Amino acids and side chains

The common backbone

Every standard amino acid has the same backbone and differs only in its side chain. Because the backbone is constant, the side chains are what give a sequence its character.

Grouping by behaviour

• Hydrophobic side chains drive aggregation and reduce water solubility
• Charged side chains — lysine, arginine, aspartate, glutamate — govern how a peptide responds to pH
• Polar uncharged side chains generally help solubility
• Cysteine can form disulfide bridges, linking parts of a chain together
• Aromatic residues absorb ultraviolet light, which is how peptides are detected during chromatography

Residues that cause trouble

Methionine oxidises readily. Asparagine and glutamine deamidate, converting to acidic residues over time. Tryptophan is photosensitive. A sequence containing several of these needs more careful storage than one that does not, which is why two peptides of similar size can have quite different shelf lives.