Lecture
Biuret reaction. To 1–2 cm3 of protein solution, add an equal volume of dilute alkali solution and a few drops of dilute pale-blue copper sulfate solution. A red-violet coloration is observed.
Xanthoproteic reaction. Add a few drops of concentrated nitric acid to the protein solution. The protein coagulates and a yellow coloration is observed.
Denaturation
Denaturation is the process of destruction of a protein's natural structure under the influence of various factors (heating, acids, alkalis, heavy metal salts, alcohols, etc.).
As a result of denaturation, a protein loses its biological activity and often precipitates.
For example, coagulation of the protein occurs upon addition of concentrated nitric acid or upon heating.
Biuret reaction
Used to detect peptide bonds in proteins.
Reaction procedure:
To 1–2 cm³ of protein solution, add an equal volume of alkali solution (usually NaOH).
Then add a few drops of pale-blue copper sulfate solution (CuSO₄).
In the presence of protein, a red-violet coloration appears.
Mechanism:
The complex of copper(II) ions with the peptide bonds of the protein gives a characteristic coloration.
Xanthoproteic reaction
Shows the presence of aromatic amino acids (for example, tyrosine, tryptophan, phenylalanine).
Reaction procedure:
Add a few drops of concentrated nitric acid (HNO₃) to the protein solution.
The protein coagulates, and a yellow coloration appears.
When alkali is added, the color intensifies (becomes orange).
Mechanism:
Nitration of the aromatic rings of the amino acids gives colored compounds.
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