How to identify complexes and chelate
Dr.Richard Murphy
(Alltech European Bioscience Center)
There are a variety of products in the form of metal complexes that supplement the animal's nutrition, and whether the trace elements are complexed or otherwise associated with organic molecules to determine whether they are named "organic trace elements". Complexation or chelation is becoming more and more well known, but its concept is often confused in the animal feed industry. For example, metal amino acid complexes, metal amino acid chelates, metal polysaccharide complexes, and metalloprotein salts are everywhere, and the official does not give a clear definition to distinguish. For example, in Table 1 are various definitions of organic trace metals used in agricultural practices as defined by the American Association of Feed Control Officials (AAFCO, 1998).
So in these fuzzy definitions, how do we distinguish between complexes and chelate?
1 complex and chelate
In general, the term "complex" can be used to describe a substance formed by the reaction of a metal ion with a molecule or ion (ligand) which contains an atom having a lone pair of electrons. Complex metal ions are bound to the ligand by electron-donating oxygen, nitrogen and sulfur. Ligands containing only one donor atom are referred to as "monoatomic chelating ligands", while those containing two or more electron-bonding bonds to metal ions are referred to as two, three or four Ligand of the base. These multi-donor species can also be referred to as multi-ligand ligands. When such a ligand is bonded to a metal ion by two or more electron donating atom bonds, the complex formed contains one or more heterocyclic rings containing a metal atom, and the complex is called "chelating". Compound."
An amino acid is an example of a di-ligand that is bonded to a metal ion via the oxygen of the carboxylic acid group and the nitrogen of the amino group. In contrast, ethylenediaminetetraacetic acid (EDTA) is a hexa ligand ligand that contains six donor atoms. It forms highly stable complexes with most metal ions, but in fact, EDTA is not particularly suitable for the formation of trace element chelates because of the extremely low bioavailability of such complexes. Although the chelate can form a four, five, six or seven membered ring, studies have shown that the chelate with a five membered ring has the best stability.
It is worth noting that although all chelate complexes are complexes, not all complexes are chelate. Indeed, the general theory of chelation is simple, but the formation of stable trace element chelate must meet the following criteria:
(1) The chelating ligand contains at least two atoms forming a bond with the metal ion;
(2) The ligand must form a closed heterocyclic ring with the metal;
(3) Space must be able to chelate metals;
(4) The ratio of the ligand to the trace element must meet the minimum stability requirement.
A true chelate has a "cyclic structure" formed by a metal ion and an amino group and a carboxyl end of an amino acid.
The chelate is prepared by enzymatic reaction of an inorganic mineral salt with an amino acid and a small peptide under certain conditions. These amino acid and peptide ligands bind to the metal through more than one point, ensuring that the metal atom becomes part of a biologically stable ring structure. Digested products of amino acids and proteins such as small peptides are ideal ligands because they have at least two functional groups (amino and hydroxyl groups) such that they form a ring structure with trace elements. Only the so-called "transition elements", such as the physicochemical properties of copper, iron, manganese and zinc, enable them to form coordinating covalent bonds with amino acids and peptides and thus form stable complexes.
2 amino acids and peptides as ligands
Many viewpoints suggest that amino acids are more suitable and advantageous than peptides in forming trace element chelates, and have higher bioavailability than other products. We have considered some of the necessary criteria to ensure the biosynthesis of mineral chelates, but many other factors involved in the chelation process must also be considered, including:
(1) Relative balance - including metal ions and ligands;
(2) Displacement reaction kinetics of hydrated metal ions and existing complexes;
(3) redox behavior of metal ions and their complexes;
(4) A reaction involving the adjustment of a ligand.
Although it is difficult to simplify and explain these complex chemical reactions, in order to prove that some amino acids are more suitable than peptides to chelate with trace element peptides, we will mainly consider factors affecting the balance and stability of complexes.
When a metal salt such as copper sulfate is dissolved in water and complexed with a bidentate ligand amino acid, a series of complexes are formed. Each complex has a constant constant and is affected by the pH of the solution. Figure 1, where copper (II) sulfate reacts with glycine, highlights many notable features:
(1) Distribution of metal ions at a given concentration of metal and amino acid concentration according to the pH of the solution;
(2) The chelate of divalent metal ions is not necessarily neutral;
(3) Different metal ions have different stability constants. Therefore, the ratio of the metal as a specific substance depends not only on the pH of the solution but also on the stability constant of the complex.
The stability of the metal complex ultimately depends on the chemical nature of the metal ion and the ligand. In terms of the effect of metal ions on stability, increasing the ionic charge and increasing the electron affinity will result in the formation of a complex with higher stability. Ligands also have characteristics that affect the stability of the complex, including: (1) the basicity of the ligand; (2) the number of metal-chelating rings per ligand; (3) the size of the chelate ring; ) steric hindrance effect; (5) resonance effect; (6) ligand atom. Since the coordination compound acts as a product of a Lewis acid-base reaction in which the metal ions are acidic and the ligands are basic, they combine with the general basic ligand to form a more stable complex. The size of the chelate ring is also an important factor affecting stability.
Further analysis of Figure 1 reveals that there is a clear distinction between the relative stability of metal amino acid chelates and metalloprotein salts. Given that metalloprote salts are products derived from the chelation of soluble salts with amino acids and partially hydrolyzed proteins, the ion distribution curve will have a much greater complexity for protein salts than the corresponding metal amino acid chelate for a given metal ion. Object. If we believe that the product profile is a relatively stable indicator at a given pH and considers the infinite binding of a single amino acid mixture in two, three or even four peptides, then the total protein salt stability is in a wide pH range. In theory, it should be much larger than a specific metal amino acid chelate.
Figure 1 Metal ion dissociation curve in a solution containing divalent copper ions (0.001M) and glycine (0.002M)
3 Biological stability
Other influencing factors previously discussed help explain the overall stability of the chelate. At the same time, these factors can also be used to assess the stability of the metallized salt physicochemical properties under conditions of varying pH. Although there are various confusions and contradictions in the industry about chelation, trace element sequestration is actually a relatively simple chemical synthesis process. In general, we are able to distinguish between two true forms of chelation, each with specific chemical and biophysical properties. By carefully studying the important factors affecting the sequestration of trace elements, the products can be distinguished on the basis of biostability and even bioavailability and efficiency.
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