What Is A Chelate Biology Essay

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The word chelate derives from the Greek word "chel", meaning a crab's claw, and refers to the pincer-like manner in which the metal is bound.  Chemically, a chelate is a compound from complexing of cations with organic compounds resulting in a ring structure.  Typical structure of chelates with known organic acids are shown below for citric acid, tartaric acid, gluconic acid and glycine.

Researchers first used the word chelate in the 1920s because it describes the principal of grasping and holding something, which is essentially what occurs in the process of chelation. Specifically, a chelate is a chemical compound in the form of a heterocyclic ring, containing a metal ion attached by coordinate bonds to at least two nonmetal ions. Generally, think of a chelate as a receptor or binding site for micronutrients that maintains their availability to the plant by forming a bridge between the nutrient and the root zone. Micronutrients are crucial partners in every metabolic function of the plant. Respiration, photosynthesis, protein synthesis, energy transfer, cell division, and cell elongation are all dependent on an adequate supply of calcium, iron, copper, manganese, zinc, and other micronutrients.

http://www.progressivegardens.com/growers_guide/plantnut.html

There are synthetic and organic forms of chelates. Ethylenediaminetetraacetate (EDTA) is the most common chelating agent found in synthetic fertilizers. Like other synthetic chelates, EDTA is an alien compound to the plant and is therefore not absorbed by the plant. When the chelated element is required, the plant will remove the element, for example iron, from the chelate and absorb the element. However, since the chelating agent is foreign to the plant, it will give up the chelating agent (EDTA) back into solution where it is free to chelate other positively charged elements. EDTA is better suited to slightly lower than neutral pH levels. Iron often becomes deficient at higher pH values such as those typically associated with uncalibrated rockwool or mineral soils. EDTA has four points of connection to the element it chelates. In some situations four points of connection mayhold the element

too tightly, where in other growing situations, it may not hold it tight enough. This is all taken into account during the production of fertilizers.

Diethylenetriaminepentaacetate (DTPA) is a chelating agent better suited to high pH levels. As the chemical name suggests, it has five (penta) points of connection to the element it chelates. DTPA is more costly than EDTA and is less soluble so it is found in smaller quantities than EDTA in most synthetic fertilizer formulations.

Studies suggest that ethylenediaminedihydroxy-phenylaceticacid (EDDHA) is the superior synthetic chelating agent. Although, its relatively high cost prohibits it from being added to many fertilizer formulations.

The first metal chelate of the un-substituted 2,6-pyridine-dicarboxamide (pdcam). Synthesis, molecular and crystal structure, and properties of [CuII(pdc)(pdcam)]·2H2O (pdc = 2,6-pyridine- dicarboxylato(2-) ligand).

http://www.lec.csic.es/~duane/publications.html

Chelating agents:

A chelating agent is a molecule or ion, called a ligand, that forms more than one coordinate bond with a metal ion (usually in water), through two or more functional groups in the ligand that donate electron pairs to the metal. Two or more electron-donor groups in the ligand thus form a chelate ring with the metal ion. A ligand may be bidentate, tridentate, tetradentate, etc., depending on whether it has two, three, or four, etc., coordination (electron-donating) groups. The number of such coordinate bonds in a complex is called the coordination number (of the metal). Below are some examples of simple chelates of metal ions with coordination numbers of 4 and 6.

Hydrated metal ion 

Metal chelate of bidentate ligand (ethylenediamine)

 

Metal chelate of tridentate ligand (diethylenetriamine)

 

Metal chelate of tetradentate ligand (triethylenetetramine)

 

Metal chelate of hexadentate ligand (pentaethylenehexamine)

 

Kirk-Othmer, Encyclopedia of Chemical Technology, 2nd Ed., Vol. 6, pp. 2-3, John Wiley & Sons, Inc., 1965

Ethylenediaminetetraacetic acid:This most ubiquitous chelating agent is found in many soaps, detergents, water softeners, shampoos, foods, and a host of other commercial products. Its purpose is to sequester metal ions in water to prevent them from forming insoluble precipitates, for example with the fatty acids in soaps:

