Pauly’s test: Principle, Reaction, Reagents, Procedure and Result Interpretation

Pauly’s test

 (Detection of tyrosine or histidine)

Pauly test is for detecting the presence of tyrosine or histidine. It is named after German chemist Hermann Pauly, who first described the reaction. Diazotized sulphanilic acid couples with amines, phenols and imidazole to form highly coloured compounds. The diazonium compound is only formed in the cold, so all solutions must be cooled in ice before diazotization.

Principle:

This test is specific for the detection of tyrosine or histidine. The reagent used for this test contains sulphanilic acid dissolved in hydrochloric acid. Sulphanilic acid upon diazotization in the presence of sodium nitrite and hydrochloric acid results in the formation of a diazonium salt. The diazonium salt formed couples with either tyrosine or histidine in an alkaline medium to give a red-colored chromogen (azo dye).

Reaction:

Reagents:

• Test solutions:
  • Amino acids: 1 % solution of amino acids like tyrosine, histidine, glycine , etc.
  • Protein solution: egg albumin in distilled water (10%)
• Sulphanilic acid:1 % sulphanilic acid in 10 % HCI
• 5 %(w/v)  sodium nitrite solution in water.
• 10 %(w/v) sodium carbonate solution in water.

Procedure:

• Mix 1 ml of sulphanilic acid with 2 ml of the test solution cooled in ice.
• Add 1 ml of sodium nitrite solution and leave in the cold for 3 min.
• Make the solution alkaline by the addition of 2 ml of sodium carbonate solution.
• Observe whether red-colored products are formed or not.

Result Interpretation:

Positive test: Formation of red color indicate presence of tyrosine or histidine.

Negative test: No formation of red color indicate absence of tyrosine or histidine.

References:

• Sawhney S.K (2016), Introductory Practical Biochemistry, Eleventh reprint, Narosa Publishing House Pvt. Ltd, 22 Delhi Medical Association Road, Daryaganj, New Delhi-110002.
• https://en.wikipedia.org/wiki/Pauly_reaction
• https://vlab.amrita.edu/?sub=3&brch=63&sim=1094&cnt=1
• https://www.onlinebiologynotes.com/paulys-test-objective-principle-reagents-procedure-and-result/
• https://quizlet.com/96596357/pauly-test-flash-cards/