1. 19.3 Formation of Peptides
A peptide bond is an amide bond that forms when the — COO− group of one amino acid reacts with the — NH3+ group of the next amino acid. The linking of two or more amino acids by peptide bonds forms a peptide.
Learning Goal Draw the structure for a peptide, give its three-letter and one-letter abbreviations, and name.

2. Formation of Peptides
The linking of two or more amino acids by peptide bonds forms a peptide.
Peptides formed from
two amino acids are called dipeptides.
three amino acids are called tripeptides.
four amino acids are called tetrapeptides.
five amino acids are called pentapeptides
longer chains of amino acids are called polypeptides.

3. Formation of Peptides A peptide bond is an amide bond.
forms between the — COO− group of one amino acid and the — NH3+ group of the next amino acid.

4. Guide to Drawing a Peptide

5. Study Check
Draw the dipeptide Ser–Thr.

6. Solution
Draw the dipeptide Ser–Thr.

7. Naming Peptides The name of a peptide
begins with the name of the N-terminal amino acid residue.
gives all the following amino acid residues up to the C-terminal amino acid in which the ine or ate endings are replaced with yl.
retains the complete name of the C-terminal amino acid.
A tripeptide consisting of alanine, glycine, and serine residues is named as
alanylglycylserine

8. Study Check
Draw the structure and give the name for the tripeptide Gly−Ser−Met.

9. Solution
Draw the structure and give the name for the tripeptide Gly−Ser−Met. STEP 1 Draw the structures for each amino acid in the peptide, starting with the N-terminus.

10. Solution
Draw the structure and give the name for the tripeptide Gly−Ser−Met. STEP 2 Remove the O atom from the carboxylate group of the N-terminal amino acid and two H atoms from the ammonium group in the adjacent amino acid. Repeat this process until the C-terminus is reached.

11. Solution
Draw the structure and give the name for the tripeptide Gly−Ser−Met. STEP 3 Use peptide bonds to connect the amino acid residues.

12. Solution
Draw the structure and give the name for the tripeptide Gly−Ser−Met.
The tripeptide is named by
replacing the last syllable of each amino acid name with yl, starting with the N-terminus.
The C-terminus retains its complete amino acid name.
N-terminus glycine is named glycyl; serine is named seryl.
C-terminus methionine keeps its full name.
The tripeptide is named glycylserylmethionine.