Peptide sequences and notation

Three-letter codes, terminal modifications and what an acetate salt means.

Reading a sequence

Sequences are written from the N-terminus to the C-terminus using three-letter or one-letter amino acid codes. Gly-His-Lys and GHK describe the same tripeptide.

Modifications

A leading Ac- means the N-terminus is acetylated. A trailing -NH2 means the C-terminus is amidated. A D- prefix marks the D-isomer of an amino acid, and Aib is the non-standard residue aminoisobutyric acid.

Salts and counter-ions

Synthetic peptides are usually supplied as acetate or trifluoroacetate salts. The counter-ion adds weight that is not part of the peptide itself, which is why molecular weight on a label refers to the free peptide.