Four figures that identify a peptide
CAS number, sequence, molecular formula and molecular mass. What each one states, how the four check one another, and where to verify them yourself.
A name is not an identity. Trade names, abbreviations and catalogue codes vary from one seller to the next, and one abbreviation is sometimes used for more than one compound. Four figures do not vary. Together they say which compound a listing means.
CAS number
A CAS Registry Number is the identifier the Chemical Abstracts Service assigns to a substance: one substance, one number. It is written in three groups — 7732-18-5 is water — and the last digit is a check digit computed from the others, so most typing errors produce a number that is not valid at all.
A salt and the free compound can carry different numbers, so the number also says which form is meant.
A number can be looked up in a public registry. CAS Common Chemistry is run by CAS itself, and PubChem by the US National Institutes of Health. Both can be searched without an account. Not every substance appears in every registry. A number that resolves in none of them is a reason to ask.
Sequence
The sequence is the order of the amino-acid residues, written from the N-terminus to the C-terminus. Listings here use the three-letter codes, separated by dashes: Ala-Gly-Ser.
Modifications are part of the sequence. An acetylated N-terminus (Ac-), an amidated C-terminus (-NH2), a D-residue or a non-standard residue each make a different compound from the unmodified chain, with a different formula, a different mass and usually a different CAS number. Two listings whose sequences differ in one of those are not the same material.
Molecular formula
The formula counts the atoms of each element in one molecule. It follows from the sequence: add up the atoms of the free amino acids, then remove one molecule of water for each peptide bond. A linear chain of n residues has n − 1 bonds.
Molecular mass
The mass follows from the formula: the sum of the atomic masses of every atom in it, in grams per mole. A specification normally states the average mass, which uses the natural isotope mix of each element. Sources differ in the last decimal, because atomic-mass tables do. A larger gap is worth a question.
A mass spectrum does not print that number directly. It reports mass-to-charge ratios of ions, often several charge states of the same molecule, and depending on the instrument the monoisotopic mass instead of the average. The figure on a spectrum and the figure on a listing agree once that is worked back, and working it back is what a laboratory's identity result states.
Why four
Because they check one another. The formula has to agree with the sequence. The mass has to agree with the formula. The CAS number has to resolve, in a registry, to a record showing the same formula and mass. An error in any one of the four shows up as a disagreement with the other three. That is the use of stating all of them.
It is also why they can be checked without taking a seller's word. None of the four is an opinion about quality. They are public facts about a compound.
How a listing here states them
A figure appears on a listing with a recorded source and the date it was read, or it does not appear. Where the source is a public registry, the page names it and links it. Where no source is recorded, the page prints a dash in place of a plausible value. Quality and methodology sets the rule out in full.
The identity figures say which compound is meant. They do not say how much of a vial is that compound. That is a different measurement: what an HPLC purity figure measures.