Peptide modifications are described by both their identity and their location. A change at a terminus is different from a modification on the side chain of a particular residue. A cyclic structure also needs a clear description of which positions are connected. This page explains common categories as technical vocabulary. Whether a specific modification is suitable for a requested sequence, and whether it is included in a project, is confirmed individually.
Terminal and site-specific changes
N-terminal acetylation and C-terminal amidation are examples of terminal descriptions. A side-chain modification instead belongs to a specified residue and position. Labels, linkers and other additions also need unambiguous identities. The purpose of the notation is to define the requested structure, not simply to give a short name to a broad modification category.
Connections need an explicit map
When a target contains a bridge or a ring, the connected positions are part of its identity. A sequence alone may not communicate that connectivity. The reference record should explain the intended connection and distinguish it from alternative arrangements, particularly where more than one pair of reactive positions exists. For a printed abbreviation, retain a separate full structural description so the project can be identified without relying on a short marketing name.