Ellman's reagent

Ellman's reagent (5, 5'-dithiobis-(2-nitrobenzoic acid) or DTNB) is a chemical used for measuring the amount of thiol groups. The compound reacts with a thiol group to release 2-nitro-5-mercaptobenzoic acid (TNB), which has a strong yellow color in water at neutral and alkaline pH. This reaction is rapid and stochiometric, with the addition of one mole of thiol releasing one mole of TNB. The TNB- is quantified in a spectrophotometer by measuring the absorbance of TNB of visible light at 412 nm, using an extinction coefficient of 14,140 M cm−1.

Ellman's reagent can be used for measuring low-molecular mass thiols such as glutathione in both pure solutions and biological samples, such as blood. It can also measure the number of thiol groups on proteins.