Biuret test
(i) On adding a dilute of copper sulphite to alkaline solution of protein, a violet colour is developed.
(ii) This test is due to the presence of peptide 
linkage
Millon’s test
(a) Millon’s reagent consists of mercury dissolved in nitric acid (forming a mixture of mercuric & mercurous nitrates).
(b) When millon’s reagent is added to a protein, a white ppt is formed, which turn brick red on heating.
(c) This test is given by protein which yield tyrosine on hydrolysis (due to the presence of phenolic group).
Nihydrin test
(i) This test is given by all proteins.
(ii) When protein is boiled with a dilute solution of ninhydrin, a violet colour is produced.
Uses of Proteins
(i) Protein constitute as essential part of our food, meat, eggs, fish, cheese provide protein to human beings.
(ii) Casein (a milk protein) is used in the manufacture of artificial wool & silk.
(iii) Amino acid needed for medicinal use & feeding experiment, are prepared by hydrolysis of proteins.
(iv) Gelatin is used in desserts, salad’s, candies bakery goods etc.
(v) Leather is obtained by tanning the protein of animal hides.
(vi) Hemoglobin present in blood is responsible for carrying oxygen and CO2.
(vii) Hormones control various process.
(viii) Enzymes are the proteins produces by living system & catalyse specific biological reaction.
Example:
Ureases (Urea -> CO2 + NH2)
Pepsin (Protein -> Amino acid)
Trypsin (Protein -> Amino acid)
Carbonic anhydride (H2CO3 -> H2O + CO2)
Nuclease (RNA, DNA -> Nucleotides)
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