9701 · 34.4
Amino acids flashcards
Revision flashcards for Cambridge 9701 Amino acids (syllabus 34.4). Flip, recall, then mark a real past-paper question.
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What is the general structure of an α-amino acid?
A central carbon atom (the α-carbon) bonded to an amine group (–NH₂), a carboxylic acid group (–COOH), a hydrogen atom (–H), and a variable side chain (–R group).
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What is a zwitterion?
A dipolar ion that has both a positive and a negative charge within the same molecule, but has an overall net charge of zero. Amino acids exist as zwitterions, H₃N⁺–CH(R)–COO⁻, at their isoelectric point.
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Define the isoelectric point (pI).
The specific pH at which an amino acid has no net electrical charge and exists predominantly in its zwitterionic form. At this pH, it will not migrate in an electric field.
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Which of the 20 common amino acids is not chiral? Explain why.
Glycine. Its R group is a hydrogen atom. This means the α-carbon is bonded to two identical groups (two H atoms), so it is not a chiral centre.
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What is a peptide bond?
An amide functional group (–CO–NH–) that links two amino acids. It is formed in a condensation reaction between the carboxyl group of one amino acid and the amine group of another.
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What are the products of the acid hydrolysis of the dipeptide Ala-Gly?
Heating with strong acid (e.g., 6 mol dm⁻³ HCl) breaks the peptide bond, producing the ammonium salts of the constituent amino acids: H₃N⁺CH(CH₃)COOH Cl⁻ (from alanine) and H₃N⁺CH₂COOH Cl⁻ (from glycine).
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What are the products of the alkaline hydrolysis of the dipeptide Ala-Gly?
Heating with strong alkali (e.g., NaOH(aq)) breaks the peptide bond, producing the sodium salts of the constituent amino acids: H₂NCH(CH₃)COO⁻Na⁺ (from alanine) and H₂NCH₂COO⁻Na⁺ (from glycine).
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What form does an amino acid take in a solution with a pH significantly lower than its pI?
It acts as a base. The carboxylate group (–COO⁻) of the zwitterion is protonated. The molecule becomes a cation with a net positive charge: H₃N⁺–CH(R)–COOH.
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What form does an amino acid take in a solution with a pH significantly higher than its pI?
It acts as an acid. The ammonium group (–NH₃⁺) of the zwitterion is deprotonated. The molecule becomes an anion with a net negative charge: H₂N–CH(R)–COO⁻.
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What is a dipeptide?
A molecule formed from two amino acids joined by a single peptide bond.
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How are amino acids classified as acidic, basic, or neutral?
Based on the nature of their R group. If the R group contains a second –COOH, it's acidic (e.g., aspartic acid). If it contains a second basic group like –NH₂, it's basic (e.g., lysine). If the R group is neutral, the amino acid is neutral (e.g., alanine).