M++ + 2 RCO2Na 2 Na+ + (RCO2)2M (a precipitate)

These precipitated salts would be responsible for the "ring around the bathtub" and unsightly haze on washed tableware:

EDTA

 

Metal ion sequestered by EDTA

(calcium edetate)

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IMPORTANCE & SIGNIFICANCE OF CHELATES:

Many essential biological chemicals are chelates. Chelates play important roles in oxygen transport and in photosynthesis. Furthermore, many biological catalysts (enzymes) are chelates. In addition to their significance in living organisms, chelates are also economically important, both as products in themselves and as agents in the production of other chemicals.

A chelate is a chemical compound composed of a metal ion and a chelating agent. A chelating agent is a substance whose molecules can form several bonds to a single metal ion. In other words, a chelating agent is a multidentate ligand. An example of a simple chelating agent is ethylenediamine.

ethylenediamine

A single molecule of ethylenediamine can form two bonds to a transition-metal ion such as nickel(II), Ni2+. The bonds form between the metal ion and the nitrogen atoms of ethylenediamine. The nickel(II) ion can form six such bonds, so a maximum of three ethylenediamine molecules can be attached to one Ni2+ ion.

 

 

chelate with one

ethylenediamine ligand

 

chelate with two

ethylenediamine ligands

 

chelate with three

ethylenediamine ligands

In the two structures on the left, the bonding capacity of the Ni2+ ion is completed by water molecules. Each water molecule forms only one bond to Ni2+, so water is not a chelating agent. Because the chelating agent is attached to the metal ion by several bonds, chelates tend to be more stable than complexes formed with monodentate ligands such as water.

Porphine is a chelating agent similar to ethylenediamine in that it forms bonds to a metal ion through nitrogen atoms. Each of the four nitrogen atoms in the center of the molecule can form a bond to a metal ion. Porphine is the simplest of a group of chelating agents called porphyrins. Porphyrins have a structure derived from porphine by replacing some of the hydrogen atoms around the outside with other groups of atoms.

porphine

 

heme

One important porphyrin chelate is heme, the central component of hemoglobin, which carries oxygen through the blood from the lungs to the tissues. Heme contains a porphyrin chelating agent bonded to an iron(II) ion. Iron, like nickel, can form six bonds. Four of these bonds tie it to the porphyrin. One of iron's two remaining bonds holds an oxygen molecule as it is transported through the blood. Chlorophyll is another porphyrin chelate. In chlorophyll, the metal at the center of the chelate is a magnesium ion. Chlorophyll, which is responsible for the green color of plant leaves, absorbs the light energy that is converted to chemical energy in the process of photosynthesis.

Another biologically significant chelate is vitamin B-12. It is the only vitamin that contains a metal, a cobalt(II) ion bonded to a porphyrin-like chelating agent. As far as is known, it is required in the diet of all higher animals. It is not synthesized by either higher plants or animals, but only by certain bacteria and molds. These are the sources of the B-12 found in animal products. Because vitamin B-12 is not found in higher plants, vegetarians must take care to include in their diets foods or supplements that contain the vitamin.

A chelating agent of particular economic significance is ethylenediaminetetraacetic acid (EDTA).

ethylenediaminetetraacetic acid (EDTA)

