Question 21.4: Locate the peptide bond in the dipeptide shown in the ball-a...
Locate the peptide bond in the dipeptide shown in the ball-and-stick model, and identify the amino acids that form the dipeptide. Name the dipeptide using the three-letter abbreviations for the amino acids.
Analysis
• Convert the ball-and-stick model to a structural formula. The peptide bond is the amide bond that joins the two amino acids together.
• Identify the side chains of the amino acids and use Table 21.2 to determine the names.
• Name the dipeptide by writing the three-letter abbreviations for the amino acids with the N-terminal amino acid first, followed by the C-terminal amino acid.
| Table 21.2 The 20 Common Naturally Occurring Amino Acids | |||||
| Neutral Amino Acids | |||||
| Name | Structure | Abbreviations | Name | Structure | Abbreviations |
| Alanine |  |
Ala A | Phenylalanine* |  |
Phe F |
| Asparagine |  |
Asn N | Proline |  |
Pro P |
| Cysteine |  |
Cys C | Serine |  |
Ser S |
| Glutamine |  |
Gln Q | Threonine* |  |
Thr T |
| Glycine |  |
Gly G | Tryptophan* |  |
Trp W |
| Isoleucine* |  |
Ile I | Tyrosine |  |
Tyr Y |
| Leucine* |  |
Leu L | Valine* |  |
Val V |
| Methionine* |  |
Met M | Arginine* |  |
Arg R |
| Aspartic acid |  |
Asp D | Histidine* |  |
His H |
| Glutamic acid |  |
Glu E | Lysine* |  |
Lys K |
| Essential amino acids are labeled with an asterisk (*). | |||||
The peptide bond is the amide bond that contains a carbon atom doubly bonded to oxygen and singly bonded to nitrogen.
The side chains of the two amino acids, the R groups that extend from the peptide backbone, identify them as alanine and cysteine ( Table 21.2 ). The dipeptide is named with the N-terminal amino acid (alanine) first, followed by the C-terminal amino acid (cysteine), using the three-letter abbreviations: Ala–Cys .
