A peptide sequence is a compact description of an ordered chain of amino acid residues. It is conventionally written from the N-terminus to the C-terminus. One-letter notation makes a sequence concise, while three-letter notation can make an unfamiliar residue easier to recognize. Neither notation removes the need to identify terminal groups, non-standard residues or modifications. This guide introduces those elements so that a sequence can be read consistently by both technical and purchasing colleagues.
Direction and notation
For a simple teaching example, Gly-Ala-Ser and GAS describe the same residue order in three-letter and one-letter notation. The example is written from the N-terminal side toward the C-terminal side. Spaces and separators can improve readability, but they should not obscure whether a character is an amino acid code, a modification label or a separate annotation.
Sequence order is only part of the record
A terminal change, a D-amino acid or a modification at a particular position can distinguish two molecules that otherwise have similar sequences. Non-standard notation should be defined alongside the sequence. Position numbering also needs a consistent convention, especially when the record contains a tag, a linker or an added terminal group. Keep the approved sequence in a controlled text record so that line breaks or graphic formatting on packaging do not change its meaning.