iodide

An iodide ion is an iodine atom with a −1 charge.^[1] Compounds with iodine in formal oxidation state −1 are called iodides. This can include ionic compounds such as caesium iodide or covalent compounds such as phosphorus triiodide. This is the same naming scheme as is seen with chlorides and bromides. The chemical test for an iodide compound is to acidify the aqueous compound by adding some drops of acid, to dispel any carbonate ions present, then adding lead(II) nitrate, yielding a bright yellow precipitate of lead iodide. Most ionic iodides are soluble, with the exception of yellow silver iodide and yellow lead iodide. Aqueous solutions of iodide dissolve iodine better than pure water due to the formation of complex ions:

I⁻(aq) + I₂(s) ⇌ I₃⁻(aq)

The color of new triiodide ions formed is brown.

Examples

Examples or common iodides include:

• hydrogen iodide (HI)
• sodium iodide (NaI)
• potassium iodide (KI)
• carbon tetraiodide (CI₄)
• silver iodide (AgI)
• nitrogen triiodide (NI₃)

Iodide as an antioxidant

Iodide can function as an antioxidant reducing species that can detoxify reactive oxygen species such as hydrogen peroxide. Over three billion years ago, blue-green algae were the most primitive oxygenic photosynthetic organisms and are the ancestors of multicellular eukaryotic algae (1). Algae that contain the highest amount of iodine (1-3 % of dry weight) and peroxidase enzymes, were the first living cells to produce poisonous oxygen in the atmosphere.^[2][3] Therefore algal cells required a protective antioxidant action of their molecular components, in which iodides, through peroxidase enzymes, seem to have had this specific role.^{[4][5] [6]} In fact, iodides are greatly present and available in the sea, where algal phytoplankton, the basis of marine food-chain, acts as a biological accumulator of iodides, selenium, (and n-3 fatty acids) ^[7][8][9]:

Antioxidant biochemical mechanism of iodides ^[10]

2 I⁻ → I₂ + 2 e⁻ (electrons) = - 0.54 Volt ;

2 I⁻ + Peroxidase + H₂O₂ + 2 Tyrosine → 2 Iodo-Tyrosine + H₂O + 2 e⁻ (antioxidants);

2 e⁻ + H₂O₂ + 2 H⁺ (of intracellular water-solution) → 2 H₂O

Antioxidant biochemical mechanism of iodides, probably one of the most ancient mechanisms of defense from poisonous reactive oxygen species:

2 I⁻ + Peroxidase + H₂O₂ + Tyrosine, Histidine, Lipids, Carbons → Iodo-Compounds + H₂O + 2 e⁻ (antioxidants)

Iodo-Compounds: Iodo-Tyrosine, Iodo-Histidine, Iodo-Lipids, Iodo-Carbons.

Clinical use

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6 mg of iodide a day can be used to treat patients with hyperthyroidism due to its ability to inhibit the organification process in thyroid hormone synthesis. This is known as the Wolff-Chaikoff Effect. In fact, prior to 1940, iodides were the predominant antithyroid agents. In large doses, iodides inhibit proteolysis of thyroglobulin. This permits TH to be synthesized and stored in colloid, but not released into the bloodstream.

Of note, this treatment is seldom used today as a stand-alone therapy despite the rapid improvement of patients immediately following administration. The major disadvantage of iodide treatment lies in the fact that excessive stores of TH accumulate, slowing the onset of action of thioamides (TH synthesis blockers). Additionally, the functionality of iodides fade after the initial treatment period. An "escape from block" is also a concern, as extra stored TH may spike following discontinuation of treatment.

See also

• "Seaweed use iodine as an antioxidant". Chemistry World blog. http://prospect.rsc.org/blogs/cw/?p=956. Retrieved 2008-05-15. 

• "Stressed seaweed contributes to cloudy coastal skies, study suggests". http://www.eurekalert.org/pub_releases/2008-05/uom-ssc050608.php. Retrieved 2008-05-15. 

• "Iodine Deficiency: studies by dr. Sebastiano Venturi (Italy)". http://web.tiscali.it/iodio/. Retrieved 2008-05-15. 

References