Mr Cole Chemistry

Amino Acids Questions

Amino Acids

Worksheet

1. Amino Acid Structure & pH

The following questions are about amino acids that are found on the A-level datasheet. They are found below for ease of use. List of amino acids from datasheet
1.1 Draw the displayed formula of phenylalanine when the amino acid is found at pH 7.0. State the name for this structure.

Zwitterion

At pH 7.0 (neutral conditions), the carboxylic acid group donates a proton to become a carboxylate ion (COO–), and the amine group accepts a proton to become an ammonium ion (NH3+).

Phenylalanine zwitterion structure
1.2 Draw the displayed formula of lysine when found at pH 3.0.

Answer:

At low pH (acidic conditions), there is a high concentration of H+ ions. All basic amine groups accept protons to become positively charged (NH3+). The carboxylic acid group remains protonated (COOH).

Lysine at pH 3 structure
1.3 Draw the displayed formula of aspartic acid when at pH 13.0.

Answer:

At high pH (alkaline conditions), there is a high concentration of OH– ions. All acidic groups (including the side chain COOH) lose protons to become carboxylate ions (COO–). The amine group remains as NH2.

Aspartic acid at pH 13 structure

2. Nomenclature & Chirality

Glycine has the structure as seen below. Glycine structure
2.1 State the IUPAC name for glycine.

2-aminoethanoic acid

2.2 Isoleucine is a more complex 2-aminoacid. It has the IUPAC name of 2-amino-3-methylpentanoic acid. Draw the skeletal formula for this molecule.

Answer:

Isoleucine skeletal formula
2.3 Explain why isoleucine is a chiral molecule whereas glycine is not. Use skeletal formulae in your answer to demonstrate.

Explanation:

A molecule is chiral if it contains a carbon atom bonded to four different groups (a chiral centre).

  • Glycine: The central carbon is bonded to two hydrogen atoms. Since two groups are the same, it is not chiral.
  • Isoleucine: Carbon 2 and Carbon 3 are both chiral centres because they each have four unique groups bonded to them. This allows for optical isomerism (enantiomers).
Chirality comparison diagram

3. Reactions of Proline

Proline is a cyclic amino acid. The skeletal formula is as follows. Proline skeletal formula
3.1 Draw the zwitterion of proline.

Answer:

The hydrogen from the carboxylic acid group moves to the amine group within the ring, creating a positive charge on the nitrogen and a negative charge on the oxygen.

Proline zwitterion
3.2 Proline can react with propanoyl chloride. Draw the products of this reaction.

Answer:

This is a nucleophilic addition-elimination reaction (acylation). The secondary amine in the proline ring reacts with the acyl chloride to form an amide linkage. HCl is produced as a byproduct.

Reaction product of proline and propanoyl chloride
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