CHEMISTRY OF PROTENS COLOUR REACTION OF PROTEIN
CHEMISTRY OF PROTENS
COLOUR
REACTION OF PROTEIN
Proteins are complex nitrogen
containing organic compounds formed by the condensation of large numbers of
alpha amino acid molecules by peptide linkages.
Proteins give characteristic
colours with certain reagents due to an amino acid or a class of amino acids
having a characteristic group or due to certain grouping in the protein
molecules. These rections are useful in the identification, characterisation
and estimation of these aminoacids or the proteins containing them.
EXPERIMENTS
Egg
white is the source of protein used for the following experiments.
(1) BIURET test:
To 1 ml of protein solution add
an equal volume of 5% NaOH and 2 drops of 1% copper sulphate, mix, purple or
violet colour is produced due to the presence of more than 2 peptide linkage.
Tripeptide give +ve test.
Explanation:
Cupric ion in alkaline medium
forms a violet coloured complex with peptide bond, nitrogens of peptides and
proteins. This reactions is so named, since biuret formed by the condensation
of 2 molecules of urea when heated to 1800C also answers this test.
The minimum requirement for a +ve test is the presence of 2 peptides bonds in
the molecule. This is a general test for proteins.
Note:
a) Proteoses and peptones give rose red or
pink colour, other proteins give violet colour.
b) Excess copper sulphate will impart its
own blue colour to solution and mask the violet or pink colour of the test.
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