Reading a peptide specification: CAS, formula, sequence and molar weight

A good specification sheet tells you exactly what a compound is. This is what each line means and why the approximations are there.

CAS number

A CAS Registry Number is a unique identifier assigned by the Chemical Abstracts Service to a specific chemical substance. It is the most reliable way to be sure two suppliers are describing the same compound, because trade names and abbreviations vary. Note that a salt form or a metal complex of a peptide can carry a different CAS number from the free peptide.

Molecular formula and molar weight

The molecular formula counts the atoms of each element in one molecule; the molar weight is the mass of one mole in grams. Molar weights for peptides are often written with a tilde — ~711.87 g/mol — because the value depends on which isotopic masses are used and, for a supplied product, whether the figure refers to the free peptide or to a salt or complex. Where a peptide is supplied as a metal complex, a specification should give both figures.

Sequence notation

Sequences are written from the N-terminus to the C-terminus using three-letter amino acid codes: Gly-His-Lys, for example. Single-letter codes (GHK) are also common. Modifications appear as prefixes and suffixes: H- and -OH mark an unmodified N- and C-terminus; -NH2 indicates a C-terminal amide; Ac- indicates an N-terminal acetyl group. These modifications change the mass and often the stability and activity of the peptide, so they are part of the identity, not a footnote.

Non-standard residues

Many research peptides include residues outside the twenty standard amino acids. D- before a code (D-Phe) denotes the D-enantiomer, often used to resist enzymatic breakdown. Aib is α-aminoisobutyric acid, a helix-stabilising residue. 2-Nal is 2-naphthylalanine. A specification that uses these abbreviations without defining them is assuming a reader who knows them; if you do not, ask.

Classification and appearance

The classification line places the compound in context — a tripeptide, a fragment of a larger hormone, a receptor agonist — and the appearance line describes what a correct vial looks like. A copper-complexed peptide is blue; most others are white to off-white. A mismatch between the stated appearance and what arrives is worth querying before use.

For laboratory research use only — not for human or veterinary use. This article is general laboratory reference information and is not advice on the use of any product.