Antioxidants metal chelation inhibition reaction

The valency of the metal ion changes in every step so that a single atom of heavy metal (Me) may produce many free radicals. Metal chelating compounds, such as citric, tartaric or phosphoric acids, ascorbic acid, phytin or phosphatidic acids, combine with metals to form non-reactive compounds so that the oxidation reactions are inhibited and natural food antioxidants are saved. 

Several studies indicate that catechins and procyanidins are powerful scavengers of ROS. Some findings regarding the antioxidant activity of proanthocyanidins are listed in Ref. [100]. Other antioxidant mechanisms are the chelation of transition metals, as well as the mediation and inhibition of enzymes. The metal-chelating activity of proanthocyanidins is thought to be due to their capacity to reduce the concentration, and thus the oxidative activity, of hydroxyl radicals formed by Fenton reaction catalyzed by iron or copper. Flavanols also influence oxidative stress via enzyme modification and modulation of cell signaling pathways the extent of the effect relies greatly on flavanol structure-related protein reactivity [101]. 

Transition metals will promote oxidative reactions by hydrogen abstraction and by hydroperoxide decomposition reactions that lead to the formation of free radicals. Prooxidative metal reactivity is inhibited by chelators. Chelators that exhibit antioxidative properties inhibit metal-catalyzed reactions by one or more of the following mechanims prevention of metal redox cycling occupation of all metal coordination sites thus inhibiting transfer of electrons formation of insoluble metal complexes stearic hinderance of interactions between metals and oxidizable substrates (e.g., peroxides). The prooxidative/antioxidative properties of a chelator can often be dependent on both metal and chelator concentrations. For instance, ethylene diamine tetraacetic acid (EDTA) can be prooxidative when EDTAiiron ratios are <1 and antioxidative when EDTAiiron is >1. The prooxidant activity of some metal-chelator complexes is due to the ability of the chelator to increase metal solubility and/or increase the ease by which the metal can redox cycle. 

As mentioned above, in contrast to classic antioxidant vitamins E and C, flavonoids are able to inhibit free radical formation as free radical scavengers and the chelators of transition metals. As far as chelators are concerned their inhibitory activity is a consequence of the formation of transition metal complexes incapable of catalyzing the formation of hydroxyl radicals by the Fenton reaction. In addition, as shown below, some of these complexes, for example, iron- and copper-rutin complexes, may acquire additional antioxidant activity. 

Chelators of transition metals, mainly iron and copper, are usually considered as antioxidants because of their ability to inhibit free radical-mediated damaging processes. Actually, the so-called chelating therapy has been in the use probably even earlier than antioxidant therapy because it is an obvious pathway to treat the development of pathologies depending on metal overload (such as calcium overload in atherosclerosis or iron overload in thalassemia) with compounds capable of removing metals from an organism. Understanding of chelators as antioxidants came later when much attention was drawn to the possibility of in vivo hydroxyl radical formation via the Fenton reaction ... 

Stabilizers inhibit the chemical reaction between two or more other chemicals, and inhibit the separation of suspensions, emulsions, or foams. Stabilizers include (1) antioxidants that prevent unwanted oxidation of food materials. (2) UV stabilizers that protect food materials from harmful effects of UV radiation, being (a) UV absorbers which absorb UV radiation and prevent it from penetrating the materials, as sunscreens, (b) Quenchers which dissipate the radiation energy as heat instead of letting it break chemical bonds, (c) Scavengers that eliminate the free radicals formed by UV radiation, as hindered-amine light stabilizers. (3) Sequestrants that inactivate traces of metal ions that would otherwise act as catalysts by forming chelate complexes. (4) Emulsifiers and surfactants that stabilize emulsions. 

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