<P> In this assay, the copper (II) binds with nitrogens present in the peptides of proteins . In a secondary reaction, the copper (II) is reduced to copper (I). Buffers, such as Tris and ammonia interfere with this assay, therefore rendering this assay inappropriate for protein samples purified from ammonium sulfate precipitation . Due to its insensitivity and little interference by free amino acids, this assay is most useful for whole tissue samples and other sources with high protein concentration . </P> <P> An aqueous sample is treated with an equal volume of 1% strong base (sodium or potassium hydroxide) followed by a few drops of aqueous copper (II) sulfate . If the solution turns purple, protein is present . 5--160 mg / mL can be determined . Peptides with the chain length of at least 3 amino acids are necessary for a significant, measurable colour shift with these reagents . </P> <P> The Biuret reagent is made of sodium hydroxide (NaOH) and hydrated copper (II) sulfate, together with potassium Chemical Reagents Potassium sodium tartrate is added to chelate and thus stabilize the cupric ions . The reaction of the cupric ions with the nitrogen atoms involved in peptide bonds leads to the displacement of the peptide hydrogen atoms under the alkaline conditions . A tri or tetra dentate chelation with the peptide nitrogen produces the "biuret" color . This is found with dipeptides (Datta, S.P., Leberman, R., and Rabin, B.R., Trans. Farad. Soc. (1959), 55, 2141 .) </P> <P> The reagent is commonly used in the biuret protein assay, a colorimetric test used to determine protein concentration by UV / VIS spectroscopy at wavelength 550 nm . </P>

In the biuret test what metallic ion does the amino group of the protein complex with