Peptide sequence, molecular weight and net peptide content: reading the notation and doing the arithmetic

Three numbers on a product page or certificate decide how much peptide is actually in a vial and how to turn milligrams into molar concentration: the sequence, the molecular weight, and, where it is reported, the net peptide content. This guide explains how sequences are written, how molecular weight is calculated and why the number on a certificate may differ slightly from the one in a database, what counter-ions and water add to the weighed mass, and how to go from a label mass to moles. For laboratory research use only.

Reading a peptide sequence

Sequences are written from the N-terminus (free amino end) on the left to the C-terminus (carboxyl end) on the right, which is also the order in which cells make them. Each residue is given either a three-letter code (Gly-Glu-Pro…) or a one-letter code (GEP…); the two are interchangeable. Modifications are written at the ends and in brackets:

Ac- at the start	N-terminal acetyl group; the free amine is capped. Common for protease resistance (thymosin β4, MT-2, PT-141).
-NH₂ at the end	C-terminal amide instead of the free acid. Also for stability (tesamorelin, MT-2).
-OH at the end	Free C-terminal carboxylic acid; the default, sometimes written to make the contrast explicit (PT-141).
cyclo[…]	The residues inside the bracket are joined in a ring, usually through side chains (MT-2’s Asp–Lys lactam).
D- before a residue	The D-enantiomer instead of the natural L form (D-Phe in MT-2), used to block enzymes.
Nle, Aib, Orn…	Non-standard residues: norleucine, aminoisobutyric acid, ornithine. Each has its own residue mass.
Cu(II), · acetate, · TFA	A complexed metal or the salt form; part of what is weighed, separate from the peptide itself.

Two peptides can share a sequence and differ only in these annotations, and the difference matters: MT-2 and PT-141 differ by one terminal group and one mass unit. Our guide to the peptide bond covers the chemistry behind the notation.

Calculating molecular weight

The molecular weight of a peptide is the sum of the residue masses plus one water molecule (18.02) for the free ends, with adjustments for any modification. A residue mass is an amino acid’s mass minus water, because each peptide bond formed released one H₂O. The arithmetic for a short example:

Gly-His-Lys (the free GHK tripeptide): residue masses 57.05 + 137.14 + 128.17 = 322.36; add 18.02 for the terminal H and OH = 340.4 g/mol. Complex it with copper(II), which replaces two protons: 340.4 + 63.55 − 2 × 1.01 ≈ 401.9 g/mol, the figure on the GHK-Cu product page.

For longer peptides, use a calculator rather than a spreadsheet; the sequence of BPC-157 (15 residues) sums to about 1,419.5, thymosin β4 (43 residues, acetylated) to about 4,963, tesamorelin (44 residues, hexenoylated, amidated) to about 5,135.9. Two conventions explain small discrepancies between sources: average mass uses the natural isotope mixture and is what “molecular weight” normally means; monoisotopic mass uses the most abundant isotope of each element and is what a mass spectrometer reports for the main peak. For a 1,400 Da peptide the two differ by under 1 Da; for a 5,000 Da peptide, by about 3 Da. A certificate’s observed mass is compared with the calculated mass of the same type.

What else is in the weighed mass

A lyophilized vial labelled “10 mg” contains 10 mg of powder, and the powder is not all peptide. Three things add to it:

Counter-ions. Peptides are purified by HPLC in trifluoroacetic acid and isolated as TFA salts unless exchanged to acetate or chloride. Each basic residue (Lys, Arg, His, the N-terminal amine) binds one TFA (114 Da) or acetate (59 Da). For an arginine- and lysine-rich peptide, TFA can be 10–20% of the powder mass.
Water. Lyophilized peptides retain a few percent of water and pick up more from humid air once opened; see what lyophilized means.
Excipients. Bulking agents such as mannitol, where used, are declared on the label and certificate.

Net peptide content (also called peptide content or NPC) is the fraction of the powder that is peptide after these are subtracted, typically 70–90% for a TFA salt and higher for acetate salts. It is measured by amino-acid analysis, by nitrogen determination or by UV absorbance against a standard, and it is a different figure from HPLC purity, which only compares peptide-related peaks to each other. A “99% pure” peptide with 80% net peptide content contains 8 mg of peptide in a 10 mg vial. When net peptide content is not reported, the label mass is gross powder mass and your concentrations are overestimates by the unreported fraction. See peptide purity explained.

From milligrams to moles

Molar quantity is mass divided by molecular weight: n (mol) = m (g) ÷ MW (g/mol). Working examples, using peptide mass (label mass × net peptide content if known):

10 mg BPC-157 ÷ 1,419.5 g/mol = 7.04 µmol. In 2 mL that is 3.52 mM.
50 mg GHK-Cu ÷ 401.9 g/mol = 124 µmol. In 5 mL that is 24.9 mM.
5 mg tesamorelin ÷ 5,135.9 g/mol = 0.97 µmol. In 1 mL that is 0.97 mM.
10 mg MOTS-c ÷ 2,174.6 g/mol = 4.60 µmol. In 2 mL that is 2.30 mM.

To go the other way, from a target molar concentration to a diluent volume: V (L) = n (mol) ÷ C (mol/L). The peptide calculator does the mass-per-volume arithmetic; convert to molar with the molecular weight from the product page. For a blend, run the calculation once per component with that component’s mass and molecular weight, as explained in peptide blends in research.

Using these numbers on a certificate

The observed mass on the mass-spectrometry panel should match the calculated mass for the sequence as annotated (including Ac-, -NH₂, the ring, the metal). A mismatch of +16 or +32 points to oxidation, +1 to deamidation, −17 or −18 to loss of ammonia or water, and larger differences to a wrong or truncated sequence.
The molecular weight on the certificate may be given as average or monoisotopic; check which before comparing.
The net peptide content, if present, is the number to multiply the label mass by before calculating concentration.

Our guide to reading a certificate of analysis shows where each appears.

Frequently asked questions

Which end of a peptide sequence is written first?

The N-terminus (amino end) on the left, the C-terminus (carboxyl end) on the right. “Ac-” before the first residue means an acetylated N-terminus; “-NH₂” after the last means an amidated C-terminus.

How is peptide molecular weight calculated?

Sum the residue masses (each amino acid minus water), add 18.02 for the free ends, and adjust for modifications: +42.04 for an N-acetyl group, −0.98 for a C-terminal amide, plus the mass of any metal minus the protons it displaces.

What is the difference between average and monoisotopic mass?

Average mass uses the natural isotope mixture and is the usual “molecular weight”. Monoisotopic mass uses the lightest common isotope of each element and is what a mass spectrometer reports. They differ by about 0.06% for peptides.

What is net peptide content?

The fraction of a lyophilized powder that is peptide, after subtracting water, counter-ions and excipients. It is different from HPLC purity and is the figure to use when converting label mass to moles.

How do I convert mg of peptide to moles?

Divide the peptide mass in grams by the molecular weight in g/mol. 10 mg of a 1,419.5 g/mol peptide is 0.010 ÷ 1,419.5 = 7.04 µmol.

Why does my certificate’s molecular weight differ slightly from the product page?

Usually because one is an average mass and the other monoisotopic, or because one includes a counter-ion or metal and the other does not. Check which convention and which form each refers to.

Quantum Peptides supplies lyophilised research peptides for in-vitro and preclinical laboratory research only. Products are not for human or veterinary use. This guide is an educational summary and is not guidance on administration of any kind.