Peptide Molecular Weight: How to Calculate It From a Sequence

Molecular weight in one sentence

IUPAC defines relative molecular mass, the quantity sometimes called molecular weight, as the “ratio of the mass of a molecule to the unified atomic mass unit”. It is printed in grams per mole or in daltons, which are interchangeable here: the CODATA 2022 molar mass constant is 1.000 000 001 05 g/mol, about one part in a billion away from 1.

Two neighbouring numbers are easily confused with it. A formula weight is the mass of whatever formula is written down, so for a salt it includes counter-ions and water. A mass read off a spectrum usually belongs to an ion carrying extra protons. Keeping the three apart settles many apparent disagreements before any arithmetic starts.

Building a peptide molecular weight from its sequence

A peptide bond forms by condensation: the carboxyl group of one amino acid joins the amino group of the next and a molecule of water is released. A chain of n amino acids has n − 1 such bonds, so it weighs the sum of its free amino acids minus n − 1 waters. The arithmetic is simpler in residue masses — an amino acid’s mass minus one water, the unit that sits inside a chain — because the rule becomes: add the residue masses, then add one water for the terminal H and OH. Expasy’s documentation for its Compute pI/Mw tool describes the same procedure, adding the residues’ average masses and “the average isotopic mass of one water molecule”.

For Pinealon (Glu-Asp-Arg), on average atomic weights: 129.115 + 115.088 + 156.189 = 400.392, and adding 18.015 for water gives 418.407, written 418.41 g/mol. The long way agrees: the free amino acids weigh 147.130, 133.103 and 174.204, a total of 454.437, and removing two waters (36.030) leaves 418.407. Two extensions cover most other sequences. A peptide of two chains joined only by disulfide bridges needs one water per chain, because each chain has its own ends; a head-to-tail cyclic peptide needs none, because its ends are joined by one more amide bond. Our chart of amino acid codes decodes the one- and three-letter symbols.

The residues used in this article, computed from CIAAW’s 2024 abridged atomic weights and NIST isotope masses:

Average or monoisotopic: choosing the inputs

Every mass in this article can be computed two ways, and the choice is about inputs, not arithmetic. IUPAC defines average mass as mass “weighted for its isotopic composition” and monoisotopic mass as mass “calculated using the mass of the most abundant isotope of each element”. Average values, built from CIAAW’s standard atomic weights, are the ones to use for anything weighed; monoisotopic values, built from NIST’s isotope masses, correspond to the first peak of the isotope pattern a high-resolution mass spectrometer resolves. For the tripeptides here the two differ by about 0.22 to 0.24, and the gap grows with size: glutathione disulfide, twice glutathione’s size, shows 0.47. How isotope patterns, charge states and the choice of mass appear on a real spectrum is covered in our guide to reading a peptide mass spectrum.

Termini, modifications and disulfide bridges

The water added in the second step supplies a hydrogen to the N-terminus and a hydroxyl to the C-terminus, the H- and -OH that full sequence notation writes at the ends. Modifications replace those groups or bridge side chains, and each moves the mass by a fixed amount. The figures below come from Unimod, “a community supported, comprehensive database of protein modifications” for mass spectrometry whose values are derived from elemental compositions; recomputed from each composition with NIST isotope masses, they agree to the sixth decimal place.

Two real molecules show the shifts at work. Thyrotropin-releasing hormone, written pGlu-His-Pro-NH₂, can be worked out from Glu-His-Pro at 381.389 g/mol on average atomic weights: amidating the C-terminus gives 380.405, and closing the N-terminal glutamate into a pyroglutamate ring removes a further water, leaving 362.390 (starting from glutamine and losing ammonia gives the same answer). That is the formula, C₁₆H₂₂N₆O₄, and the average mass that ChEBI lists, and the monoisotopic value, 362.1703, matches ChEBI’s 362.17025. Glutathione disulfide is two glutathione molecules joined by one bridge: 2 × 307.083806 − 2.015650 = 612.151962, matching ChEBI’s monoisotopic 612.15196. One caution from Expasy applies to any residue-sum tool: its calculator “does not account for the effects of post-translational modifications”, so shifts like these have to be added by hand.

Molecular weight vs formula weight: salts and water

Peptides purified by reversed-phase HPLC often leave as salts. Sikora and colleagues’ review of counter-ions explains that cleavage and purification use TFA, so “the cationic peptides are obtained as trifluoroacetate salts”, with counter-ions associated with basic sites — histidine, lysine, arginine and the N-terminal amino group — and some TFA possibly “adsorbed in lyophilizate as a contaminant”. TFA is often exchanged for chloride, the review adds, and most peptide drugs are acetate salts.

So one vial has two honest masses. The molecular weight belongs to the peptide; the formula weight belongs to the salt, and it is what a balance actually weighs. Each counter-ion adds the mass of its acid — 114.02 g/mol for trifluoroacetic acid, 60.05 for acetic acid and 36.46 for hydrogen chloride on CIAAW’s abridged atomic weights — and each water of hydration adds 18.02.

