o-Nitrophenol (ONP substrates)

2-Nitrophenol or ONP (product code 24XXX) substrates are colourless compounds which offer a diffuse yellow endpoints following enzymatic cleavage, making them best suited for use in liquid media applications. The coloration of nitrophenols in aqueous solution is pH-dependent as deprotonation of the hydroxyl group is required for generation of a chromophore. The strongest colouration is achieved at pH values above 9.0, this is typically achieved by the addition of a base after a defined reaction time at neutral pH. The colouration of the phenolate ion can be measured using a spectrophotometer at 405 nm.

The substrates are widely used in diagnostic microbiology as they have the advantage of being stable and offering a district colour change on hydrolysis.

Glycosynth offers a wide range of nitrophenyl based substrates including ONPG (product code 24045) is used to detect beta-galactosidase activity, often being employed as an indicator of total coliforms in dual substrate systems.