Chemistry
Sequence
Research use only. For in vitro research use only. Not for human or animal consumption. Not for human consumption, medical use, or personal application.
A sequence is the ordered list of residues in a peptide, written from the N-terminus to the C-terminus. It appears in three-letter code, such as Gly-His-Lys, or in single-letter code as GHK, with direction implicit in the convention rather than marked. Sequence fixes composition and calculable mass, which makes it the primary statement of what a molecule is. It does not describe terminal modifications, cyclisation or salt form.
Notation details that change the meaning
Small markers carry real information. A trailing "-NH2" indicates C-terminal amidation, "Ac-" indicates N-terminal acetylation, a D- prefix marks an inverted stereocentre, and brackets denote cyclisation. Dropping any of them describes a different compound.
Because each amino acid contributes a fixed mass, a stated sequence yields a theoretical molecular weight that can be compared against measurement. That comparison is the reason to record sequence precisely.
Using the stated sequence at goods-in
A useful check is whether the sequence on the catalogue page, on the lot documentation and in the published literature agree, modifications included. Disagreement usually points to a naming problem worth resolving first.
Where the documentation supports it, mass spectrometry data on the Certificate of Analysis is the evidence that delivered material matches the stated sequence.
Frequently asked questions
- Does mass spectrometry confirm the sequence itself?
Intact mass confirms composition, not order — two peptides with the same residues in different positions share a mass. Establishing order needs fragmentation data, and routine documentation reports intact mass only.
- Why do catalogue and literature sequences differ in length?
Fragments, analogues and truncated variants circulate under similar names, and numbering is sometimes quoted relative to a precursor. Comparing full residue lists rather than names identifies the molecule reliably.
- Is single-letter code interchangeable with three-letter code?
For standard residues, yes. Modified residues have no agreed single-letter form, so three-letter code with explicit modifiers is safer once a sequence leaves the standard twenty.
Updated 2026-06-01