Английская Википедия:Chelation

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Chelation is a type of bonding of ions and molecules to metal ions. It involves the formation or presence of two or more separate coordinate bonds between a polydentate (multiple bonded) ligand and a single central metal atom. These ligands are called chelants, chelators, chelating agents, or sequestering agents. They are usually organic compounds, but this is not a necessity.

The word chelation is derived from Greek χηλή, chēlē, meaning "claw"; the ligands lie around the central atom like the claws of a crab. The term chelate was first applied in 1920 by Sir Gilbert T. Morgan and H. D. K. Drew, who stated: "The adjective chelate, derived from the great claw or chele (Greek) of the crab or other crustaceans, is suggested for the caliperlike groups which function as two associating units and fasten to the central atom so as to produce heterocyclic rings."

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 scanning, in manufacturing using homogeneous catalysts, in chemical water treatment to assist in the removal of metals, and in fertilizers.

Chelate effect

Файл:Me-EN.svg

Ethylenediamine ligand chelating to a metal with two bonds

Файл:Cu chelate.svg

Cu²⁺ complexes with nonchelating methylamine (left) and chelating ethylenediamine (right) ligands

The chelate effect is the greater affinity of chelating ligands for a metal ion than that of similar nonchelating (monodentate) ligands for the same metal.

The thermodynamic principles underpinning the chelate effect are illustrated by the contrasting affinities of copper(II) for ethylenediamine (en) vs. methylamine. Шаблон:NumBlk Шаблон:NumBlk In (Шаблон:EquationNote) the ethylenediamine forms a chelate complex with the copper ion. Chelation results in the formation of a five-membered CuC₂N₂ ring. In (Шаблон:EquationNote) the bidentate ligand is replaced by two monodentate methylamine ligands of approximately the same donor power, indicating that the Cu–N bonds are approximately the same in the two reactions.

The thermodynamic approach to describing the chelate effect considers the equilibrium constant for the reaction: the larger the equilibrium constant, the higher the concentration of the complex. Шаблон:NumBlk Шаблон:NumBlk Electrical charges have been omitted for simplicity of notation. The square brackets indicate concentration, and the subscripts to the stability constants, β, indicate the stoichiometry of the complex. When the analytical concentration of methylamine is twice that of ethylenediamine and the concentration of copper is the same in both reactions, the concentration [Cu(en)] is much higher than the concentration [Cu(MeNH₂)₂] because β₁₁ ≫ β₁₂.

An equilibrium constant, K, is related to the standard Gibbs free energy, Шаблон:Tmath by

<math>\Delta G^\ominus = - RT \ln K = \Delta H^\ominus - T \Delta S^\ominus</math>

where R is the gas constant and T is the temperature in kelvins. Шаблон:Tmath is the standard enthalpy change of the reaction and Шаблон:Tmath is the standard entropy change.

Since the enthalpy should be approximately the same for the two reactions, the difference between the two stability constants is due to the effects of entropy. In equation (Шаблон:EquationNote) there are two particles on the left and one on the right, whereas in equation (Шаблон:EquationNote) there are three particles on the left and one on the right. This difference means that less entropy of disorder is lost when the chelate complex is formed with bidentate ligand than when the complex with monodentate ligands is formed. This is one of the factors contributing to the entropy difference. Other factors include solvation changes and ring formation. Some experimental data to illustrate the effect are shown in the following table.

Equilibrium	log β	Шаблон:Tmath	<math>\Delta H^\ominus \mathrm{/kJ\ mol^{-1}}</math>	<math>-T\Delta S^\ominus \mathrm{/kJ\ mol^{-1}}</math>
Cu2+ + 2 MeNH2 Шаблон:Eqm Cu(MeNH2)22+	6.55	−37.4	−57.3	19.9
Cu2+ + en Шаблон:Eqm Cu(en)2+	10.62	−60.67	−56.48	−4.19

These data confirm that the enthalpy changes are approximately equal for the two reactions and that the main reason for the greater stability of the chelate complex is the entropy term, which is much less unfavorable. In general it is difficult to account precisely for thermodynamic values in terms of changes in solution at the molecular level, but it is clear that the chelate effect is predominantly an effect of entropy.

