{{Use dmy dates|date=August 2023}} {{More citations needed|date=February 2022}} {{DISPLAYTITLE:''ortho''-Nitrophenyl-β-galactoside}} {{Chembox | Name=''ortho''-Nitrophenyl-β-galactoside | ImageFile = ONPG structure.png | ImageSize = | IUPACName = 2-Nitrophenyl β-<small>D</small>-galactopyranoside | SystematicName = (2''R'',3''R'',4''S'',5''R'',6''S'')-2-(Hydroxymethyl)-6-(2-nitrophenoxy)oxane-3,4,5-triol | OtherNames = 2-Nitrophenylgalactoside, Ortho-Nitrophenyl-β-D-Galactopyranoside | Section1 = {{Chembox Identifiers | Abbreviations = ONPG | CASNo = 369-07-3 | CASNo_Ref = {{cascite|correct|CAS}} | ChEBI = 90144 | ChEMBL = 1229648 | ChemSpiderID = 87255 | DrugBank = DB01920 | EINECS = 206-716-1 | PubChem = 123646 | UNII_Ref = {{fdacite|correct|FDA}} | UNII = 8RFX6AT9WP | StdInChI=1S/C12H15NO8/c14-5-8-9(15)10(16)11(17)12(21-8)20-7-4-2-1-3-6(7)13(18)19/h1-4,8-12,14-17H,5H2/t8-,9+,10+,11-,12?/m1/s1 | StdInChIKey = KUWPCJHYPSUOFW-SCWFEDMQSA-N | SMILES = O=[N+]([O-])C1=CC=CC=C1O[C@@H]2O[C@H](CO)[C@H](O)[C@H](O)[C@H]2O | MeSHName = 2-nitrophenylgalactoside }} | Section2 = {{Chembox Properties | C=12|H=15|N=1|O=8 | Appearance = | Density = | MeltingPt = | BoilingPt = }} | Section3 = {{Chembox Hazards | MainHazards = | FlashPt = | AutoignitionPt = }} }}

'''''ortho''-Nitrophenyl-β-galactoside''' ('''ONPG''') is a colorimetric and spectrophotometric substrate for detection of β-galactosidase activity.<ref>{{cite web | website=sigmaaldrich.com | url=https://www.sigmaaldrich.com/NO/en/product/sial/73660 | access-date=14 August 2023|title=2-Nitrophenyl β-D-galactopyranoside}}</ref> This compound is normally colorless. However, if β-galactosidase is present, it hydrolyzes the ONPG molecule into galactose and ortho-nitrophenol. The latter compound has a yellow color that can be used to check for enzyme activity by means of a colorimetric assay (at 420&nbsp;nm wavelength). β-Galactosidase is required for lactose utilization, so the intensity of the color produced can be used as a measure of the enzymatic rate.

Though ONPG mimics lactose and is hydrolyzed by β-galactosidase, it is unable to act as an inducer for the lac operon. Without another lactose analog that can act as an inducer, such as isopropyl β-<small>D</small>-1-thiogalactopyranoside (IPTG), β-galactosidase will not be transcribed and ONPG will not be hydrolyzed.

{{Glycosides}}

==References== {{Reflist}}

{{DEFAULTSORT:Nitrophenyl-beta-galactoside, ortho-}} Category:Galactosides Category:2-Nitrophenyl compounds