Hydroxyl radical

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. 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. 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. == Importance in the Earth's atmosphere ==

The hydroxyl radical (•OH) 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 most air pollutants. Even though •OH is of order 10−13 of the abundance of atmospheric oxygen, it is still the most widespread oxidizer in the troposphere, the lowest part of the atmosphere. it has a lifetime in the Earth's atmosphere of less than one second. Understanding the role of •OH 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 main production pathway of the OH radical in the troposphere comes from the photolysis of ozone at wavelengths less than 320 nm. The excited atomic oxygen formed as a result, O(1D), reacts very quickly with water vapor, H2O, forming two hydroxyl radicals. {O3} + h\nu \longrightarrow {O2} + {O(^1D)} \quad \lambda {O(^1D)} + {H2O} \longrightarrow 2 {.}{OH} Because •OH production in the troposphere relies on the abundance of UV-B radiation, its formation rate is maximized around the equator. The abundance of water vapor around the ITCZ, near the equator, helps the second reaction to evolve quickly. The photolysis of hydrogen peroxide (H2O2) is another common way hydroxyl radicals are produced. This occurs at wavelengths less than 300 nm, maximizing its photolysis at 250 nm. 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) show that the interannual variability in •OH estimated from CH3CCl3 measurements is small, less than 2%, indicating that global •OH is generally well buffered against perturbations. This small variability is consistent with measurements of methane and other trace gases primarily oxidized by •OH, as well as global photochemical model calculations. == Astronomical importance ==

First detection of interstellar •HO 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. Important subsequent reports of •HO astronomical detections Energy levels •HO is a diatomic molecule. The electronic angular momentum along the molecular axis is +1 or −1, and the electronic spin angular momentum S=1/2. Because of the orbit-spin coupling, the spin angular momentum can be oriented in parallel or anti-parallel directions to the orbital angular momentum, producing the splitting into Π1/2 and Π3/2 states. The 2Π3/2 ground state of •HO is split by lambda doubling interaction (an interaction between the nuclei rotation and the unpaired electron motion around its orbit). Hyperfine interaction with the unpaired spin of the proton further splits the levels. Chemistry of the molecule •HO In order to study gas phase interstellar chemistry, it is convenient to distinguish two types of interstellar clouds: diffuse clouds, with T=30–100 K, and n=10–1000 cm−3, and dense clouds with T=10–30K and density n=–. Ion-chemical routes in both dense and diffuse clouds have been established for some works (Hartquist 1990). •HO production pathways The •HO radical is linked with the production of H2O in molecular clouds. Studies of •HO distribution in Taurus Molecular Cloud-1 (TMC-1) suggest that in dense gas, •HO is mainly formed by dissociative recombination of . Dissociative recombination is the reaction in which a molecular ion recombines with an electron and dissociates into neutral fragments. Important formation mechanisms for •HO are: (1a) Dissociative recombination (1b) Dissociative recombination (2a) Dissociative recombination (3a) Neutral-neutral (4a) Ion-molecular ion neutralization •HO destruction pathways Experimental data on association reactions of •H and •HO suggest that radiative association involving atomic and diatomic neutral radicals may be considered as an effective mechanism for the production of small neutral molecules in the interstellar clouds. The formation of O2 occurs in the gas phase via the neutral exchange reaction between •O and •HO, which is also the main sink for •HO in dense regions. Rate constants have the form: :k(T) = \alpha\left(\frac{T}{300}\right)^\beta e^{-\frac{\gamma}{T}} The following table has the rate constants calculated for a typical temperature in a dense cloud (10 K). Formation rates (rix) can be obtained using the rate constants k(T) and the abundances of the reactant species C and D: :rix = k(T)ix[C] [D] where [Y] represents the abundance of the species Y. In this approach, abundances were taken from the 2006 UMIST database, and the values are relative to the H2 density. The following table shows rates for each pathway relative to pathway 1a (as the ratio rix/r1a) in order to compare the contributions of each to hydroxyl formation. The results suggest that pathway 1a is the most prominent mode of hydroxyl formation in dense clouds, which is consistent with the report from Harju et al.. The •HO molecule has been observed in the interstellar medium since 1963 through its 18-cm transitions. In the subsequent years, •HO was observed by its rotational transitions at far-infrared wavelengths, mainly in the Orion region. Because each rotational level of •HO is split by lambda doubling, astronomers can observe a wide variety of energy states from the ground state. •HO as a tracer of shock conditions Very high densities are required to thermalize the rotational transitions of •HO, so it is difficult to detect far-infrared emission lines from a quiescent molecular cloud. Even at H2 densities of 106 cm−3, dust must be optically thick at infrared wavelengths. But the passage of a shock wave through a molecular cloud is precisely the process which can bring the molecular gas out of equilibrium with the dust, making observations of far-infrared emission lines possible. A moderately fast shock may produce a transient raise in the •HO abundance relative to hydrogen. So, it is possible that far-infrared emission lines of •HO can be a good diagnostic of shock conditions. In diffuse clouds Diffuse clouds are of astronomical interest because they play a primary role in the evolution and thermodynamics of the ISM. Observation of the abundant atomic hydrogen in 21 cm has shown good signal-to-noise ratio in both emission and absorption. Nevertheless, HI observations have a fundamental difficulty when they are directed to low-mass regions of the hydrogen nucleus, such as the center part of a diffuse cloud: the thermal width of hydrogen lines are of the same order as the internal velocity structures of interest, so cloud components of various temperatures and central velocities are indistinguishable in the spectrum. Molecular line observations in principle do not suffer from these problems. Unlike HI, molecules generally have an excitation temperature Tex kin, so that emission is very weak even from abundant species. CO and •HO are considered to be the most easily studied candidate molecules. CO has transitions in a region of the spectrum (wavelength 2O masers form, and where total densities drop rapidly and UV radiation from young stars can dissociate H2O molecules. So, observations of •HO masers in these regions can be an important way to probe the distribution of the important H2O molecule in interstellar shocks at high spatial resolutions. == Application in water purification ==

Hydroxyl radicals also play a key role in the oxidative destruction of organic pollutants. == See also ==