Chelation

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Chelation (USA: , UK: ) is a type of ionic and molecular bond with metal ions. This involves the formation or presence of two or more separate coordinate bonds between the polydentate ligands (multiple bonds) and one central atom. Usually these ligands are organic compounds, and are called chelants, chelators, chelating agents, or extraction agents.

Chelation is useful in applications such as providing nutritional supplements, in chelation therapy to remove toxic metals from the body, as contrast agents in MRI scans, in manufacture using homogeneous catalysts, in chemical water treatment to aid in metal removal, and in fertilizers.

Video Chelation

Chelate effect

The chelate effect is an enhanced affinity of chelating ligands for metal ions compared to the similar affinity of latches of nonchelating (monodentat) ligands for the same metal.

The thermodynamic principle underlying the chelate effect is illustrated by the contrast affinity of copper (II) for ethylenediamine (en) vs methylamine.

In ( 1 ) ethylenediamine forms a chelate complex with copper ions. The chelation results in the formation of five CuC rings ₂ N ₂ . In ( 2 ) the bidentate ligand is replaced by two methylamin monodentate ligands around the same donor power, indicating that the Cu-N bond is more or less the same in the two reactions.

The thermodynamic approach to describe the chelate effect considers the equilibrium constant for the reaction: the greater the equilibrium constant, the higher the complex concentration.

The electrical charge has been removed for the simplicity of the notation. The square brackets indicate the concentration, and the subscript to the stability constant ,,, indicates complex stoichiometry. When the analytical concentration of methylamine doubled from ethylenediamine and copper concentration were the same in both reactions, the concentration of [Cu (en)] was much higher than that of Cu [ ] because? ₁₁ >>? ₁₂ .

An equilibrium constant, K , is related to the standard free Gibbs energy, ${\ displaystyle \ Delta G ^ {\ ominus}}$ by

${\ displaystyle \ Delta G ^ {\ ominus} = - RT \ ln K = \ Delta H ^ {\ ominus} -T \ Delta S ^ {\ ominus}}$

where R is the gas constant and T is the temperature in the kelvin. ${\ displaystyle \ Delta H ^ {\ ominus}}$ is the standard enthalpy change of the reaction and ${\ displaystyle \ Delta S ^ {\ ominus}}$ is a standard entropy change.

Since the enthalpy must be roughly the same for two reactions, the difference between the two stability constants is due to the entropy effect. In the equation ( 1 ) there are two particles on the left and one on the right, whereas in equation ( 2 ) there are three particles on the left and one on the right. This difference means that the entropy of the disorder is reduced when the chelate complex is formed rather than when complexes with monodentate ligands are formed. This is one of the factors that contribute to the difference in entropy. Other factors include changes in solvation and ring formation. Some experimental data to illustrate the effect are shown in the following table.

This data confirms that the enthalpy changes are approximately equal for two reactions and that the main reason for greater chelate complex stability is the term of entropy, which is much less favorable. It is generally difficult to calculate precisely for thermodynamic values ​​in terms of changes in solution at the molecular level, but it is clear that the chelate effect is dominated by the entropy effect.

Other explanations, including Schwarzenbach, are discussed in Greenwood and Earnshaw ( loc.cit ).

Maps Chelation

In nature

Many biomolecules show the ability to dissolve certain metal cations. Thus, proteins, polysaccharides, and polynucleic acid are excellent polydentate ligands for many metal ions. Organic compounds such as amino acids glutamic acid and histidine, organic acids such as malate, and polypeptides such as phytochelatin are also typical chelators. In addition to these adventitious chelators, some biomolecules are specially produced to bind certain metals (see next section).

In biochemistry and microbiology

Almost all metalloenzymes feature chelated metal, usually for peptides or cofactors and prosthetic groups. Such chelating agents include a porphyrin ring in hemoglobin and chlorophyll. Many microbial species produce water-soluble pigments that act as chelating agents, called siderophores. For example, the species Pseudomonas is known to emit pyochelin and pyoverdine which bind iron. Enterobactin, produced by E. coli , is the strongest known chelating agent. Sea shells use metal chelation esp. Fe ³ chelation with a Dopa residue in the clam-leg protein 1 to increase the strength of the yarn used to secure itself to the surface.

In geology

In geography, heat chemical weathering is associated with organic chelating agents (eg, peptides and sugars) that extract metal ions from minerals and rocks. Some metallic complexes in the environment and in nature are not found in some form of a chelate ring (eg, with humic acid or protein). Thus, metal chelates are relevant to the mobilization of metals in the soil, uptake and accumulation of metals into plants and microorganisms. Selective chelas of heavy metal is relevant to bioremediation (eg, removal of ¹³⁷ Cs from radioactive waste).

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Medical applications

Nutritional supplements

In the 1960s, scientists developed the concept of chelating metal ions before feeding the elements in animals. They believe that this will create a neutral compound, protecting minerals from being complexed with insoluble salts in the stomach, which will make the metal unavailable for absorption. Amino acids, being effective metal binders, are selected as prospective ligands, and studies are carried out on a combination of amino-metal acids. This study supports that amino-amino acid chains are able to increase mineral absorption.

