= Cyclic peptide
A <cyclic peptide> contains a covalently closed ring incorporating part or all of its <amino acid residue> chain. A head-to-tail ring joins the usual terminal amino and carboxyl groups through a <peptide bond>. Other ring architectures involve side chains or different linkages, so the positions and chemical types of the closures must be specified.<Nomenclature of Organic Chemistry: IUPAC Recommendations and Preferred Names 2013 by IUPAC>[Peptide nomenclature, homodetic and heterodetic cyclic peptides]{ref}
For a topology example, a linear unbranched chain of six <amino acid residues> has five backbone <peptide bonds>. Closing that chain head-to-tail adds a sixth bond and removes its free <N-terminus> and <C-terminus>. It preserves the residue order around the ring, although choosing a different residue as the written starting point does not create a new ring. This counting example does not imply that every chosen sequence readily forms such a ring.
<Alpha-amanitin> supplies a biological example: it combines a cyclic eight-residue framework with an additional sulfur-containing bridge. Its shape is therefore more constrained than residue count alone reveals.<Alpha-Amanitin in PubChem>[CID 9543442, structural description]{ref} Ring closure changes accessible conformations; its effects on stability, binding and biological activity depend on the particular <peptide> and its surroundings.
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