USA: +1-585-535-1023

UK: +44-208-133-5697

AUS: +61-280-07-5697

56.9 TESTS OF PROTEINS

1. BIURET TEST

To the dispersion of the substance to be tested (say 5% solution of egg albumin) add about 2 ml of NaOH solution. Now add 4-5 drops of 1 % CuSO4 solution.

Bluish violet colouration indicates the presence of protein.

2. MILLON’S TEST

This test is given by proteins containing phenolic amino acids. Gelatin does not give this test.

To 1-2 ml of egg albumin dispersion add 2 drops of Millon’s reagent.

White ppt. which changes to brick red on boiling, confirms the presence of proteins.

Millon’s Reagent is prepared by dissolving 5 g each of HgNO3 and Hg(NO3)2 in 100 ml of dil.
HNO3.

Caution: Millon’s reagent is poisonous. Avoid contact with skin and do not inhale fumes.

3. NINHYDRIN TEST

Take about 2 ml of egg albumin dispersion in a test-tube and add 1-2 ml of Ninhydrin solution. Boil the contents.

Intense blue or purple colouration confirms the presence of proteins.

Ninhydrin Solution is prepared by dissolving 0.1 g of ninhydrin in about 100 m1 of distilled water. This solution is unstable and can be kept only for two days.

 

SUMMARY

  • α-Amino Acids: The organic compounds containing -COOH group and an amino group at the α-carbon atom. These are the building blocks of proteins and peptides.
  • Zwitter ion or dipolar ion structure of α-amino acids

+

H3N -  CH – COO-

|

R

  • Proteins: The complex nitrogenous organic molecules which are essential for the growth and maintenance of life.
  • Proteins are condensation polymers of α-amino acids.

In proteins various amino acids are joined by peptide linkage.

  • Hydrolysis product of peptides and proteins is α-amino acids
  • Denaturation. It is the process that brings about changes in the physical as well as biological properties of the protein.
  • Enzymes: The biological catalysts produced by living cells which catalyse the biochemical reactions in living organism. Most enzymes are globular proteins.
  • Presence of proteins in a food item can be detected by

- Biuret test

- Millon’s test

- Ninhydrin test.

 

EVALUATE YOURSELF

I. Objective Type Questions

Select the most appropriate choice from the options given as (a), (b), (c) and (d) after each question:

1.      Which of the following is not a source of proteins?

a)      fish

b)      potato

c)      egg

d)     milk.

2.      The number of naturally occurring α-amino acids is about

a)      10

b)      50

c)      40

d)     20

3.      The structure of phenylalanine is 

 

Its IUPAC name is

a)      2-amino-l-phenylpropanoic acid

b)      1-phenyl-2-aminopropanoic acid

c)      3-phenyl-2-aminopropanoic acid

d)     2-amino-3-phenylpropanoic acid

4.      The correct statement in respect of protein haemoglobin is that it

a)      maintains blood sugar level

b)      acts as an oxygen carrier in the blood

c)      forms antibodies and offers resistance to diseases

d)     functions as a catalyst for biological reactions.

5.      Which one of the following structures represents the peptide chain?

6.      The helical structure of protein is stabilized by

a)      hydrogen bonds

b)      ether bonds

c)      peptide bonds

d)     dipeptide bonds.

7.      The presence of an amino acid or protein will change the colour of ninhydrin solution from red to

a)      green

b)      yellow

c)      blue

d)     orange.

 

II. Discussion Questions

8.      Give examples of two natural polymers.

9.      What is the final product obtained by hydrolysis of proteins?

10.      Draw structures of glycine and alanine, and give their IUPAC names.

11.      List any five sources of proteins.

12.      Discuss the following:

i.            zwitter ion structure of amino acids

ii.            peptide linkage

iii.            isoelectric point.

13.      List four important physical properties of a-amino acids.

14.      Explain briefly:

i.            primary structure of proteins

ii.            denaturation of proteins

iii.            enzymes

15.  List any four functions of proteins in human body.

16.  Describe two tests to detect the presence of proteins in a food item.

 

Answers

I. Objective Type Questions

1.      (b)

2.      (d)

3.      (a)

4.      (b)

5.      (b)

6.      (a)

7.      (c)