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

In nature

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

Virtually all metalloenzymes feature metals that are chelated, usually to peptides or cofactors and prosthetic groups. Such chelating agents include the porphyrin rings in hemoglobin and chlorophyll. Many microbial species produce water-soluble pigments that serve as chelating agents, termed siderophores. For example, species of Pseudomonas are known to secrete pyochelin and pyoverdine that bind iron. Enterobactin, produced by E. coli, is the strongest chelating agent known. The marine mussels use metal chelation esp. Fe³⁺ chelation with the Dopa residues in mussel foot protein-1 to improve the strength of the threads that they use to secure themselves to surfaces.

In earth science, chemical weathering is attributed to organic chelating agents (e.g., peptides and sugars) that extract metal ions from minerals and rocks. Most metal complexes in the environment and in nature are bound in some form of chelate ring (e.g., with a humic acid or a protein). Thus, metal chelates are relevant to the mobilization of metals in the soil, the uptake and the accumulation of metals into plants and microorganisms. Selective chelation of heavy metals is relevant to bioremediation (e.g., removal of ¹³⁷Cs from radioactive waste).

Applications

In the 1960s, scientists developed the concept of chelating a metal ion prior to feeding the element to the animal. They believed that this would create a neutral compound, protecting the mineral from being complexed with insoluble salts within the stomach, which would render the metal unavailable for absorption. Amino acids, being effective metal binders, were chosen as the prospective ligands, and research was conducted on the metal–amino acid combinations. The research supported that the metal–amino acid chelates were able to enhance mineral absorption.Шаблон:Citation needed During this period, synthetic chelates such as ethylenediaminetetraacetic acid (EDTA) were being developed. These applied the same concept of chelation and did create chelated compounds; but these synthetics were too stable and not nutritionally viable. If the mineral was taken from the EDTA ligand, the ligand could not be used by the body and would be expelled. During the expulsion process the EDTA ligand randomly chelated and stripped another mineral from the body. According to the Association of American Feed Control Officials (AAFCO), a metal–amino acid chelate is defined as the product resulting from the reaction of metal ions from a soluble metal salt with amino acids, with a mole ratio in the range of 1–3 (preferably 2) moles of amino acids for one mole of metal.Шаблон:Citation needed The average weight of the hydrolyzed amino acids must be approximately 150 and the resulting molecular weight of the chelate must not exceed 800 Da.Шаблон:Citation needed Since the early development of these compounds, much more research has been conducted, and has been applied to human nutrition products in a similar manner to the animal nutrition experiments that pioneered the technology. Ferrous bis-glycinate is an example of one of these compounds that has been developed for human nutrition. Because of their wide needs, the overall chelating agents growth was 4 % annually during 2009-2014 and the trend is likely to increase.

Dentin adhesives were first designed and produced in the 1950s based on a co-monomer chelate with calcium on the surface of the tooth and generated very weak water-resistant chemical bonding (2–3 MPa).

Chelation therapy is an antidote for poisoning by mercury, arsenic, and lead. Chelating agents convert these metal ions into a chemically and biochemically inert form that can be excreted. Chelation using calcium disodium EDTA has been approved by the U.S. Food and Drug Administration (FDA) for serious cases of lead poisoning. It is not approved for treating "heavy metal toxicity". Although beneficial in cases of serious lead poisoning, use of disodium EDTA (edetate disodium) instead of calcium disodium EDTA has resulted in fatalities due to hypocalcemia. Disodium EDTA is not approved by the FDA for any use, and all FDA-approved chelation therapy products require a prescription.

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

Chelation in the intestinal tract is a cause of numerous interactions between drugs and metal ions (also known as "minerals" in nutrition). As examples, antibiotic drugs of the tetracycline and quinolone families are chelators of Fe²⁺, Ca²⁺, and Mg²⁺ ions.

EDTA, which binds to calcium, is used to alleviate the hypercalcemia that often results from band keratopathy. The calcium may then be removed from the cornea, allowing for some increase in clarity of vision for the patient.Шаблон:Citation needed

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

Citric acid is used to soften water in soaps and laundry detergents. A common synthetic chelator is EDTA. Phosphonates are also well-known chelating agents. Chelators are used in water treatment programs and specifically in steam engineering.Шаблон:Cn Although the treatment is often referred to as "softening," chelation has little effect on the water's mineral content, other than to make it soluble and lower the water's pH level.

