Protecting groups are chemical fragments used to block reactive functional groups during synthesis. In peptide synthesis, they help ensure that coupling occurs only at the intended positions.
Common examples include acid-labile groups such as Boc and tert-butyl esters, base-labile groups such as Fmoc, and other chemistry-specific protecting strategies. To be useful, a protecting group should be easy to install and easy to remove without damaging the rest of the peptide.