OxfordAQA International A Level Chemistry
OxfordAQA International A Level Chemistry 3.3.13 Amino Acids and Proteins Exam Questions
Free OxfordAQA International A Level Chemistry revision for 3.3.13 Amino Acids and Proteins: what you need to know, common mistakes and exam-style questions with answers. Covers amino acid structure, zwitterions, peptide links, primary, secondary and tertiary protein structure, hydrolysis, thin-layer chromatography with ninhydrin, and cisplatin.
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About OxfordAQA International A Level Chemistry 3.3.13 Amino Acids and Proteins
Amino acids and proteins is an A2 organic topic examined in Unit 4. You need to explain why amino acids exist as zwitterions and how they form different ions in acidic and alkaline solutions, describe how peptide links form, and explain primary, secondary (alpha helix and beta pleated sheet) and tertiary structure, including hydrogen bonds and disulfide bridges. You also need to describe the hydrolysis of proteins and how thin-layer chromatography with ninhydrin or UV light identifies the amino acids produced from their Rf values. The OxfordAQA specification includes cisplatin, which binds to nitrogen atoms by ligand replacement, and its adverse effects.
What you need to know
- Amino acids Amino acids contain an amine group and a carboxylic acid group. In a solid or at a certain pH they exist as zwitterions, H3N+CHRCOO-, giving high melting points.
- Effect of pH In acidic solution the COO- gains a proton to give H3N+CHRCOOH. In alkaline solution the NH3+ loses a proton to give H2NCHRCOO-.
- Peptides and proteins Amino acids join by condensation to form peptide (amide) links. Primary structure is the sequence of amino acids; secondary structure is the alpha helix or beta pleated sheet held by hydrogen bonds; tertiary structure involves hydrogen bonds and disulfide bridges.
- Hydrolysis Heating a protein with concentrated hydrochloric acid hydrolyses the peptide links, giving a mixture of amino acids.
- Thin-layer chromatography Amino acids are separated by TLC and located with ninhydrin or UV light. Rf = distance moved by spot / distance moved by solvent front; compare with standards.
- Cisplatin Cisplatin is an anti-cancer drug that binds to nitrogen atoms in DNA by ligand replacement, preventing DNA replication in cancer cells. It can also affect healthy cells, causing side effects such as hair loss.
Common mistakes in 3.3.13 Amino Acids and Proteins
- Drawing a zwitterion without both charges. It has NH3+ and COO- in the same ion.
- Saying disulfide bridges hold secondary structure. They are part of tertiary structure; secondary structure is held by hydrogen bonds.
- Measuring Rf to the edge of the spot rather than its centre, or measuring the solvent front from the bottom of the plate. Measure both from the baseline.
- Saying cisplatin binds to the phosphate groups. It binds to nitrogen atoms on bases in DNA.
Try three 3.3.13 Amino Acids and Proteins exam-style questions
1. Draw the structure of glycine, H2NCH2COOH, in a solution of low pH. (1 mark)
Show the mark scheme answer
H3N+CH2COOH.
2. A spot moved 3.6 cm and the solvent front moved 8.0 cm. Calculate the Rf value. (1 mark)
Show the mark scheme answer
3.6 / 8.0 = 0.45.
3. Explain how cisplatin prevents cancer cells from dividing. (2 marks)
Show the mark scheme answer
A chloride ligand is replaced by a nitrogen atom on a DNA base; this distorts DNA and prevents replication / cell division.
3.3.13 Amino Acids and Proteins FAQs
What does OxfordAQA International A Level Chemistry 3.3.13 Amino Acids and Proteins cover?
It covers amino acid structure, zwitterions, peptide links, primary, secondary and tertiary protein structure, hydrolysis, thin-layer chromatography with ninhydrin, and cisplatin.
Which unit is 3.3.13 Amino Acids and Proteins examined in?
It is examined in Unit 4, an International A2 paper covering organic and physical chemistry. The Unit 5 practical and synoptic paper can also assess it.
Why is ninhydrin used in chromatography of amino acids?
Amino acids are colourless, so the spots are invisible. Ninhydrin reacts with them to form coloured spots, or UV light can be used with a fluorescent plate, so Rf values can be measured.
