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A hydroxyl is a chemical functional group containing an oxygen atom connected by a covalent bond to a hydrogen atom. It is sometimes called the alcohol functional group because when bonded to carbon in a molecule that otherwise contains only hydrogen and carbon the hydroxyl group defines the molecule as an alcohol, resulting in a name ending in -ol. A hydroxyl group bonded covalently to the carbon of a carbonyl group (-C=O) produces a carboxyl group (-COOH) that is the defining group of a carboxylic acid. When the -OH group participates in an ionic bond, the [OH−] anion is called the "hydroxide" ion.
The oxygen atom in the hydroxyl group is much more electronegative than the hydrogen, so that the electric charge is closer to the oxygen atom. Similar to water, compounds containing the hydroxyl group are capable of forming hydrogen bonds. Primary alcohols are miscible with water. In complex compounds, the portion of the molecule containing the hydroxyl group is hydrophilic.
The hydroxyl group appears throughout organic chemical structure and biochemical structure. It appears covalently bonded to carbon in sugars. The hydroxyl group appears on the second carbon of the hydroxyl amino acids serine and threonine. It appears on the phenol side chain of tyrosine. The capacity of the hydroxyl group to form hydrogen bonds imparts water solubility and internal structural stability to proteins that include these amino acids.
Hydroxyl groups participate in the dehydration reactions that link simple biological molecules into long chains. The joining of a fatty acid to glycerol to form a triacylglycerol removes the -OH from the carboxyl end of the fatty acid. The joining of two aldehyde sugars to form a disaccharide removes the -OH from the carboxyl group at the aldehyde end of one sugar. The creation of a peptide bond to link two amino acids to make a protein removes the -OH from the carboxyl group of one amino acid.
Hydroxyl radicals are highly reactive and undergo chemical reactions that make them short-lived. When biological systems are exposed to hydroxyl radicals, they can cause damage to cells, including those in humans, where they react with DNA, lipids, and proteins.
In 2009, India's Chandrayaan-1 satellite, NASA's Cassini spacecraft and the Deep Impact probe have each detected the presence of water by evidence of hydroxyl fragments on the Moon. As reported by Richard Kerr, "A spectrometer (the Moon Mineralogy Mapper, aka "M3"), detected an infrared absorption at a wavelength of 3.0 micrometers that only water or hydroxyl—a hydrogen and an oxygen bound together—could have created." NASA also reported in 2009 that the LCROSS probe revealed an ultraviolet emission spectrum consistent with hydroxyl presence. The Venus Express orbiter has been continuously sending back Venus science data since April 11, 2006. Results from Venus Express include the detection of hydroxyl in the atmosphere.
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