The molecular weight on a peptide specification is the number an identity measurement is checked against. It looks like a single fixed value and it is not: the same molecule has at least two legitimate masses, and the form in the vial can differ from the form in the calculation.

Calculating it from the sequence

A peptide is built by joining residues and losing a molecule of water at each bond. The mass follows directly: add up the residue masses for the sequence, then add the mass of one water for the two ends that were never joined. Nothing else is required, which is why a sequence alone determines a mass.

Residue masses are tabulated, and they are not the masses of the free amino acids. A free amino acid weighs about eighteen more, because it still has the water that condensation removes. Confusing the two is the most common arithmetic error in this calculation. Our note on amino acids, peptides and proteins covers the distinction.

Two masses, both correct

Elements occur as mixtures of isotopes, and the standard atomic weights are averages over those mixtures. (Reference: Wieser, 2006) That gives two ways to add up a formula.

  • Average mass uses those weighted values. It is the number to quote when you are weighing out material, because a milligram of powder contains the natural isotope mixture.
  • Monoisotopic mass uses the lightest abundant isotope of each element: carbon-12, hydrogen-1, nitrogen-14, oxygen-16. It is the number a mass spectrometer reports for a resolved peak, because the instrument separates the isotopes instead of averaging them.

For a short peptide the two differ by a fraction of a dalton. The gap widens with size, since a larger molecule has more carbon atoms available to carry a heavier isotope, and above a few thousand daltons it exceeds a dalton. (Reference: Senko et al., 1995)

Where residue masses come from

The tabulated residue mass for each amino acid is the mass of the free amino acid minus one water, because that is what condensation leaves behind in a chain. Glycine contributes about 57, alanine about 71, tryptophan about 186. The spread across the twenty standard residues runs from roughly 57 to roughly 186, which is why sequences of the same length can differ substantially in mass.

Two consequences follow for anyone reading a spectrum. The difference between consecutive fragments in a tandem measurement equals the residue that was lost, which is how a sequence is read from a spectrum. And because leucine and isoleucine share a residue mass exactly, a mass measurement cannot separate them.

Modifications change the number

Anything attached to the chain adds its own mass. A fatty acid on an acylated peptide contributes a large increment; acetylation at the amino terminus adds about 42; forming a disulfide bond removes two hydrogens. Cyclisation removes a water, about 18, because closing the ring is another condensation.

Each of these is large enough to see, so a specification that names a modification is naming something a measurement can confirm. It also means a calculated mass taken from the bare sequence will disagree with a modified compound, which is a common source of apparent mismatches.

Why an observed mass can sit beside the expected one

A measured value that differs from the specification is not automatically a problem. Several ordinary explanations come first.

  • The two conventions. A monoisotopic measurement compared against an average calculation will differ by about the gap described above.
  • The C-terminus. A chain ending in an amide weighs about one dalton less than the same chain ending in a free acid. They are different compounds, and the linker used in synthesis decides which one you have.
  • The salt form. Peptides are usually isolated with a counterion from purification. The calculated mass of the free peptide does not include it.

A specification that states which convention it uses and which form it describes removes all three ambiguities at once.

Mass and mass-to-charge are different numbers

An instrument using electrospray does not report mass. It reports mass-to-charge ratio, and a peptide in that source picks up several protons, so one molecule produces a series of peaks at different charge states. Software combines that series into a single mass.

The series is why a spectrum with several evenly spaced peaks is usually one compound rather than several. The detail matters when reading a raw spectrum on a certificate instead of the summary figure. Our note on mass spectrometry covers the ionisation side.

What the number is doing on a certificate

Identity on a certificate of analysis is a comparison between a calculated mass and a measured one. The calculation comes from the intended sequence; the measurement comes from the batch. Agreement is evidence that the material is the intended molecule.

It is evidence and not proof. Two sequences built from the same residues in a different order weigh the same, and a D-amino acid weighs exactly what its L counterpart weighs. Mass narrows the possibilities sharply without closing them, which is why purity and identity are reported together and why a batch can be tied to its paperwork by batch number.

Resolution decides which of the two conventions an instrument can report. An instrument that separates the isotope peaks gives a monoisotopic value; one that does not gives an average-like figure for the unresolved envelope. That is why the same batch measured on two instruments can yield two numbers that are both honest.

Weight is not content

One last distinction, because it causes real confusion. Molecular weight describes one molecule. It says nothing about how much of the powder in a vial is that molecule. Counterion, absorbed water and any excipients are part of the weighed mass, which is why net peptide content is a separate figure from both the molecular weight and the chromatographic purity.

Check this against a real batch

Every compound in the catalog lists its formula, molecular weight and CAS number, and every batch we ship has a certificate you can look up by number.

Frequently asked questions

How is the molecular weight of a peptide calculated?

Add the residue masses for the sequence and add one water for the two ends that were never joined. Residue masses are about eighteen less than the corresponding free amino acids, because condensation removes a water at every bond.

What is the difference between average and monoisotopic mass?

Average mass uses the standard atomic weights, which are averages over natural isotope mixtures, and is the number for weighing material. Monoisotopic mass uses the lightest abundant isotope of each element and is what a mass spectrometer reports for a resolved peak.

Why do the two masses diverge as a peptide gets larger?

A larger molecule has more atoms that can carry a heavier isotope, so the average pulls further from the all-light value. For a short peptide the difference is a fraction of a dalton; above a few thousand daltons it exceeds one.

Why might an observed mass differ from the specification?

Usually for one of three ordinary reasons: a monoisotopic measurement compared against an average calculation, a C-terminal amide instead of a free acid, which is about a dalton lighter, or a salt form whose counterion is not in the calculated mass.

Does a mass spectrometer measure mass directly?

No. An electrospray instrument reports mass-to-charge ratio, and a peptide picks up several protons, producing a series of peaks at different charge states that software combines into one mass.

Does molecular weight tell me how much peptide is in the vial?

No. It describes one molecule. Counterion, absorbed water and any excipients are part of the weighed powder, which is why net peptide content is a separate figure from both molecular weight and chromatographic purity.

References

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