ortho-Nitrophenyl-β-galactoside

ortho-Nitrophenyl-β-galactoside (ONPG) is a colorimetric and spectrophotometric substrate for detection of β-galactosidase activity.[1] 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
3D model (JSmol)
Abbreviations ONPG
ECHA InfoCard 100.006.106
MeSH 2-nitrophenylgalactoside
UNII
  • O=[N+]([O-])C1=CC=CC=C1O[C@@H]2O[C@H](CO)[C@H](O)[C@H](O)[C@H]2O
Properties
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.

References
  1. "2-Nitrophenyl β-D-galactopyranoside". sigmaaldrich.com. Retrieved 14 August 2023.
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