Synthetic Chelators

Reactions of the Hydroxyl Group. The hydroxyl proton of hydroxybenzaldehydes is acidic and reacts with alkahes to form salts. The lithium, sodium, potassium, and copper salts of sahcylaldehyde exist as chelates. The cobalt salt is the most simple oxygen-carrying synthetic chelate compound (33). The stabiUty constants of numerous sahcylaldehyde—metal ion coordination compounds have been measured (34). Both sahcylaldehyde and 4-hydroxybenzaldehyde are readily converted to the corresponding anisaldehyde by reaction with a methyl hahde, methyl sulfate (35—37), or methyl carbonate (38). The reaction shown produces -anisaldehyde [123-11-5] in 93.3% yield. Other ethers can also be made by the use of the appropriate reagent. 

It is interesting to observe that response mechanisms to Fe deficiency have been studied almost exclusively using synthetic chelates such as EDTA and ED-... 

Treeby M., Marschner H., Romheld V. Mobilisation of iron and other micronutrient cations from a calcareous soil by plant borne, microbial and synthetic chelators. Plant Soil 1989 114 217-226. 

At present, many natural and synthetic chelators have been studied as potential pharmacological agents for the treatment of iron or copper overload under various pathophysiological conditions. (The chelating activity of flavonoids was already discussed above and the application of chelators in thalassemia and some other pathologies is considered in Chapter 31.) There are specific thermodynamic demands to chelators to be efficient antioxidants [369], We will consider just several chelators of potential therapeutic importance. 

Three methods for trace metal preconcentration were examined liquid-liquid extraction aided by a chelating agent, concentration on a synthetic chelating resin and reductive precipitation with NaBTLt. The latter method gave 1000-fold preconcentration factors with total recovery of Pb and other elements17. Preconcentration of nanogram amounts of lead can be carried out with a resin incorporating quinolin-8-ol (3)18. Enhancement factors of 50-100 can be achieved by such preconcentration procedures followed by determination in a FLA (flow injection analysis) system limits of detection are a few pg Pb/L19. 

Formation of L-lysinehydroxamato-iron(III) complexes occurs by an interchange mechanism formation and dissociation (acid-catalyzed) are significantly affected by charge repulsion (the ligand is H3N+(CH2)4CH(NH3 )CONHOH (335). Rate constants for complex formation between Feaq and two synthetic chelators of the dicatecholsper-midine family are, at 450 and 500M-1s-1 (336), similar to that for desferrioxamine. 

Yelle, D., Goodell, B., Gardner, D.J., Amirbahman, A., Winistorfer, P. and Shaler, S. (2004). Bonding of wood fiber composites using a synthetic chelator-lignin activation system. Forest Products Journal, 54(4), 73-78. 

El Hage Chahine JM, Bauer AM, Baraldo K, Lion C, Ramiandrasoa F, Kunesch G. (2001) Kinetics and Thermodynamics of Complex Formation between Fe and Two Synthetic Chelators of the Dicatecholspermidine Family. Eur J Inorg Chem 2287... 

Wallace, A. K Decade of Synthetic Chelating Agents in Inorganic Plant Nutrition. Los Angeles A. Wallace 1962. 

Figure 12-3 Synthetic chelate covalently attached to an antibody carries a metal isotope (M) to deliver lethal doses of radiation to tumor cells...

Aminocarboxylic acids in Figure 12-4 are synthetic chelating agents. Amine N atoms and carboxylate O atoms are the potential ligand atoms in these molecules (Figures 12-5 and 12-6). When these molecules bind to a metal ion. the ligand atoms lose their protons. 

A number of synthetic chelating agents have been developed. They are substances like ethylenediaminelelraaceiic acid lEDTA) (Structure 1) and N-hydroxyethylelhylenediamineuiacetic acid (HEDTAt and their salts, usually sodium salts. Many of these compounds and mixtures of these... 

