Hydroxyl radical

HydroxylThe hydroxyl radical, •OH, is the neutral form of the hydroxide ion (OH−). Hydroxyl radicals are highly reactive (easily becoming hydroxyl groups) and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H2O2 (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds. H3O+ + e− → •OH + H2    (Dissociative recombination:           1a) H3O+ + e− → •OH + •H + •H    (Dissociative recombination:           1b) HCO+2 + e− → •OH + CO    (Dissociative recombination:           2a) •O + HCO → •OH + CO    (Neutral–neutral:           3a) H− + H3O+ → •OH + H2 + •H     (Ion–molecular ion neutralization:          4a) •OH + O → O2 + •H    (Neutral–neutral:           1A) •OH + C+ → CO+ + •H    (Ion–neutral           2A) •OH + •N → NO + •H    (Neutral–neutral:           3A) •OH + C → CO + •H    (Neutral–neutral:          4A) •OH + •H → H2O + photon    (Neutral–neutral:           5A) The hydroxyl radical, •OH, is the neutral form of the hydroxide ion (OH−). Hydroxyl radicals are highly reactive (easily becoming hydroxyl groups) and consequently short-lived; however, they form an important part of radical chemistry. Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water. It is also an important radical formed in radiation chemistry, since it leads to the formation of hydrogen peroxide and oxygen, which can enhance corrosion and SCC in coolant systems subjected to radioactive environments. Hydroxyl radicals are also produced during UV-light dissociation of H2O2 (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds. In organic synthesis, hydroxyl radicals are most commonly generated by photolysis of 1-hydroxy-2(1H)-pyridinethione. The hydroxyl radical is often referred to as the 'detergent' of the troposphere because it reacts with many pollutants, decomposing them through 'cracking', often acting as the first step to their removal. It also has an important role in eliminating some greenhouse gases like methane and ozone. The rate of reaction with the hydroxyl radical often determines how long many pollutants last in the atmosphere, if they do not undergo photolysis or are rained out. For instance methane, which reacts relatively slowly with hydroxyl radical, has an average lifetime of >5 years and many CFCs have lifetimes of 50 years or more. Other pollutants, such as larger hydrocarbons, can have very short average lifetimes of less than a few hours. The first reaction with many volatile organic compounds (VOCs) is the removal of a hydrogen atom, forming water and an alkyl radical (R•). The alkyl radical will typically react rapidly with oxygen forming a peroxy radical. The fate of this radical in the troposphere is dependent on factors such as the amount of sunlight, pollution in the atmosphere and the nature of the alkyl radical that formed it. Hydroxyl radicals can occasionally be produced as a byproduct of immune action. Macrophages and microglia most frequently generate this compound when exposed to very specific pathogens, such as certain bacteria. The destructive action of hydroxyl radicals has been implicated in several neurological autoimmune diseases such as HAND when immune cells become over-activated and toxic to neighboring healthy cells. The hydroxyl radical can damage virtually all types of macromolecules: carbohydrates, nucleic acids (mutations), lipids (lipid peroxidation), and amino acids (e.g. conversion of Phe to m-Tyrosine and o-Tyrosine). The hydroxyl radical has a very short in vivo half-life of approximately 10−9 seconds and a high reactivity. This makes it a very dangerous compound to the organism. Unlike superoxide, which can be detoxified by superoxide dismutase, the hydroxyl radical cannot be eliminated by an enzymatic reaction.