1. 27.7 Peptides 4

2. Peptides
Peptides are compounds in which an amide bond links the amino group of one a-amino acid and the carboxyl group of another.
An amide bond of this type is often referred to as a peptide bond.

3. Alanine and Glycine
CH3 O C + H –
H3N O C H
H3N + –

4. Alanylglycine
CH3 O C
H3N + H N –
Two a-amino acids are joined by a peptide bond in alanylglycine. It is a dipeptide.

5. Alanylglycine
CH3 O C
H3N + H N –
N-terminus
C-terminus
Ala—Gly AG

6. Alanylglycine and glycylalanine are constitutional isomers
CH3 O C
H3N + H N –
Alanylglycine
Ala—Gly AG H O C
H3N + N
CH3 –
Glycylalanine
Gly—Ala GA

7. Alanylglycine
CH3 O C
H3N + H N –
The peptide bond is characterized by a planar geometry.

8. Higher Peptides
Peptides are classified according to the number of amino acids linked together.
dipeptides, tripeptides, tetrapeptides, etc.
Leucine enkephalin is an example of a pentapeptide.

9. Tyr—Gly—Gly—Phe—Leu YGGFL
Leucine Enkephalin
Tyr—Gly—Gly—Phe—Leu YGGFL

10. Oxytocin is a cyclic nonapeptide.
Ile—Gln—Asn
Tyr
Cys S
Cys—Pro—Leu—GlyNH2 1 2 3 4 5 6 7 8 9
C-terminus
N-terminus
Oxytocin is a cyclic nonapeptide.
Instead of having its amino acids linked in an extended chain, two cysteine residues are joined by an S—S bond.

11. Oxytocin
S—S bond
An S—S bond between two cysteines is often referred to as a disulfide bridge.