Qualitative Analysis of Proteins – MeitY OLabs


QUALITATIVE ANALYSIS OF PROTEINS Proteins are large biological molecules composed
of one or more long chain amino acid residues linked together by peptide bonds. Protein
can be found in meat, fish, cheese, eggs and milk, as well as many other foods like beans,
lentils, nuts and seeds . There are four aspects to protein’s structure designated as: primary
structure, secondary structure, tertiary structure and quaternary structure. Primary structure
refers to the sequence of amino acids in proteins. Secondary refers to the shape in which a long
polypeptide chain can exist. The most common are alpha-helix and beta -pleated sheet structure,
which are stabilized by hydrogen bonding. Tertiary structure refers to the overall shape
of a simple protein molecule and the quaternary structure is formed by several protein molecules.
Our aim here is to determine the presence of protein in a given sample. The tests to determine the presence of protein
are: Biuret test
Xanthoproteic Test Ninhydrin Test
Millon’s Test Biuret Test Materials required: Egg albumin dispersion, gelatin dispersion,
sodium hydroxide solution, 1% copper sulphate solution, test tubes, test tube holder, droppers
and Bunsen burner. Procedure: Take a small quantity of egg albumin dispersion
and gelatin dispersion in two test tubes labeled A and B.
Using a dropper, add a small quantity of sodium hydroxide solution to the egg albumin dispersion
and gelatin dispersion. Now, add few drops of 1% copper sulphate solution
to each test tube. Warm the contents of both the test tubes for
few minutes over the Bunsen burner. In presence of alkali, proteins react with
copper (II) ions to form a violet colored complex called Biuret. Xanthoproteic Test: Materials required: Egg albumin dispersion, gelatin dispersion,
concentrated nitric acid, test tubes, test tube holder, dropper and Bunsen burner. Procedure: Take a small quantity of egg albumin dispersion
and gelatin dispersion in two test tubes labeled A and B.
Using a dropper, add a few drops of concentrated nitric acid to the egg albumin dispersion
and gelatin dispersion. Heat the contents of both the test tubes for
few minutes over the Bunsen burner. In presence of concentrated nitric acid, certain
amino acids present in proteins undergo nitration to form yellow colored xanthoproteic acid. Ninhydrin Test: Materials required: Egg albumin dispersion, gelatin dispersion,
ninhydrin reagent, test tubes, test tube holder, dropper and Bunsen burner. Procedure: Take a small quantity of egg albumin dispersion
and gelatin dispersion in two test tubes A and B.
Using a dropper add a few drops of ninhydrin reagent to egg albumin dispersion and gelatin
dispersion. Boil the contents of both the test tubes over
the Bunsen burner. Proteins react with ninhydrin reagent to form
an intense blue colored compound. Millon’s Test: Materials required: Egg albumin dispersion, gelatin dispersion,
millon’s reagent, test tubes, test tube holder, dropper and Bunsen burner. Procedure: Take a small quantity of egg albumin dispersion
and gelatin dispersion in two test tubes labeled A and B.
Using a dropper, add a few drops of Millon’s reagent to egg albumin dispersion and gelatin
dispersion. A white precipitate is formed in egg albumin
dispersion, whereas no such precipitate is formed in gelatin dispersion.
Boil the contents of both the tests tubes over the Bunsen burner.
The white precipitate in albumin changes to brick red on boiling, whereas no characteristic
changes are observed in gelatin. Precautions: Use droppers to take the reagents from the
bottles. Heating and boiling should be done very carefully.

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