AQA A Level Chemistry 3.3.13 Amino Acids, Proteins and DNA Exam Questions

About AQA A Level Chemistry 3.3.13 Amino Acids, Proteins and DNA

This A Level topic on the AQA course applies organic chemistry to biological molecules. You need to describe amino acids as zwitterions, explain how peptide links form and are hydrolysed, describe primary, secondary and tertiary protein structure, and use thin-layer chromatography to identify amino acids. You also study enzymes as stereospecific catalysts and how drugs can be designed to block active sites, the structure of DNA from nucleotides with complementary base pairing, and how the anticancer drug cisplatin works and why it has side effects. It draws on 3.3.7, 3.3.9, 3.3.12 and 3.2.5. Examiners like zwitterion structures and explanations involving hydrogen bonding.

What you need to know

• Amino acids Each has an amine group and a carboxylic acid group. In the solid state and at the isoelectric point they exist as zwitterions, e.g. H3N+CH2COO- for glycine, giving high melting points. In acid they are protonated (-NH3+); in alkali they lose H+ (-COO-). Most are chiral.
• Proteins Amino acids join by condensation to form peptide (amide) links. Primary structure is the sequence of amino acids; secondary structure is the α-helix or β-pleated sheet held by hydrogen bonds; tertiary structure is the 3D folding held by hydrogen bonds, ionic interactions and disulfide bridges.
• Hydrolysis and TLC Proteins are hydrolysed by heating with hydrochloric acid to give amino acids. The amino acids can be separated by thin-layer chromatography, made visible with ninhydrin or UV light, and identified by Rf values.
• Enzymes Enzymes are proteins with stereospecific active sites. A drug can act as an inhibitor by binding to the active site, and computer modelling helps design such drugs.
• DNA Each nucleotide contains a phosphate group, the sugar 2-deoxyribose and one of four bases. Strands are joined by hydrogen bonds between complementary base pairs: adenine with thymine and guanine with cytosine, forming a double helix.
• Cisplatin A square planar platinum(II) complex. A chloride ligand is replaced by a water ligand, then platinum binds to a nitrogen atom on guanine in DNA, preventing replication of cancer cells. It also affects healthy cells, so it has side effects such as hair loss.

Common mistakes in 3.3.13 Amino Acids, Proteins and DNA

• Drawing amino acids in the neutral form when a zwitterion is asked for. Show -NH3+ and -COO- on the same molecule.
• Giving the wrong form at low pH. In acid, the amino acid is fully protonated as -NH3+ and -COOH.
• Saying cisplatin bonds to the phosphate groups in DNA. It forms a coordinate bond with nitrogen atoms on guanine.
• Measuring Rf to the edge of a spot or reporting an Rf value greater than 1. Measure to the centre of the spot, and Rf is always less than 1.
• Mixing up base pairs. Adenine pairs with thymine and guanine pairs with cytosine.

Try three 3.3.13 Amino Acids, Proteins and DNA exam-style questions