biuret reaction

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a urea derivative; its presence is detected after addition of sodium hydroxide and copper sulfate solutions by a pinkish-violet color (protein test) or a pink and finally a bluish color (urea test).
biuret reaction the reaction in biuret tests.
biuret test either of the tests done with biuret.

bi·u·ret re·ac·tion

the formation of biuret that gives a violet color as a result of the reaction of a polypeptide of more than three aminoacyl residues with CuSO4 in strongly alkaline solution; dipeptides and amino acids (except histidine, serine, and threonine) do not so react; used for the detection and quantification of polypeptides, or proteins, in biologic fluids.

bi·u·ret re·ac·tion

, biuret test (bī'yŭr-et' rē-ak'shŭn, bī'yŭr-et' test)
The formation of biuret, which gives a violet color because of the reaction of a polypeptide of more than three amino acids with CuSO4 in strongly alkaline solution; used for the detection and quantitation of polypeptides or protein in biologic fluids.

Biuret reaction

a chemical change used as a quantitative method for protein determination. The reaction takes place when dilute copper sulphate solution is added to a protein solution which is then made alkaline by the addition of sodium hydroxide. A copper hydroxide precipatate forms and a purple/violet colour is produced, the strength of which indicates the quantity of protein present. The reaction only works if at least two PEPTIDE BONDS are present in the protein. Thus single AMINO ACIDS and DIPEPTIDES with only one peptide bond will not give a positive Biuret reaction.
References in periodicals archive ?
Carboxy groups probably are not significant elements in chelation for the biuret reaction; copper in the biuret reaction is stabilized by chelation with carboxyl groups of tartrate, and the absorbance of the reagent with no added biuret-reactive components probably reflects copper interacting with carboxyl groups.
Ethanolamine serves as a model for the chelate-forming portions of threonine and serine, and its reactivity in the biuret reaction was of similar magnitude.
This suggests that absorbance for the biuret reaction product of albumin was the sum of peptide backbone and sidechain interactions with copper.
The biuret reaction of amino acids and small peptides was completed by the time of the first postbaseline reading: within ~2-4 s depending on the speed of mixing.
The biuret reaction commonly has been considered to selectively measure peptides 3 amino acid residues in length or longer (3), although there have been reports extending back many years indicating that amino acids, dipeptides, and other compounds are reactive (30-33).
For many years, variations of the biuret reaction were applied to measure urine protein, but it was considered necessary to separate protein from other low-moleculeweight components by methods such as acid precipitation or gel filtration (35-38).