Chelate Test Page

This page will test the loading and presentation of a number of animated GIF files created for an agricultural chelate page. Report any difficulties to Phillip Barak

Note: the animated GIFs have proven to be too large for practical use and have been largely replaced shortly with Chime and VRML representions. See the Virtual Museum of Minerals and Molecules.

structural diagram of EDTAThe archtypal synthetic chelating agent is EDTA, commonly known as ethylenediamine tetraacetic acid. The coordinating groups in EDTA are two amine nitrogens and four carboxylic oxygens, which are capable of wrapping around a central metal ion, such as Fe(III), and satisfying the octahedral coordination requirements of Fe(III).

EDTA was the first synthetic chelate used in nutrient solutions for hydroponics, although it was earlier noted that its effectiveness was limited to pH 6 or lower. It is occasionally used in foliar application of micronutrients. Soil applications are likewise limited to acidic and slightly acidic soils, which makes its useful limited since most micronutrient deficiencies that would call for chelate therapy appear in calcareous soils, with pH greater than 7.5.

structural diagram of EDTAA closely related synthetic chelate is DTPA, commonly known as diethylenetriamine pentaacetic acid. The coordinating groups in DTPA are, as in EDTA, two amine nitrogens and four carboxylic oxygens, which are capable of wrapping around a central metal ion, such as Fe(III), and satisfying the octahedral coordination requirements of Fe(III).

DTPA is widely used as an iron chelate in hydroponic solutions. It is occasionally soil-applied as an iron or zinc source.

structural model of EDDHAAn aromatic chemical relative of EDTA is EDDHA, commonly known as ethylenediaminedi(o-hydroxyphenylacetic) acid, or EHPG (N,N'-ethylenebis-2-(o-hydroxyphenyl) glycine). This molecule offers two amine nitrogens, two carboxylic oxygens, and two phenolic oxygens to satisfy octahedral coordination requirements. Because of the strong bonds between phenolic groups and Fe(III), chelates of this type are much stronger than purely carboxylic chelates. The phenol-Fe(III) bond gives a red to purple color to the ferrated chelate.

Examination of the chemical structure of EDDHA shows that there are two asymmetrical carbon atoms around which are arranged the acetic acid, phenol, and amino groups, along with an additional H to make up the tetrahedral requirement of the carbon. The rules of stereochemistry indicate that there are potentially 2n stereoisomers, where n is the number of asymmetrical atoms, so EDDHA could have four different stereoisomers. The rules of stereochemistry assign a counting system around the asymmetrical atom, based on various priorities, that yield either an R or S designation for a particular arrangement around an asymmetric atom. When EDDHA has one carbon with R and one S, it is identical to EDDHA with one S and one R, so the total number of stereoisomers is reduced by one. Stereoisomers that are superimposable on their own mirror images are called meso compounds, and one of the stereoisomers of EDDHA is therefore a meso stereoisomer.

Fe(III)-meso-EDDHA: ball and stick representation. [animated gif, 113 kb] Fe(III)-meso-EDDHA: space-filling representation. [animated gif, 290 kb]

When the meso-EDDHA coordinates with Fe(III), one of the phenolic oxygens coordinates with Fe(III) in an equatorial position and the other in a polar position, as shown in the animations above.

Fe(III)-rac-EDDHA: ball and stick representation. [animated gif, 113 kb] Fe(III)-rac-EDDHA: space-filling representation. [animated gif, 290 kb]

Two other stereoisomers are the R,R and S,S isomers. These are enantiomers, stereoisomers that are not superimposable on their mirror images. Equal amounts of enantiomers are termed a racemic mixture, and lab synthesis of molecules capable of stereoisomers typically produce racemic mixtures. (Biological syntheses, on the other hand, are highly directed in terms of specific stereochemistry.) The phenolic groups of the R,R and S,S isomers are both arranged equatorially.

The racemic and meso isomers of Fe-EDDHA can be separated from each other using paper chromatography, ion chromatography, and ion pair chromatography, as well as by careful crystallization. The stability of Fe(III)-rac-EDDHA has been found to be 2.26 log units higher than for the meso type, indicating a 500-fold difference in iron chelating ability due to stereoisomerism! Whether this makes one stereoisomer a better chelate for plant growth than the other is still unresolved.

Although EDDHA was originally "advertised" as the phenolic analog of EDTA, the truth is that EDDHA is an -amino acid, i.e., the amino and the carboxyl groups are bonded to the same carbon (adjacent to the carboxyl C), while in EDTA, one acetic acid group can be thought to be bonded directly to the amino group rather than the carbon adjacent to the carboxyl C.

structural diagram of HBEDThe true phenolic analog to EDTA, then, is a different compound: HBED, N,N'-bis(2-hydroxybenzyl)ethylenediamine-N,N'-diacetic acid. Examination reveals that it does not have asymmetric carbons. However, chromatographic evidence shows that Fe(III)-HBED may behave as if it has two asymmetric N due to the very stable chelatation causing very slow equilibrium, effectively producing two conformational isomers in equilibrium in a ~10:1 ratio.

This page was last modified by Phillip Barak, Univ. of Wisconsin, on 14 Oct 1998. All rights reserved.