Amino Acids, Peptides and Proteins
Amino acids are the monomers of proteins, joined by peptide bonds. This domain tests the 20 standard amino acids, zwitterions, peptide bond formation, protein structure levels, denaturation and simple colour tests.
Core rules
- A standard amino acid has an α-carbon attached to −NH2, −COOH, −H and a variable R group.
- At physiological pH, amino acids exist mainly as zwitterions; the isoelectric point is the pH at which net charge is zero.
- Peptide bonds form between the α-carboxyl of one amino acid and the α-amino of the next, releasing water.
- Protein structure has primary (sequence), secondary (α-helix, β-sheet), tertiary (3D folding) and quaternary (subunit assembly) levels.
- Denaturation disrupts non-covalent bonds and disulfides but not peptide bonds. Biuret test detects two or more peptide bonds; ninhydrin detects amino groups.
Worked example
For the tripeptide Ala-Gly-Ser, count residues, peptide bonds and water molecules removed. Three amino acids form two peptide bonds and release two water molecules. The peptide has one free N-terminal amino group and one free C-terminal carboxyl group. Complete hydrolysis gives three amino acids, not two.
Traps that cost marks
A peptide bond is not a bond to the R group. Denaturation does not hydrolyse peptide bonds, so primary structure remains intact. The Biuret test is not specific for one amino acid. Do not confuse essential amino acids with non-essential ones; essential amino acids must come from the diet.