Tests for Proteins -1

1.The Biuret Test

Solution contents:
KOH, CuSO_4, Potassium sodium tartarate

Tested for:
Peptide bonds. (Proteins and peptones give positive results)

Theory and observations:
Following compound is formed between the -C-N-C-back bone of the proteins and the copper(II) ions in the solution which results in a violet colour complex.

2. The Ninhydrin Test

Solution contents:
Ninhydrin

Tested for:
Amine group (mainly primary amine groups in proteins, peptones and amino acids)

Theory and observations:
Any primary amino acid will undergo the I^st reaction and will produce a blue colour and secondary amino group containing amino acid, proline will give a yellow/orange colour (II).

3. The Millon’s Test

Solution contents:
Mercury (Hg₂²⁺, Hg²⁺) in nitric acid

Tested for:
Phenol groups (Tyrosine)

Theory and observations:
Firstly the phenol ring is nitrated with conc. Nitric acid and the nitrated product will react with the mercury ions in the solution to give in either a deep red colour solution or precipitate. [Hg²⁺](Some tyrosine containing proteins will give in a white precipitate [Hg₂²⁺] which would turn red when heated.