Structured Questions
Question 1: Amino Acid Chemistry
6 marks(a) Glycine and alanine are α-amino acids. Draw the zwitterion structure of glycine. [1]
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Structure: H₃N⁺-CH₂-COO⁻ [1]
(b) Explain how zwitterions can act as buffers. Write chemical equations showing the reaction of a general zwitterion (H₃N⁺-CHR-COO⁻) with H⁺ ions and with OH⁻ ions. [3]
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- Zwitterions contain both acidic groups (−NH₃⁺) which can donate protons to neutralise bases, and basic groups (−COO⁻) which can accept protons to neutralise acids [1]
- Reaction with H⁺: H₃N⁺-CHR-COO⁻ + H⁺ → H₃N⁺-CHR-COOH [1]
- Reaction with OH⁻: H₃N⁺-CHR-COO⁻ + OH⁻ → H₂N-CHR-COO⁻ + H₂O [1]
(c) Two molecules of glycine undergo a condensation reaction to form a dipeptide. Draw the structure of this dipeptide, clearly indicating the peptide bond. [2]
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Structure: H₂N-CH₂-CONH-CH₂-COOH [1]
Indication: Clearly highlighting or circling the −CONH− linkage as the peptide bond [1]
Question 2: Thin-Layer Chromatography (TLC)
6 marks(a) A mixture of amino acids was separated using thin-layer chromatography. The solvent front moved 12.0 cm. A spot corresponding to serine moved 4.8 cm. Calculate the \(R_f\) value for serine. [2]
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- Calculation: \(\frac{4.8}{12.0}\) [1]
- \(R_f = 0.40\) [1] (Accept 0.4. \(R_f\) values have no units)
(b) Explain the factors that determine the distance moved by different amino acids on a TLC plate. [3]
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- Separation depends on the relative solubility in the mobile phase (solvent) and the relative adsorption to the stationary phase (silica plate) [1]
- The stationary phase is highly polar, while the mobile phase has a different polarity [1]
- Amino acids with more polar side chains (e.g. lysine or serine) form stronger hydrogen bonds / dipoles with the polar stationary phase, moving more slowly / traveling a shorter distance [1]
(c) Suggest why lysine (R = −(CH₂)₄NH₂) has a different \(R_f\) value than alanine (R = −CH₃) in a polar mobile phase. [1]
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Lysine has a highly polar amine group in its side chain, allowing it to form stronger electrostatic interactions / hydrogen bonds with the stationary phase (or dissolve better in the polar solvent) compared to the non-polar hydrophobic methyl group in alanine [1]
Question 3: DNA & Cisplatin
5 marks(a) Describe the components of a DNA nucleotide and state the type of bond that links nucleotides in the sugar-phosphate backbone. [2]
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- Components: A phosphate group, a 2-deoxyribose sugar, and a nitrogenous base (Adenine, Thymine, Cytosine, or Guanine) [1]
- Bond in backbone: Covalent phosphodiester bond [1]
(b) Explain the mechanism by which cisplatin functions as an anticancer drug, and explain why it is associated with severe side effects. [3]
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- Cisplatin undergoes ligand substitution in cells, replacing one or both chloride ligands with water [1]
- It then binds to nitrogen atoms on two adjacent Guanine bases in a DNA strand, distorting the double helix shape which prevents DNA replication and triggers cell death [1]
- Side effects occur because cisplatin is not selective; it also binds to and damages the DNA of healthy cells that divide rapidly (such as hair follicle cells or lining of the stomach) [1]
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