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 carbon tetraiodide. 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. Iron(III) iodide does not exist because iron(III) ions oxidize iodide ions in aqueous solution. Aqueous solutions of iodide dissolve iodine better than pure water due to the formation of complex ions:

I⁻(aq) + I₂(s) ⇌ I₃⁻(aq)

The colour of new triiodide ions formed are brown.

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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 as it is a 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]}. 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) ^{[6][7] [8]}:

Antioxidant biochemical mechanism of iodides ^[9]

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

Iodide (>6mg/day) can be used to treat patients with hyperthyroidism due to its ability to block the release of thyroid hormone (TH), known as the Wolff-Chaikoff Effect, from the thyroid gland. 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.

References