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An amino acid contains an amino group and a carboxylic acid group. In many aqueous or solid contexts, proton transfer gives a zwitterion with NH3+ and COO⁻ groups and zero overall charge. In acidic conditions the carboxylate becomes COOH, producing a positive species; in alkaline conditions the ammonium group loses H+, producing a negative species. The side chain may add further acid–base behaviour. State the pH conditions and account for all ionisable groups rather than assigning one fixed form to every solution.
Video by Eliot Rintoul. The video’s own board label is retained; follow the specification reference and scope notes on this lesson for assessed content.
Viewing focus: Predict amino-acid forms at different pH values
After watching, attempt these questions before opening the answers. You can also use the written lesson below.
After watching 1: What is the overall charge of a simple zwitterion?
Zero. Its positive and negative charges balance.
After watching 2: Which bond connects amino-acid residues?
A peptide bond. The covalent link is –CO–NH–.
What you'll be able to do
An amino acid contains an amino group and a carboxylic acid group. In many aqueous or solid contexts, proton transfer gives a zwitterion with NH3+ and COO⁻ groups and zero overall charge. In acidic conditions the carboxylate becomes COOH, producing a positive species; in alkaline conditions the ammonium group loses H+, producing a negative species. The side chain may add further acid–base behaviour. State the pH conditions and account for all ionisable groups rather than assigning one fixed form to every solution.
Condensation between the amino group of one amino acid and the carboxyl group of another forms a peptide link, –CO–NH–. Two different amino acids can form different sequences, so a dipeptide order matters. Hydrolysis breaks peptide links; acid hydrolysis produces protonated amino groups, while alkaline hydrolysis produces carboxylate salts. Protein folding is stabilised by interactions including hydrogen bonding, ionic attractions, disulfide links and hydrophobic effects. Breaking those interactions is different from hydrolysing every peptide bond.
DNA contains a sugar–phosphate backbone and nitrogenous bases. Phosphodiester bonds connect nucleotides within a strand; hydrogen bonds connect complementary bases between strands. Complementary pairing supports sequence copying and recognition. In chemical contexts such as anticancer compounds, bonding to particular sites can alter DNA shape and interfere with replication. Explain which molecular interaction changes rather than claiming that all treatments destroy every bond. A structural formula, charge pattern and accessible donor atoms provide evidence for a proposed interaction.
Attempt these problems on paper before opening the worked reasoning. Explain your steps as well as giving a final answer.
Common mistakes to avoid
Key takeaways
Question 1: What small molecule is eliminated when two amino acids form one peptide link?
Water.
Check the assessed scope and practical or mathematical requirements.
An external explanation or interactive activity to use alongside the lesson. It may include extension material.
Close the notes and answer from memory. Revisit any explanation you cannot yet reconstruct.
Test yourself
Answer a few questions, then review anything you missed.
Start with the quiz above to check the key ideas. You can also choose another topic from the practice library.