biochemistry

1. Peptide linkage and chemical
properties of amino acids
An overview
1

2. Peptide linkage
• Peptide linkage or peptide bond or amide
bond: Proteins are linear polymers of L-α-amino
acids in which the carboxyl group of one amino
acid is linked to the amino group of another
amino acid.
• The peptide bond is an amide bond formed
between the -COO - group of one amino acid and
the -NH3
+ group of another amino acid.
• The bond is covalent and rather strong which
serve as cementing material between the
individual amino acids (bricks) 2

3. Peptide linkage
• Formation of peptide bond: The linkage is
formed by removal of the elements of water
(dehydration) from the α-carboxyl group of one
amino acid and the α-amino group of another.
• It is a dehydration reaction, a common class of
reactions in living cells.
• The molecule formed by condensing two amino
acids is called a dipeptide
• Three amino acids can be joined by two peptide
bonds to form a tripeptide and so on. 3

4. Peptide linkage
• Both –C=O and
-NH groups of
peptide bonds are
polar and are
involved in
hydrogen bond
Formation. The peptide bond is planer. 4

5. Peptide linkage
• Writing of peptide structures: In a peptide, the amino
acid residue at the end with a free α- amino group is
the amino-terminal (or N-terminal) residue
• And the amino acid with a free -COO - group at the
other end, is known as the carboxyl, or C-terminal
amino acid.
• The peptide chains are conventionally written with
their N-terminal amino acid on the left and free
carboxyl end (C-terminal amino acid) to the right.
Proteins biosynthesis also starts from N-terminal
amino acid. 5

6. • Naming of peptides: Peptides are named as
derivatives of the C-terminal amino acid, which
receives its entire name. For all other amino acids,
the ending -ine is changed to -yl. Thus, the
dipeptide alanyl-glycine has
• glycine as its C-terminal amino acid, as indicated
by its full name, glycine: 6

7. Peptide linkage
• Chemical properties of amino acids:
• Amino acids participate in variety of reactions
involving -COO – group, -NH3
+ group , SH group
and R group.
A. Reactions involving -COO – group:
1. Reaction with alkalies (salt formation):
• The carboxylic group release H+ ion and become
carboxylate (-COO –) ions. 7

8. chemical
properties of amino acids
• These may be neutralized by cation like Na+
and Ca2+ to form salt. e.g., sodium salt of
glutamic acid (monosodium glutamate), which
is used as flavouring agent to impart meat like
flavour to soups. 8

9. chemical
properties of amino acids
2. Reaction with alcohols (esterification):
• With alcohol corresponding ester are formed.
• The ester is more volatile than free amino acid.
3. Reaction with amines: Amino acids react with
amines to form amides.
B. Reactions involving NH2 group:
1. Reaction with mineral acids: when treated with
mineral acid like HCl, the acid slats are formed 9

10. chemical
properties of amino acids
+HO―C2H5→ R―CH―COO―C2H5 + H2O
NH2
Ethyl ester of amino acid
R―CH―COOH + HNH—R R―CH―CO―NH—R + H2O
NH2 NH2
amine 10 amide

11. chemical
properties of amino acids
R―CH―COOH + HCL R―CH―COOH + H2O
NH2 NH2.HCL
amino acid hydrochloride
2. Reaction with nitrous acid: Reaction with HNO2
liberate N2 gas with formation of corresponding
α-hydroxy acids.
• The reaction is used for the estimation of amino
acid by measuring the volume of N2 gas. The
imino acid proline don not respond to this
reaction. 11

12. chemical
properties of amino acids
3. Reaction with FDNB or Sanger’s reagent:
• FDNB (1-fluoro- 2,4-dinitrobenzene) reacts with
α-amino acids to produce yellow cloured
derivatives, DNB- amino acids.
• The reaction was first used by Sanger in
determining the sequence of amino acids in
insulin. 12

13. chemical
properties of amino acids
• DNB-amino acids (yellow) 13

14. chemical
properties of amino acids
C. Reactions involving both COOH and NH2
groups:
1. Reaction with ninhydrin: ninhydrin or
triketohydrindene hydrate (2,2-
dihydroxyindane-1,3-dione) is a powerful
oxidizing agent and cause oxidative
decarboxylation of α-amino acids producing
CO2, NH3 and aldehyde with one less carbon
atom than the parent amino acid. 14

15. chemical
properties of amino acids
II. Ninhydrin I. amino acid VIII. Hydrindantin
VII. Diketohydrindylidene-diketohydrindamine (Ruheman’s
purnple) 15

16. chemical
properties of amino acids
• The reduced ninhydrin (hydrindantin) the reacts
with the liberated NH3 and a mole of ninhydrin,
forming blue-coloured Ruheman’s complex.
• Van Slyke used this reaction for quantitative
estimation of amino acids.
2. Reaction with phenylisothiocyanate or Edam
reagent: phenylisothiocyanate reacts with N-
teminal amino acid to produce thiohydantion.
This reaction is useful in studies of protein structure.
16

17. chemical
properties of amino acids

18. chemical
properties of amino acids
D. Reactions involving side chain or R-group:
1. Biuret test: alkaline 0.2% copper sulphate
solution (biuret reagent) reacts with peptide
bonds containing compound to produce purple
colour.
• If the test is negative, the CuSO4 solution stays
blue
• The reaction is called biuret reaction because it is
also given by substance biuret.
• The test is used both in qualitative and
quantitative purposes 18

19. chemical
properties of amino acids
Biuret

20. chemical
properties of amino acids
2. Millon’s test: Red colour develops when
proteins are heated with HgNO3 in HNO2.
The reactions is specific for tyrosine and
takes place between mercuric and mercurous
nitrates and tyrosine residue of protein.
Tryptophan also respond to this reaction. 20