A hydroxy or hydroxyl group is the entity with the formula OH. It contains oxygen bonded to hydrogen. In organic chemistry, alcohols and carboxylic acids contain hydroxy groups. Both the negatively charged anion OH−, called hydroxide, and the neutral radical ·OH, known as the hydroxyl radical, consist of an unbounded hydroxyl group.
Water, alcohols, carboxylic acids, and many other hydroxy-containing compounds can be deprotonated readily. This behavior is rationalized by the disparate electronegativities of oxygen and hydrogen. Hydroxyl-containing compounds engage in hydrogen bonding, which causes them to stick together, leading to higher boiling and melting than found for compounds that lack this functional group. Organic compounds, which are often poorly soluble in water, become water-soluble when they contain two or more hydroxy groups, as illustrated by sugars and amino acid.
The hydroxy group is pervasive in chemistry and biochemistry. Many inorganic compounds contain hydroxy groups, including sulfuric acid, the chemical compound produced on the largest scale industrially.
Hydroxy 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 carboxy end of the fatty acid. The joining of two aldehyde sugars to form a disaccharide removes the −OH from the carboxy 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 carboxy 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 can react with DNA, lipids, and proteins.
Airglow of the EarthEdit
The Earth's night sky is illuminated by diffuse light, called airglow, that is produced by radiative transitions of atoms and molecules. Among the most intense such features observed in the Earth's night sky is a group of infrared transitions at wavelengths between 700 nanometers and 900 nanometers. In 1950, Aden Meinel showed that these were transitions of the hydroxyl molecule, OH.
Surface of the MoonEdit
In 2009, India's Chandrayaan-1 satellite and the National Aeronautics and Space Administration (NASA) Cassini spacecraft and Deep Impact probe each detected evidence of water by evidence of hydroxyl fragments on the Moon. As reported by Richard Kerr, "A spectrometer [the Moon Mineralogy Mapper, a.k.a. "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.
On October 26, 2020, NASA reported definitive evidence of water on the sunlit surface of the Moon, in the vicinity of the crater Clavius (crater), obtained by the Stratospheric Observatory for Infrared Astronomy (SOFIA). The SOFIA Faint Object infrared Camera for the SOFIA Telescope (FORCAST) detected emission bands at a wavelength of 6.1 micrometers that are present in water but not in hydroxyl. The abundance of water on the Moon's surface was inferred to be equivalent to the contents of a 12-ounce bottle of water per cubic meter of lunar soil. 
Atmosphere of VenusEdit
The Venus Express orbiter collected Venus science data from April 2006 until December 2014. In 2008, Piccioni, et al. reported measurements of night-side airglow emission in the atmosphere of Venus made with the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS) on Venus Express. They attributed emission bands in wavelength ranges of 1.40 - 1.49 micrometers and 2.6 - 3.14 micrometers to vibrational transitions of OH. This was the first evidence for OH in the atmosphere of any planet other than Earth's.
Atmosphere of MarsEdit
In 2013, OH near-infrared spectra were observed in the night glow in the polar winter atmosphere of Mars by use of the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM).
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- A. B. Meinel (1950). "OH Emission Bands in the Spectrum of the Night Sky. I". Astrophysical Journal. 111: 555–564.
- Richard A. Kerr (24 September 2009). "A Whiff of Water Found on the Moon". Science Now. Retrieved 2016-06-01.
- Jonas Dino (13 November 2009). "LCROSS Impact Data Indicates Water on Moon". NASA. Retrieved 2009-11-14.
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- Felicia Chou; Alison Hawkes (26 October 2020). "NASA's SOFIA Discovers Water on Sunlit Surface of Moon". NASA. Retrieved 2020-10-26.
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- R. Todd Clancy; Brad J.Sandor; Antonio García-Muñoz; Franck Lefèvre; Michael D.Smith; Michael J.Wolff; Franck Montmessin; Scott L.Murchie; Hari Nair (2013). "First detection of Mars atmospheric hydroxyl: CRISM Near-IR measurement versus LMD GCM simulation of OH Meinel band emission in the Mars polar winter atmosphere". Icarus. 226: 272–281.
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