ortho-Nitrophenyl-β-galactoside

ortho-Nitrophenyl-β-galactoside (ONPG) is a colorimetric and spectrophotometric substrate for detection of β-galactosidase activity. 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 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.

ortho-Nitrophenyl-β-galactoside

Names
IUPAC name 2-Nitrophenyl β-D-galactopyranoside
Systematic IUPAC name (2R,3R,4S,5R,6S)-2-(Hydroxymethyl)-6-(2-nitrophenoxy)oxane-3,4,5-triol
Other names 2-Nitrophenylgalactoside, Ortho-Nitrophenyl-β-D-Galactopyranoside
Identifiers
CAS Number	369-07-3 Y
3D model (JSmol)	Interactive image
Abbreviations	ONPG
ECHA InfoCard	100.006.106
MeSH	2-nitrophenylgalactoside
PubChem CID	123646
UNII	8RFX6AT9WP Y
SMILES O=[N+]([O-])C1=CC=CC=C1O[C@@H]2O[C@H](CO)[C@H](O)[C@H](O)[C@H]2O
Properties
Chemical formula	C12H15NO8
Molar mass	301.251 g·mol−1
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

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-D-1-thiogalactopyranoside (IPTG), β-galactosidase will not be transcribed and ONPG will not be hydrolyzed.