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

A chelating agent is the main component of some rust removal formulations.

Green chelators

Шаблон:Main Aminopolycarboxylates chelators (like EDTA and NTA) and phosphonates have strong chelation effects for metals. Unfortunately, most of these compounds have severe disadvantages:

• They are not readily biodegradable .
• The infiltration of these chelants into the environment could cause dissolution of heavy metals from the sediments and soils, thereby mobilizing them thus leading to increased levels of metals, except phosphonates that do not mobilise toxic metals.
• These strong chelants persist in the environment due to their high solubility in water and low biodegradability (except NTA). It has been stated that 800 μg/L of EDTA has been found in some U.S. industrial and municipal wastewater treatment plants and up to 12 mg/L in European bodies of water. EDTA is now among the EU priority list of substances for risk assessment.
• According to Sillanpaa, ethylenediamine tetraacetic acid (EDTA) contains 10% nitrogen which could harm aquatic organisms.
• Furthermore, the majority of the traditional chelating agents (aminopolycarboxylates and phosphonates) are petroleum derived.
• Another concern is that most of these common chelants are produced from toxic substances like cyanide.
• Their persistence in the environment is because of their low biodegradability and high water solubility.
• In addition, studies have shown that there is a decline in the high quality phosphorus rock reserves used to produce phosphate chelants which could lead to higher costs associated with obtaining phosphates and phosphonate products. The continuous dependence on phosphates and phosphonate chelators will further accelerate the decline of finite high quality phosphate rocks. Furthermore, phosphates are essential components in fertilisers (used for food production) and therefore the utilisation of phosphates as chelators is in direct competition with the food industry.

Therefore, it is essential to look for Greener alternative chelating agents in order to reduce the reliance on these traditional chelants. The consumption of traditional aminopolycarboxylates chelators is indeed declining (–6% annually), because of the persisting concerns over their toxicity and negative environmental impact. In addition, the EU is regulating the use of phosphates in consumer laundry detergents and consumer dishwasher detergents in order to reduce the eutrophication risks and costs of phosphate removal by wastewater treatment plants.

Aminopolycarboxylic acids chelators are the most widely consumed chelating agents; however, the percentage of the Greener alternative chelators in this category continues to grow. In 2013, these Greener alternative chelants represented approximately 15% of the total aminopolycarboxylic acids demand. This is expected to rise to around 21% by 2018, replacing in particular the EDTA (ethylenediaminetetraacetic acid), NTA (nitrilotriacetic acid) and aminophosphonic acids used in cleaning applications. This is because of issues like non-biodegradability, toxicity, and mobilization of toxic metals by these traditional chelants as earlier mentioned. In addition, more than 90% of organic chemicals are derived from fossil fuel refineries which is not sustainable. The continuous depletion of petroleum resources coupled with a shift to Greener products by consumers means that it is vital to look for alternative Greener chelating agents. Therefore, in order to replace traditional chelants, the alternative chelating agents must have a strong ability to form complexes, as well as possess low nitrogen content so as to reduce the loading of nitrogen. In addition, they should be readily or at least inherently biodegradable. These alternative chelants are well favored by environmental protection policies. Examples of some Greener alternative chelating agents include ethylenediamine disuccinic acid ([S, S]-EDDS), polyaspartic acid (PASA), methylglycinediacetic acid (MGDA), glutamic diacetic acid (L-GLDA), citrate, gluconic acid, amino acids, plant extracts etc. Asemave and Asemave et al. reported the use of lipophilic β-diketone, 14,16-hentriacontanedione as Greener alternative chelator for metals recovery. These have been proposed to replace the classical EDTA and diethylenetriaminepentaacetic acid (DTPA) chelators in various applications. According to Hyvönen, alternative chelants have a lower chelating ability when compared to the traditional chelators, notwithstanding, this will make them less toxic.

Reversal

Dechelation (or de-chelation) is a reverse process of the chelation in which the chelating agent is recovered by acidifying solution with a mineral acid to form a precipitate.Шаблон:Rp

See also

• Шаблон:Annotated link

External links

• Шаблон:Wiktionary-inline