Definition
Capping a peptide's free N-terminal amine with an acetyl group to remove its charge and slow aminopeptidase cleavage.
N-terminal acetylation replaces the free amine at the start of a peptide chain with an acetyl group, written Ac-. In synthesis this is done on the resin, typically with acetic anhydride, before final cleavage. Acetylation removes a positive charge, which can change solubility and HPLC behaviour, and it blocks aminopeptidases that degrade peptides from the N-terminus. Many native proteins are acetylated, so capping can also make a synthetic fragment better resemble its position within a larger protein. Acetyl-capped sequences include thymosin β4 and the tetrapeptide Ac-SDKP derived from it. Acetylation adds 42 Da to the molecular mass, so a certificate of analysis should show it in both the sequence and the theoretical mass.