Fig. 2. Postulated role of trihydroxamates in heme synthesis in microorganisms. (Note in some instances the test organisms will respond to synthetic chelating agents. J. Bact. 89, 1630, 1965) the observation that heme will not reverse the toxicity of the ferric trihydroxamic antibiotics suggests the latter do not act by blocking the insertion of iron into porphyrin (Nature 208, 674, 1965).

The uptake of iron from transferrin by reticulocytes is a time-, temperature-, and energy-dependent process in which integrity of both protein and cells is required (64, 65). Synthetic iron chelates, once thought to be effective iron donors (66), appear to depend on membrane-bound transferrin as an intermediate agent cells depleted of the protein by preincubation and washing no longer accept iron from such complexes (67). When such cells are reincubated with transferrin, their capacity to accept iron initially bound to synthetic chelators is largely restored. 

Wongkongkatep, J., Parkpian, P., Polprasert, C., Supaibulwatana, K., lida, T., and Fukushi, K. 2004. Phytoremediation of arsenic by Pityrogramma calomelanos Do synthetic chelating agents increase or decrease arsenic phytoextraction efficiency Annual Report of Interdisciplinary Research Institute of Environmental Sciences, 22 61-72. 

Multidentate synthetic chelating agents are used for the decontamination of nuclear reactors and for nuclear waste processing, because they form stable soluble complexes with many radionuclides. Unfortunately, their joint disposal has sometimes resulted in increased radionuclide mobility, with the concomitant contamination of groundwater. An attempt to lessen this problem involves degrading the chelating agents with bacteria (see Chapter 11). 

The use of acid solutions (mainly hydrochloric, nitric, and sulfuric acids) offers quite high removal yields because metal ions tend to be more soluble at low pH values. A drawback is that the soil becomes acidified—this can introduce major alterations in its chemical, physical, and biological properties in addition, carbonate-rich soils strongly interact with the acids and produce CO2. As discussed in Chapter 3, metal complexes with natural or synthetic chelating agents tend to be soluble and stable. Then, heavy metals in soils can also be solubilized by chelation and extraction followed by an ex situ treatment. 

Even though the complexation approach has less impact on the soil properties than acid washing, it poses problems of its own as well. For instance, synthetic chelating agents [e.g., aminopolycarboxylic... 

Fig. 16. Structures of different types of ligands acting as two-electron redox relays in natural and artificial systems Flavins such as (11) are the essential constituents of flavodoxines and flavoproteins 137). The a-ketoglutarate anion (a-KG, 12) is a typical example of a sacrificial redox mediator which decomposes during catalysis (72). Synthetic chelates such as bis-arylimino-acenaphthene (BIAN, 13) have been proposed for the development of bio-inspired multielectron transfer photosensitizers 138).

One of the best purification processes so far discovered is based on the coordination properties of synthetic chelating polymers containing functional thiol groups [1]. Typically, effluents rejected by industries where mercury electrodes are extensively used have been purified through resins prepared with a styrene/diyjnylbenzene copolymer functionalized with thiol groups [2]. 

Organic sources include natural organic complexes that are byproducts of the wood pulp industry, as well as synthetic chelates which are generally more effective than inorganic salts. The synthetic chelates are relatively expensive to use. 

Huang, J. W. W., Chen, J. J., Berti, W. R., and Cunningham, S. D. (1997). Phytoremediation of lead-contaminated soils role of synthetic chelates in lead phytoextraction. Environ. Sci. Technol. 31, 800-805. 

Chelation by LMMO substances is a major factor in the transport of micronutrient cations such as Cu, Zn, Ni, Fe, and Mn to root surface by mass flow and diffusion. In soil solutions of calcareous soils 40 to 75% of tlie zinc and 98% of copper have been found in organic complexes (Hodgson et al., 1966 Sanders, 1982, 1983 McGrath et al., 1988). For plants the importance of complexed micronutrients in the soil solution is particularly evident in calcareous soils. This is also indicated by the fact that soil extractions with synthetic chelators such as EDTA and DTPA provide suitable soil tests for estimation of available micronutrieiits (Sims and Johnson, 1991) (summarized in detail in Section 11.3). 

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