PL aijq: Amino acids; peptide bonding; optical isomerism

Question 1

Aminocarboxylic acids are more commonly known as what?

Answer 1

α-amino acids

Question 2

Draw the general structure of an α-amino acid.

Answer 2

alpha carbon attached to both -NH₂ + -COOH groups

Question 3

Amino acids are bifunctional compounds. What is meant by “bifunctional”?

Answer 3

Has 2 functional groups (i.e. NH₂ + COOH)

Question 4

Name a property of amino acids which is derived from their:

• amine group
• carboxyl group

Answer 4

• Amine: basic: react with acids to form salts
• Carboxyl: acidic: react with alkalis to form salts

Question 5

What are zwitterions?

Answer 5

Neutral molecules containing both negatively and positively charged groups.

Question 6

How do amino acids behave when dissolved in water?

Answer 6

• Form a (mostly) zwitterion solution, which is neutral (unless extra COOH / NH₂ in R group)
• Highly soluble since effectively ionic

​COOH is proton-donating + NH₂ is proton-accepting

Question 7

Give the ionic form in which amino acids exist in:

• neutral solution
• acid solution
• alkaline solution

Answer 7

• Neutral: H₃N⁺-CHR-COO^-
• Acid: H₃N⁺-CHR-COOH
• Alkaline: H₂N-CHR-COO^-

Question 8

Adding small quantities of acid or alkali to an amino acid solution has little effect on the pH. Explain why, using equations to support your answer.

Answer 8

Solutions of amino acids are buffer solutions, since they contain zwitterions, which can either donate or accept protons.

H₃N⁺-CHR-COO^- + OH^- → H₂O + H₂N-CHR-COO^-

H₃N⁺-CHR-COO^- + H₃O⁺ → H₂O + H₃N⁺-CHR-COOH

Question 9

Why do amino acids exhbit optical isomerism?

Answer 9

There are 4 different groups arranged tetrahedrally around a carbon atom.

Question 10

What is the other term for “optical isomer”?

Answer 10

Enantiomer

Question 11

Explain why 2-aminopropanoic acid has an enantiomer but aminoethanoic acid doesn’t.

Answer 11

All 4 groups on the former are different, whereas the latter’s carbon is bonded to 2 hydrogens.

Question 12

State what is meant by the term “chiral molecule”.

Answer 12

A molecule with non-superimposable mirror images.

Question 13

What is the chiral centre?

Answer 13

The carbon in a chiral molecule which is bonded to 4 different groups.

Question 14

Proteins are built from only one enantiomer of each amino acid. What are these known as?

Answer 14

L-enantiomers

Question 15

How do enantiomers compare in their properties?

Answer 15

• Most physical properties (melting/boiling point, solubility, density) and chemical properties are the same
• Behaviour differs in presence of other chiral molecules
• Sometimes smell/taste different (due to enzymes also being chiral)

Question 16

How are peptide bonds formed?

Answer 16

• NH₂ of one amino acid reacts with COOH group of another
• Secondary amide group, CONH, is formed
• H₂O molecule eliminated: condensation reaction

Question 17

How are dipeptides named?

Answer 17

• Acid with bonded COOH group written first (CONH - CO comes first)
• E.g. GlyAla

Question 18

What is an amino acid residue?

Answer 18

An α-amino acid which has lost the elements of water in forming a peptide.

Question 19

What is a more chemical name for peptides?

Answer 19

Secondary amides

Question 20

How can the amino acids in a protein be identified?

Answer 20

• Peptides are secondary amides
• Hydrolyse by heating by reflux with moderately concentrated acid/alkali to release individual amino acids
• Use paper chromatography: compare results to known samples of pure amino acids

Question 21

Write the equation for the reaction of serine, shown below, with NaOH solution.

Answer 21

H₂N-CH(CH₂OH)-COOH + NaOH → H₂N-CH(CH₂OH)-COO^-Na⁺ + H₂O

Question 22

Write the equation for the reaction of lysine, shown below, with excess hydrochloric acid.

Answer 22

H₂N-CH((CH₂)₄NH₂)-COOH +2HCl→ Cl^-H₃N⁺-CH((CH₂)₄NH₃⁺Cl^-)-COOH

Question 23

Write the equation for the reaction of aspartic acid, shown below, with excess NaOH solution.

Answer 23

H₂N-CH(CH₂(COOH))-COOH + 2NaOH → H₂N-CH(CH₂(COO^-Na⁺))-COO^-Na⁺ + 2H₂O

Question 24

Write the equation for the hydrolysis of the dipeptide AlaGly, shown below, using moderately concentrated hydrochloric acid.

Answer 24

H₂N-CH(CH₃)-CONH-CH₂-COOH + H₂O + 2HCl → Cl^-H₃N⁺-CH(CH₃)-COOH + Cl^-H₃N⁺-CH₂COOH

Question 25

Which statement(s) is/are correct about amino acids?

1. They all have a chiral centre
2. They form buffer solutions in aqueous solution
3. They form crystalline solids

Answer 25

2 and 3

1 is incorrect because glycine is not chiral