EDTA is a versatile chelating agent. It can form four or six bonds with a metal ion, and it forms chelates with both transition-metal ions and main-group ions. EDTA is frequently used in soaps and detergents, because it forms a complexes with calcium and magnesium ions. These ions are in hard water and interfere with the cleaning action of soaps and detergents. The EDTA binds to them, sequestering them and preventing their interference. In the calcium complex, [Ca(EDTA)]2-, EDTA is a tetradentate ligand, and chelation involves the two nitrogen atoms and two oxygen atoms in separate carboxyl (-COO-) groups. EDTA is also used extensively as a stabilizing agent in the food industry. Food spoilage is often promoted by naturally-occurring enzymes that contain transition-metal ions. These enzymes catalyze the chemical reactions that occur during spoilage. EDTA deactivates these enzymes by removing the metal ions from them and forming stable chelates with them. It promotes color retention in dried bananas, beans, chick peas, canned clams, pecan pie filling, frozen potatoes, and canned shrimp. It improves flavor retention in canned carbonated beverages, salad dressings, mayonnaise, margarine, and sauces. It inhibits rancidity in salad dressings, mayonnaise, sauces, and sandwich spreads. EDTA salts are used in foods at levels ranging from 33 to 800 ppm.

In other applications, EDTA dissolves the CaCO3 scale deposited from hard water without the use of corrosive acid. EDTA is used in the separation of the rare earth elements from each other. The rare earth elements have very similar chemical properties, but the stability of their EDTA complexes varies slightly. This slight variation allows EDTA to effectively separate rare-earth ions. EDTA is used as an anticoagulant for stored blood in blood banks; it prevents coagulation by sequestering the calcium ions required for clotting. As an antidote for lead poisoning, calcium disodium EDTA exchanges its chelated calcium for lead, and the resulting lead chelate is rapidly excreted in the urine. The calcium salt of EDTA, administered intravenously, is also used in the treatment of acute cadmium and iron poisoning.

Dimercaprol (2,3-dimercapto-1-propanol) is an effective chelating agent for heavy metals such as arsenic, mercury, antimony, and gold. These heavy metals form particularly strong bonds to the sulfur atoms in dimercaprol.

Dimercaprol was originally employed to treat the toxic effects of an arsenic-containing mustard gas called Lewisite [dichloro(2-chlorovinyl)arsine], which was used in World War I. The chelated metal cannot enter living cells and is rapidly excreted from the body. Since dimercaprol is water insoluble, it is dissolved in an oil base (often peanut oil) and injected intramuscularly.

http://scifun.chem.wisc.edu/CHEMWEEK/Chelates/Chelates.html

LANTHANIDE CHELATES:

Lanthanide chelates have unique fluorescence properties that serve them well as sensitive labels. For example, they have long fluorescence decay after excitation so it is possible to detect fluorescence signals even after a long time delay, which virtually eliminates all background fluorescence. Lanthanide chelates also display a large Stoke's Shift compared to traditional labels. Such a large Stoke's Shift minimizes crosstalk between excitation and emission signals and contributes to a high signal-to-noise ratio. Moreover lanthanides are suitable for quantitative multianalyte assays because of their narrow emission peaks at different wavelengths and their different fluorescence lifetime. The combination of spectral window and time window can be utilized for the optimization of the measurement parameters in order to obtain maximal sensitivity and to minimize signal spillover.

Labels include Europium (Eu), Samarium (Sm), Dysprosium (Dy), and Terbium (Tb) chelates.  The plot below illustrates the fluorescence emission of these chelates.

http://www.case.edu/med/pathology/faculty/cobblab/TRF.html

Rare earth chelates with identical organic ligands have been synthesized and characterized optically. The low- lying ligand triplet state prevents rare earth emission from the visible emitting rare earth ions, whilst a chemiluminescent process is feasible for Yb.

The Figure shows a Yb chelate structure.

http://www3.interscience.wiley.com/journal/109716480/issue

The significance of chelation process in soil are:

1.  Increase the availability of nutrients.  

Chelating agents will bind the relatively insoluble iron in high pH soil and make it available to plants.

 

2.  Prevent mineral nutrients from forming insoluble precipitates.

The chelating agents of the metal ions will protect the chelated ions from unfavorable chemical reactions and hence increase the availability of these ions to plants.  One example is iron in high pH soil.  In high pH soil, iron will react with hydroxyl group (OH-) to form insoluble ferric hydroxide (Fe(OH)3) which is not available to plants.       

