Nitrophenol - Substrato cromogénico

Nitrophenols are a family of nitrated phenols with the formula HOC6H4NO2. Three isomeric nitrophenols exist: o-Nitrophenol (1-hydroxy-2-nitrobenzene; OH and NO2 groups are neighboring; CAS number: 88-75-5), a yellow crystalline solid (m.p. 46 °C). m-Nitrophenol (1-hydroxy-3-nitrobenzene, CAS number: 554-84-7), a yellow solid (m.p. 97 °C) and precursor to the drug mesalazine (5-aminosalicylic acid). p-Nitrophenol (1-hydroxy-4-nitrobenzene, CAS number: 100-02-7), yellow crystals (m.p. 114 °C). It is a precursor to the rice herbicide fluorodifen and the pesticide parathion. Nitrophenols are poisonous. Occasionally, nitrophenols contaminate the soil near former explosives or fabric factories and military plants, and current research is aimed at remediation. para-Nitrophenylphosphate (pNPP) is a chromogenic substrate for acid and alkaline phosphatase in ELISA assays. Under their influence the decay to yellow para-nitrophenol is catalysed. This product can be measured with a 405 nm spectrophotometer. ortho-Nitrophenyl-β-galactoside (ONPG) is a colorimetric and spectrophotometric substrate for detection of beta-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.