Mr Cole Chemistry

Dipeptides and Tripeptides Questions

Peptides

Worksheet

1. Formation and Hydrolysis of Dipeptides

Many of the following questions are about amino acids that are found in the AQA A-level Chemistry Datasheet. They are copied below for your convenience. Amino acid datasheet
Dipeptides can form between amino acids.
a. Draw the displayed formula of the two possible structures of a dipeptide formed on cysteine and serine.

Answer:

There are two possible isomers because either amino acid can act as the N-terminal (amine end) or the C-terminal (carboxylic acid end).

  • Cys-Ser: Cysteine provides the NH2 and Serine provides the COOH.
  • Ser-Cys: Serine provides the NH2 and Cysteine provides the COOH.
Structures of Cys-Ser and Ser-Cys dipeptides
b. State the name of the inorganic molecule that is formed when serine and cysteine react to form the dipeptide.

Water (H2O)

This is a condensation reaction where a molecule of water is eliminated for every peptide bond formed.

c. Circle the peptide link in the molecules above and state the name for this bond.

Peptide Link (or Amide Linkage)

The bond corresponds to the -CONH- group connecting the two amino acid residues.

d. State the conditions required for the dipeptide to return to being individual amino acids.

Hydrolysis Conditions:

  • Acid Hydrolysis: Heat/Reflux with concentrated HCl (forms ammonium salts).
  • Alkaline Hydrolysis: Heat/Reflux with concentrated NaOH (forms carboxylate salts).

2. Tripeptides and Chirality

A tripeptide can form between three amino acids, resulting in the following structure: Phe-ala-lys.
a. Draw the skeletal formula of the molecule when at pH 2.0.

Answer:

At pH 2.0 (acidic), all amine groups are fully protonated (NH3+), including the amine group on the Lysine side chain. The terminal carboxylic acid remains protonated (COOH).

Skeletal formula of Phe-Ala-Lys at pH 2.0
b. On the formulae you drew, circle the carbons that can cause plane polarised light to be rotated.

Chiral Centres

The carbons responsible for rotating plane-polarised light are the α-carbons (the central carbon bonded to the R-group) of each amino acid residue. In this tripeptide, the α-carbons of Phenylalanine, Alanine, and Lysine are all chiral. The chiral centres have been highlighted in the image in 2a.

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