Sequence
In short The order of the amino acids that make up a peptide, written from the N-terminus to the C-terminus.
A peptide is a chain of amino acids, and its sequence lists them in order from the free amino end (N-terminus) to the free carboxyl end (C-terminus). The order determines the peptide's shape and behaviour; two peptides with the same amino acids in a different order are different molecules.
Sequences are written with three-letter codes, such as Gly-Glu-Pro, or one-letter codes, such as GEP. Longer peptides are usually given in one-letter form; MOTS-c, for example, is MRWQEMGYIFYPRKLR.
Many research peptides are modified. Ac- at the start means an acetylated N-terminus; -NH2 at the end means an amidated C-terminus. Non-standard building blocks also appear: Aib (aminoisobutyric acid) and D-amino acids in ipamorelin, Nle (norleucine) in the melanotans, and cyclic structures shown with "cyclo". Each modification is part of the identity of the compound.
UKPeps product pages give the sequence or, for very long or modified peptides, a description of the structure, in the specification table.