Galactose

Galactose (gəˈlæktoʊs, galacto- + -ose, "milk sugar"), sometimes abbreviated Gal, is a monosaccharide sugar that is about as sweet as glucose, and about 65% as sweet as sucrose. It is an aldohexose and a C-4 epimer of glucose. A galactose molecule linked with a glucose molecule forms a lactose molecule. Galactan is a polymeric form of galactose found in hemicellulose, and forming the core of the galactans, a class of natural polymeric carbohydrates. D-Galactose is also known as brain sugar since it is a component of glycoproteins (oligosaccharide-protein compounds) found in nerve tissue. The word galactose was coined by Charles Weissman in the mid-19th century and is derived from Greek γαλακτος, galaktos, (of milk) and the generic chemical suffix for sugars -ose. The etymology is comparable to that of the word lactose in that both contain roots meaning "milk sugar". Lactose is a disaccharide of galactose plus glucose. Galactose exists in both open-chain and cyclic form. The open-chain form has a carbonyl at the end of the chain. Four isomers are cyclic, two of them with a pyranose (six-membered) ring, two with a furanose (five-membered) ring. Galactofuranose occurs in bacteria, fungi and protozoa, and is recognized by a putative chordate immune lectin intelectin through its exocyclic 1,2-diol. In the cyclic form there are two anomers, named alpha and beta, since the transition from the open-chain form to the cyclic form involves the creation of a new stereocenter at the site of the open-chain carbonyl. The IR spectra for galactose shows a broad, strong stretch from roughly wavenumber 2500 cm−1 to wavenumber 3700 cm−1. The Proton NMR spectra for galactose includes peaks at 4.7 ppm (D2O), 4.15 ppm (–CH2OH), 3.75, 3.61, 3.48 and 3.20 ppm (–CH2 of ring), 2.79–1.90 ppm (–OH). Galactose is a monosaccharide. When combined with glucose (another monosaccharide) through a condensation reaction, the result is a disaccharide called lactose. The hydrolysis of lactose to glucose and galactose is catalyzed by the enzymes lactase and β-galactosidase.