Example 1
Give the zwitterionic form of glycine, H2NCH2COOH.
Transfer H+ from COOH to NH2 to give +H3NCH2COO−. The+1 and −1 charges sum to zero.
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Year 12 · SACE Stage 2
Amino acids, peptide chains and protein shape
Read amino-acid charges and peptide links and apply protein-shape rules.
Method and worked examples
An α−amino acid has an amino group and carboxyl group attached to the same adjacent carbon, commonly written H2N–CH(R)–COOH. In its zwitterionic form it is+H3N–CH(R)–COO−: charges are separated but total charge is zero. Acidic conditions favour protonation; alkaline conditions favour deprotonation. Condensation between amino and carboxyl groups creates peptide, or amide, links.
Protein properties depend on amino-acid sequence and three-dimensional folding. Hydrogen bonds, ionic attractions and non-polar interactions help stabilise the shape. Temperature and pH can disrupt these interactions, altering an enzyme’s active site and function. Denaturation does not necessarily break the peptide backbone. Hydrolysis is a distinct process that breaks peptide links and can release amino acids.
Example 1
Example 2
How many peptide links and water molecules occur when six amino acids form one unbranched peptide?
Six units require five links. Condensation releases five H2O molecules; complete hydrolysis would consume five H2O to reverse those links.
Watch out: Denaturation changes folding and interactions; peptide-bond hydrolysis is a separate chemical change.