During this period, synthetic chelates such as ethylenediaminetetraacetic acid (EDTA) are being developed. It applies the same chelation concept and creates chelated compounds; but this synthetic is too stable and not nutritious. If the mineral is taken from EDTA ligand, the ligand can not be used by the body and will be removed. During the process of ejection the EDTA ligands are randomly chelated and peel other minerals from the body.

According to the Association of American Feed Control Officials (AAFCO), amino acid amino chains are defined as products resulting from the reaction of metal ions from a soluble metal salt with a mole ratio of one to three (preferably two) moles of amino acids. The average weight of a hydrolysed amino acid should be about 150 and the molecular weight generated from chelate should not exceed 800 Da.

Since the beginning of development of this compound, more research has been done, and has been applied to human nutrition products in a manner similar to animal nutrition experiments that pioneered the technology. Ferrous bis-glycinate is an example of one of the compounds that has been developed for human nutrition.

Dental and Mouth Application

First-Generation Dentin Adhesives was first designed and manufactured in the 1950s. This system is based on chelate co-monomer with calcium on tooth surface and produces very weak water resistance chemical bond (2-3 MPa).

Detoxification of heavy metals

Chelation therapy is used as an antidote for poisoning by mercury, arsenic, and lead. The chelating agent converts this metal ion into an excretable chemical and biochemical inert form. Chelation using EDTA disodium calcium has been approved by the US Food and Drug Administration (FDA) for serious lead poisoning cases. Not approved to treat "heavy metal poisoning".

Although useful in cases of serious lead poisoning, the use of EDTA (edetate disodium) disodium rather than EDTA disodium calcium has resulted in death due to hypocalcemia. Disodium EDTA is not FDA approved for any use, and all FDA approved chelation therapy products require a prescription.

Pharmacy

The chatez complex of gadolinium is often used as a contrast agent in MRI scans, although iron particles and manganese chelate complexes have also been explored. The bifunctional chelate complexes of zirconium, gallium, fluorine, copper, yttrium, bromine, or iodine are often used for conjugation to monoclonal antibodies for use in PET-based antibody imaging. This chelate complex often uses the use of hexadentate ligands such as desferrioxamine B (DFO), according to Meijs et al., And the gadolinium complex often uses the use of octadentate ligands such as DTPA, according to Desreux et al. Auranofin, a golden chelate complex, is used in the treatment of rheumatoid arthritis, and penicillamine, which forms a copper chelate complex, is used in the treatment of Wilson's disease and cystinuria, as well as refractory rheumatoid arthritis.

Other medical applications

Chelation in the intestinal tract is a cause of many interactions between drugs and metal ions (also known as "minerals" in nutrients). For example, antibiotic drugs from tetracycline and quinolones are chelators of Fe ² , Ca ² , and Mg ² .

EDTA, which binds calcium, is used to relieve hypercalcimia that is often produced from band keratopati. Calcium can then be removed from the cornea, allowing for increased visual clarity for the patient.

Chelation metal can be used as a carcinostatic agent. An anti-cancer drug, which is a metal complex, frees the drug on the cancer site and binds the virus, which causes cancer, removing it from the body.

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Industrial and agricultural applications

Catalysis

Homogeneous catalysts are often chelated complexes. A representative example is the use of BINAP (bidentate phosphine) in asymmetric hydrogenation of Noyori and asymmetric isomerization. The latter has the practical use of synthetic (-) - mentol.

Water softening

Citric acid is used to soften water in soaps and detergents. Common synthetic chelator is EDTA. Phosphonate is also a famous chelating agent. Chelators are used in water treatment programs and particularly in steam engineering, for example, boiler water treatment systems: Chelant Water Treatment Systems. Although this treatment is often referred to as "softening", chelation has little effect on the mineral content of water, in addition to making it dissolve. What changed was the pH level of the water, which was lowered.

Fertilizer

Metal chelate compounds are a common component of fertilizers to provide micronutrients. These micronutrients (manganese, iron, zinc, copper) are necessary for plant health. Most of the fertilizers contain phosphate salts which, in the absence of chelating agents, usually convert these metal ions into insoluble solids that have no nutritional value for the plant. EDTA is a typical chelating agent that makes these metal ions in a soluble form.

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Etymology

Ligands form complexes chelate with substrate. The Chelate complex is contrasted with a coordination complex consisting of a monodentate ligand, which forms only one bond with a central atom. The word chelation comes from the Greek ????, ch? L? , which means "claw"; The ligand is located around a central atom like a lobster claw. The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and HDK Drew, which states: "The adjective chelate, derived from large claws or (Greek) from lobsters or other crustaceans, it is advisable for groups such as caliper which function as two association units and bind to the central atom resulting in a heterocyclic ring. "

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References

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External links

• Definition of dictionary from chelate in Wiktionary

Source of the article : Wikipedia