Fe+3+ 3 OH-

-------->

Fe (OH)3

Soluble

 

Insoluble

Chelation will prevent this reaction from happening and hence render iron available to plants.

 

3.  Reduce toxicity of some metal ions to plants.

Chelation in the soil may reduce the concentration of some metal ions to a non-toxic level.  This process is usually accomplished by humic acid and high-molecular-weight components of organic matter.

 

4.  Prevent nutrients from leaching.

Metal ions forming chelates are more stable than the free ions.  Chelation process reduces the loss of nutrients through leaching.

 

5.  Increase the mobility of plant nutrients.

Chelation increases the mobility of nutrients in soil.  This increased mobility enhances the uptake of these nutrients by plants.

 

6.  Suppress the growth of plant pathogens.

Some chelating agents may suppress the growth of plant pathogens by depriving iron and hence favor plant growth.

http://www.scielo.br/img/revistas/sa/v63n3/29832f1.gif

APPLICATIONS OF CHELATES:

1.IRON CHELATES BEAT CANCER:

Cancer cells need more iron than normal body cells to sustain their abnormally rapid growth. A range of organic molecules that can bind iron in a tight chelate complex and thus deplete tumours of this vital element. While many such compounds show some activity against tumour cells, the researchers identified one particularly potent substance, Dp44mT (di-2-pyridylketone-4,4,-dimethyl-3-thiosemicarbazone). Studying the activity of this substance in vivo, in human tumours implanted into mice (xenografts), the researchers found that Dp44mT has promising anti-tumour activity at concentrations so low it does not cause side-effects in the rest of the body. Surprisingly, the researchers found that at the lowest active doses, even the tumour tissue wasn't significantly depleted of iron. 

Thus, the authors suggest two other mechanisms which might account for the remarkable anti-tumour activity of this compound. One direct way in which it might harm the tumour cells is by entering the cell and forming iron chelate complexes that are cytotoxic due to the redox chemistry they engage in. Additionally, the researchers suspect that the presence of the chelator leads to the upregulation of a tumour-suppressor gene known as Ndrg1 (N-myc downstream regulated gene-1). The only worrying side effect the researchers observed, after administration of higher doses, were heart problems. The researchers also attribute these effects to the redox chemistry of the chelate complex. However, they are confident that additional chelators or scavengers can help control these effects. The most promising aspect of their findings, the researchers say, is the observation that Dp44mT is active against a broad range of human tumours including those that have shown resistance towards conventional chemotherapy.

http://www.rsc.org/chemistryworld/news/2006/september/18090601.asp

M Whitnall et al, Proc. Natl. Acad. Sci. USA, DOI: 10.1073/pnas.0604979103

2.The most common applications of chelates is in water softening and treatment of poisoning. In the former instance, a compound such as sodium tripolyphosphate is added to water. That compound forms chelates with calcium and magnesium ions, ions responsible for the hardness in water. Because of their ability to "tie up" metal ions in chelates, compounds like sodium tripolyphosphate are sometimes referred to as sequestering agents.

3. A typical sequestering agent used to treat poison victims is ethylenediaminetetraacetic acid, commonly known as EDTA. Suppose that a person has swallowed a significant amount of lead and begins to display the symptoms of lead poisoning. Giving the person EDTA allows that molecule to form chelates with lead ions, removing that toxic material from the bloodstream.

http://science.jrank.org/pages/1375/Chelate.html

4.EDTA is also used in root canal treatment as a way to irrigate the canal. EDTA softens the dentin facilitating access to the entire canal length and to remove the smear layer formed during instrumentation.

5.Chelate complexes of gadolinium are often used as contrast agents in MRI scans.

6. Antibiotic drugs of the tetracycline family are chelators of Ca2+ and Mg2+ ions.

7. Chelation therapy is the use of chelating agents to detoxify poisonous metal agents such as mercury, arsenic, and lead by converting them to a chemically inert form that can be excreted without further interaction with the body.

http://tuberose.com/Metal_Detoxification.html

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