Lab # 10
Proteins
Mahwish Siddiqui
1
Introduction
▪ Proteins are polymers of amino acids, which are joined together by
the formation of amide bonds between the amino group of one
amino acid and the carboxylic acid group of another. The amide
joining two amino acids in a protein is called a peptide bond.
▪ Although all proteins are constructed from the same 20 amino
acids, there are a vast number of ways in which these 20 subunits
can be combined. As a result, there are millions of different protein
polypeptide sequences
Zwitterionic property
▪ A zwitterion is a molecule with functional groups, of which at least
one has a positive and one has a negative electrical charge. The
net charge of the entire molecule is zero. Amino acids are the best-
known examples of zwitterions. They contain an amine group
(basic) and a carboxylic group (acidic). The -NH2 group is the
stronger base, and so it picks up H+ from the -COOH group to
leave a zwitterion. The (neutral) zwitterion is the usual form amino
acids exist in solution.
Adding an alkali to an amino acid solution
▪ If you increase the pH of a solution of an amino acid by adding
hydroxide ions, the hydrogen ion is removed from the -NH3+ group.
Adding an acid to an amino acid solution
▪ If you decrease the pH by adding an acid to a solution of an amino acid,
the -COO- part of the zwitterion picks up a hydrogen ion.
▪ Some proteins play important roles in the structure of animals
bodies. For example, hair, skin and tendons are composed
primarily of proteins. These structural proteins are mainly fibrous
proteins.
▪ Other proteins are globular proteins, these are often water
soluble and occur naturally in the fluids of the body. Globular
proteins
include,
for
example,
albumin,
antibodies
(immunoglobins) in the blood serum, and enzymes in the
intracellular fluid.
Peptide linkage
Biuret test for proteins
13
Introduction
▪ Biuret test is used for the detection of proteins. The test is
positive if at least two peptide linkages are present. A
dipeptide therefore doesn't give positive result.
▪ The test is so named since this test is also given positive by
the biuret compound, which is obtained by condensation of 2
molecules of urea when heated to 180⁰C.
Procedure
Interpretation.
▪ Purple color indicates the presence of proteins.
▪ Blue color indicates the absence of proteins.
Principle
▪ Under alkaline conditions substances containing two or more
peptide bonds form a purple complex with copper salts in the
reagent.