Hydroxyl radical
The hydroxyl radical, •HO, is the neutral form of the hydroxide ion (HO–). Hydroxyl radicals are highly reactive 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 stress corrosion cracking 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, often acting as the first step to their removal. It also has an important role in eliminating some greenhouse gases like methane and ozone.[3] 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. 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 (See chapters 12 & 13 in External Links "University Lecture notes on Atmospheric chemistry)
Biological significance[edit]
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 HIV-associated dementia, when immune cells become over-activated and toxic to neighboring healthy cells.[4]
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.[5] This makes it a very dangerous compound to the organism.[6][7]
Unlike superoxide, which can be detoxified by superoxide dismutase, the hydroxyl radical cannot be eliminated by an enzymatic reaction. Mechanisms for scavenging peroxyl radicals for the protection of cellular structures include endogenous antioxidants such as melatonin and glutathione, and dietary antioxidants such as mannitol and vitamin E.[6]
Importance in the Earth's atmosphere[edit]
The hydroxyl •HO radicals is one of the main chemical species controlling the oxidizing capacity of the Earth's atmosphere, having a major impact on the concentrations and distribution of greenhouse gases and pollutants. It is the most widespread oxidizer in the troposphere, the lowest part of the atmosphere. Understanding •HO variability is important to evaluating human impacts on the atmosphere and climate. The •HO species has a lifetime in the Earth atmosphere of less than one second.[8] Understanding the role of •HO in the oxidation process of methane (CH4) present in the atmosphere to first carbon monoxide (CO) and then carbon dioxide (CO2) is important for assessing the residence time of this greenhouse gas, the overall carbon budget of the troposphere, and its influence on the process of global warming. The lifetime of •HO radicals in the Earth atmosphere is very short, therefore •HO concentrations in the air are very low and very sensitive techniques are required for its direct detection.[9] Global average hydroxyl radical concentrations have been measured indirectly by analyzing methyl chloroform (CH3CCl3) present in the air. The results obtained by Montzka et al. (2011)[10] shows that the interannual variability in •HO estimated from CH3CCl3 measurements is small, indicating that global •HO is generally well buffered against perturbations. This small variability is consistent with measurements of methane and other trace gases primarily oxidized by •HO, as well as global photochemical model calculations.
Astronomical importance[edit]
First detection of interstellar •HO[edit]
The first experimental evidence for the presence of 18 cm absorption lines of the hydroxyl (•HO) radical in the radio absorption spectrum of Cassiopeia A was obtained by Weinreb et al. (Nature, Vol. 200, pp. 829, 1963) based on observations made during the period October 15–29, 1963.[11]