CCEA A Level Chemistry 5.9 Amino Acids Exam Questions

About CCEA A Level Chemistry 5.9 Amino Acids

Section 5.9 of Unit A2 2 covers amino acids and proteins from a chemist's point of view. You need the formulae of glycine and alanine, why alanine is optically active and glycine is not, and how zwitterions explain the high melting points and water solubility of amino acids. CCEA requires the reactions of amino acids with sodium carbonate, copper(II) sulfate and nitrous acid. You must describe primary, secondary and tertiary protein structure and the peptide link, and explain enzymes as biological catalysts using induced fit and lower activation energy, including enzymes in washing powders and the effect of pH and temperature.

What you need to know

• Glycine and alanine Glycine is H2NCH2COOH and has no chiral centre. Alanine is H2NCH(CH3)COOH; its central carbon has four different groups, so it is optically active.
• Zwitterions In the solid and in neutral solution, amino acids exist as zwitterions, +H3NCHRCOO-. Strong ionic attractions between zwitterions give high melting points and good solubility in water.
• Reactions With sodium carbonate, the COOH group gives effervescence (CO2). With copper(II) sulfate, a deep blue complex forms. With nitrous acid, the NH2 group reacts to release nitrogen gas, which can be used to estimate the amino acid.
• Peptides and proteins Amino acids join by condensation to form peptide (amide) links, -CONH-. Primary structure is the sequence of amino acids; secondary structure is the alpha-helix or beta-pleated sheet held by hydrogen bonds; tertiary structure is the overall folding held by hydrogen bonds, ionic attractions and disulfide links.
• Enzymes Enzymes are protein catalysts that provide a lower activation energy route. The substrate binds to the active site, which changes shape slightly (induced fit). High temperature or extreme pH disrupts tertiary structure and denatures the enzyme. Enzymes in biological washing powders work at lower temperatures, saving energy.

Common mistakes in 5.9 Amino Acids

• Saying glycine is optically active. Its central carbon has two hydrogens, so it is not chiral.
• Explaining high melting points of amino acids with hydrogen bonding only. The main reason is ionic attraction between zwitterions.
• Drawing a peptide link as an ester link. It is -CO-NH-, an amide link.
• Saying enzymes are killed by heat. Enzymes are denatured as the tertiary structure and active site shape change.
• Forgetting to show the water lost when drawing a dipeptide. Condensation releases one H2O per peptide link.

Try three 5.9 Amino Acids exam-style questions