Elementary plant Biochemistry and Biotechnology (2+1)

Chemical properties of proteins

Colour reactions of proteins

• The colour reactions of proteins are of importance in the qualitative detection and quantitative estimation of proteins and their constituent amino acids.
• Biuret test is extensively used as a test to detect proteins in biological materials.

Biuret reaction

• A compound, which is having more than one peptide bond when treated with Biuret reagent, produces a violet colour.
  
• This is due to the formation of coordination complex between four nitrogen atoms of two polypeptide chains and one copper atom

Xanthoproteic reaction

• Addition of concentrated nitric acid to protein produces yellow colour on heating, the colour changes to orange when the solution is made alkaline.
  
• The yellow stains upon the skin caused by nitric acid are the result of this xanthoproteic reaction.
  
• This is due to the nitration of the phenyl rings of aromatic amino acids.

Hopkins-Cole reaction

• Indole ring of tryptophan reacts with glacial acetic acid in the presence of concentrated sulphuric acid and forms a purple coloured product.
  
• Glacial acetic acid reacts with concentrated sulphuric acid and forms glyoxalic acid, which in turn reacts with indole ring of tryptophan in the presence of sulphuric acid forming a purple coloured product.