As an illustration, not a statement about any product: Pinealon has two basic sites, its N-terminal amine and its arginine. Isolated with one trifluoroacetate on each, it would have a formula weight of 418.41 + 2 × 114.02 = 646.45, of which the peptide would be 64.7%. As a bis-acetate the figures would be 538.51 and 77.7%, and two waters of hydration would take the bis-trifluoroacetate’s peptide share down to 61.3%. That arithmetic underlies net peptide content, which our guide to counter-ions and net peptide content explains.

Worked examples: glutathione and Pinealon

Glutathione

ChEBI defines glutathione as “a tripeptide compound consisting of glutamic acid attached via its side chain to the N-terminus of cysteinylglycine” — the gamma linkage. The unusual bond changes which carboxyl group condenses, not how many atoms are lost, so the ordinary arithmetic applies: 129.115 + 103.139 + 57.052 + 18.015 = 307.321 g/mol, or 129.042593 + 103.009185 + 57.021464 + 18.010565 ≈ 307.083806 Da monoisotopic. ChEBI lists C₁₀H₁₇N₃O₆S and a monoisotopic mass of 307.08381, which matches. Its average, 307.328, is 0.007 higher than our sum: a difference of the size that comes from the choice of atomic weights (sulfur’s abridged value alone carries ±0.02), and well inside the 307.30 to 307.35 range that CIAAW’s intervals allow. Carrying extra digits through the arithmetic is good practice, but quoting a sulfur-containing peptide’s final average mass to three decimals claims more precision than the inputs support. An alpha-linked isomer would have the same formula and mass, so a correct mass cannot confirm the gamma bond.

Pinealon

Khavinson and colleagues identify Pinealon as “EDR (Glu-Asp-Arg, Pinealon)”, and PubChem lists Glu-Asp-Arg as C₁₅H₂₆N₆O₈. With free ends, its average molecular weight is 418.41 g/mol (CIAAW range 418.38 to 418.43) and its monoisotopic mass 418.1812 Da; a singly protonated ion would appear at m/z 419.1885 and a doubly protonated one at 210.0979.

Computed for this article from CIAAW and NIST values; [M+H]⁺ adds the CODATA 2022 proton mass, 1.007276.

Checking the mass on a certificate

A stated mass is worth checking in a fixed order:

1. Identify the basis. Neutral molecule or an ion such as [M+H]⁺? Average or monoisotopic? Free peptide or salt? If a document does not say, the number itself is your only clue.
2. Recompute from the sequence, adding every modification the notation shows. A residue-sum tool that ignores modifications will be out by exactly the missing shifts in the table above.
3. Compare at a sensible precision. Average masses cannot be pinned down much better than a few hundredths, because of the atomic-weight intervals. Monoisotopic masses are compared in parts per million: one published peptide quality-control method confirmed identities within a 5 ppm tolerance and saw a worst case of 2.4 ppm (Bosc-Bierne and colleagues), and at 5 ppm Pinealon’s [M+H]⁺ of 419.1885 must match within ±0.0021. Our explainer on LC-MS identity testing covers how mass accuracy is reported.
4. Decode the offset. A difference of 1.0073 points to a protonated ion and 42.011, 0.984, 2.016, 17.027 or 18.011 to a modification (monoisotopic values); a difference equal to a counter-ion’s added mass, to a salt’s formula weight quoted as a molecular weight.
5. Ask which basis and formula were used if the numbers still do not reconcile.

What’s established and what isn’t

Using masses when comparing products in Canada

For a Canadian lab comparing research reagents, these checks take a few minutes with a sequence and a spreadsheet:

• Recompute every stated molecular weight from the sequence, end groups and disulfides included. A listing whose formula or mass does not follow from its own sequence deserves a question.
• Note whether a figure is for the free peptide or a salt. A gram of a bis-trifluoroacetate contains less peptide than a gram of the bis-acetate of the same peptide, so comparisons need a common basis.
• Use average masses for anything weighed, and the monoisotopic value or the matching ion for identity checks.
• Keep the sequence, formula and both masses with your records for each lot.

We are a supplier ourselves, so apply the same checks to our listings, documents and the certificates on our Lab Testing page. If the salt form or mass basis of a product is not clear from its documentation, ask through the Contact page.

Try the arithmetic on a catalogue peptide

Pinealon and glutathione, two of the worked examples in this guide, are both in our catalogue alongside other short research peptides. Their test reports are listed on our Lab Testing page, where Pinealon’s is available on request.

PinealonGlu-Asp-Arg tripeptideView product Glutathione (GSH)Reduced glutathione tripeptideView product GHK-CuCopper tripeptideView product KPVLys-Pro-Val tripeptideView product

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