 

Aluminum appears to be Involved in Alzheimer s disease and concentrates in the nucleus therefore, we have looked at aluminum binding to DNA, and we find that aluminum forms crosslinks with the DNA strands. Metal ions generally produce deleterious effects in genetic information transfer when present in the wrong kind or concentration. They are, however, essential in genetic information transfer. The concentration of metal ions in various cells changes with age, and we have hypothesized that perhaps these metal ions have an Impact on genetic information transfer processes... 

Metal ions not only play a role in structure, transport or catalysis, but they may also have deleterious effects as a toxic metal (see above), or play an important role as a metal-containing drug or diagnostic agent. Examples of the last category are ... 

As to potential imparities involved in PBT, ether groups are a possibility, at least via hydroxyl end dehydration in the melt. The presence of such groups in the as-made polymer, from 1,4-butanediol, is imlikely, due to the greater likelihood of TFIF formation than formation of the etherdiol. In the case of metal impurities, the discussion of this aspect in the previous section would seem to suggest that, at least by complexation between metal ions and carbonyl groups, there is unlikely to be a deleterious effect of catalyst residues on PBT thermal stability. The fact that use of titanium catalysts for both parts of PBT synthesis is well known and practised, while use of titanium catalyst in PET manufacture results in severe yellowing, would seem to support this hypothesis. 

Since the pH defines the potential of hydrogen evolution, its variation may be used to extend negatively the potential window for metal ion discharge. One may also use hydrogen evolution for depolarizing or for preventing deposition of less noble metals, subject to the deleterious effects on the quality of the metal deposit (see above). 

Thermodynamically, dissolution should occur whenever the fiee energy of the solvated metal ion at a givai concentration is lower than the free energy of the atom in the metal, plus the thermodynamic potential of the electrons exchanged during the reaction via the standard hydrogen electrode. Such thermodynamic conditions arc summarized in the series of potential- H phase diagrams that have been extensively collated by Poutbaix [69]. Even in cases where a metal is coveted by a thin protective oxide, deleterious corrosion effects can... 

The deleterious effects of heavy metal impurities can be eliminated by the inclusion of chelating agents. A chelating agent refers to a compound, which can form a metal ion complex having two or more... 

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