Proteins and peptides are polymers of amino acids. They are chains of amino acids as well as other biomolecules or ions or compounds. The amino acids are covalently bound to each other by a covalent bond, called a peptide bond, between the carbon number one (C1) of one amino acid and nitrogen number two (N2) of adjacent amino acid. The formation of a peptide bond is a condensation reaction.
In the process carboxylic acid moiety of one amino acid loose hydrogen and oxygen, the amino moiety of another amino acid loses hydrogen and the exposed carbon of the 1 st amino acid and the exposed nitrogen of the 2 nd amino acid join to form a dipeptide with a peptide bond (-CO-NH-).
The nitrogen atom in a peptide bond of proteins and peptides contains unshared electrons. These unshared electrons of the peptide bonds, in an alkaline environment, can be used by cupric ion (Cu +2 ) present in the Biuret reagent to form a violet or purple-colored complex.
This colorimetric chemical test used to detect the peptide bond using the Biuret reagent is called the Biuret test. It is also called Piotrowski’s reaction after the name of the Polish physiologist Gustaw Piotrowski who observed this phenomenon in 1857 and used it to detect proteins in samples.
In the Biuret reagent, the compound Biuret is not actually used. Biuret is a chemical compound having a molecular formula of HN(CONH2)2 which is formed by the condensation of two urea molecules when urea is heated at 150°C. A similar reaction producing a purple-colored complex compound was first noted when biuret reacts with Cu +2 ions because biuret has bonds similar to peptide bonds. Hence the test is named Biuret test due to the similarity in the end products.
It is used in labs to detect the presence of peptides or proteins in a sample. It is a qualitative test, and can only state the presence or absence of the peptide bonds but demonstrate nothing about the exact quantity and type of proteins.
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