How it works
Residues are conventionally written in order from the N-terminus (the free-amine end) to the C-terminus (the free-acid or amide end), often in three-letter or one-letter code. Direction matters — the same residues in the opposite order are a different peptide.
A sequence is more than the standard twenty letters. It carries the non-standard residues, terminal modifications, bridges, and attached groups a research peptide may have, and those are part of the identity, not footnotes to it.
From the sequence and its modifications, the molecular formula and mass follow by construction. That is why identity work is sequence-anchored: the expected mass a spectrometer confirms is computed from the sequence, and a structure is derived from it rather than assumed.
Why it matters
The sequence is the anchor for everything downstream. Formula, mass, and the expected ESI-MS series all derive from it, so an error in the sequence propagates into every computed value.
It is also where subtle errors hide. A single substituted residue — an isomeric swap that leaves the formula and mass unchanged — is invisible to a mass check, which is why sequence and structure must agree and be derived, not guessed.
Comparing sequences is how near neighbours are told apart: two peptides in the same class can differ by one residue or one modification, a difference the sequence makes explicit.
At Lineará
The compound pages draw each peptide’s sequence residue-by-residue, highlighting terminal chemistry, non-standard residues, bridges, and any metal coordination.
Because formula and mass are computed from the sequence, the sequence is where the identifier work begins — and where it is reconciled against an independent computation.