Biuret Reaction

Biuret Reaction, one of the most characteristic color reactions that proteins and higher products of their decomposition give. To the test solution is added one-third volume of strong sodium alkali and a small amount of highly diluted (1:500 or weaker) solution of CuSO₄ (avoid an excess of the latter, as the precipitated copper hydrate darkens the reaction). In the presence of native proteins, a violet coloring is obtained; albumoses and peptones give more reddish tones. The biuret reaction is most sensitive toward peptones; it is still obtained with their content of 1:100,000. With very small amounts, it is better to perform it as a so-called ring test: a very dilute solution of copper sulfate is carefully layered over the alkalized test solution. With a positive reaction, a ring of one shade or another appears at the boundary of the liquids. The biuret reaction received its name because it is given by biuret (see). In general, all compounds in which two CONH₂ groups are connected to the same C or N atom, or directly to each other, give the biuret reaction, e.g., CONH₂-N-CONH₂ (biuret); CONH₂-C-CONH₂ (malonamide); CONH₂-NH₂-CONH₂ (oxamide); one of the CONH₂ groups can be replaced by the CH₂NH₂ or CSNH₂ group. In proteins, the biuret reaction is apparently due to the -CHNH-CONH- group. The significance of the biuret reaction, besides its sensitivity, lies in its great specificity—of the compounds found in nature, only proteins and their decomposition products give it. Similar reactions, but of a different shade, are obtained with cobalt and nickel salts. The matter apparently comes down to the formation of complex complex compounds, and the shade of color may depend not only on the chemical nature of the test substance but also on its colloid-chemical state. Lit.: Cohnheim O., Chemie der Eiweisskörper, Braunschweig, 1911.