Abstract
A peptide's molecular weight can be calculated from its sequence and measured by mass spectrometry. This article explains monoisotopic and average mass, charge states, common mass differences, and the limits of mass-based identification.
A peptide is a defined sequence of amino acids joined by peptide bonds. Because each residue contributes a known mass, the mass of the whole molecule can be calculated before any laboratory work begins. This makes molecular weight the most direct link between a written sequence and a physical sample, and it is why mass measurement sits at the centre of peptide identity testing.
Theoretical mass from a sequence
The calculation sums the residue masses and adds the mass of a water molecule, accounting for the water lost at each bond formed during synthesis. Modifications change the result predictably: acetylation, amidation at the C-terminus, or an internal disulfide bond each shift the expected mass by a known amount. A correctly stated theoretical mass therefore already encodes the intended structure, including its modifications.
Monoisotopic versus average mass
Two different masses are commonly quoted. Monoisotopic mass uses the lightest stable isotope of each element and corresponds to the first peak of an isotope cluster in a high resolution spectrum. Average mass uses natural isotopic abundances and corresponds to what lower resolution instruments report. For small peptides the two are close; for larger molecules they diverge by several mass units. Comparing a monoisotopic observation against an average theoretical value, or vice versa, produces an apparent discrepancy that has nothing to do with the sample.
How the measurement is made
Mass spectrometry converts molecules into ions, separates those ions according to mass-to-charge ratio, and records their abundance. Electrospray ionisation is the usual technique for peptides because it transfers them into the gas phase from solution without extensive fragmentation, and because it readily produces multiply charged ions.
Multiple charging is a practical advantage: it brings large molecules into an instrument's accessible range. It also requires interpretation. A doubly charged ion appears near half the molecular mass plus the mass of the added protons, and a triply charged ion near a third. Recognising charge states is part of reading a spectrum correctly.
Interpreting a mass difference
Where an observed mass differs from the expected value, the size of the difference is often informative. Certain differences recur so consistently in peptide work that they function as diagnostic signatures.
- Around +16: oxidation, frequently at methionine
- Around −2: formation of a disulfide bond between two cysteine residues
- Around +42: acetylation, or in some cases a residual protecting group
- A residue-sized deficit: a deletion sequence from incomplete coupling during synthesis
- Around +18: hydrolysis of an amide, adding a water molecule
An unexpected mass is a finding, not a failure of the instrument. Its magnitude usually points to a specific chemical explanation.
Where mass alone is insufficient
Mass measurement has a structural blind spot: it does not read sequence order. Two peptides containing the same residues in different arrangements have identical molecular formulas and identical masses. Isomeric substitutions, such as leucine and isoleucine, are indistinguishable by intact mass alone. Where sequence confirmation is required, tandem mass spectrometry fragments the molecule and reads the resulting series of fragment masses, allowing the order of residues to be inferred.
Mass accuracy and tolerance
Reports often express accuracy in parts per million (ppm), a scale-independent measure of deviation. A high resolution instrument may achieve a few ppm, narrowing the range of formulas consistent with an observation. A lower resolution instrument reports a wider tolerance, which is adequate to confirm a known target but less able to discriminate between near-identical candidates.
Practical role in documentation
On a Certificate of Analysis, a useful mass result states the theoretical value, which mass convention it follows, the observed value, the ionisation technique, and ideally shows the spectrum. Together with a chromatographic purity determination, that establishes both what the material is and how much of the sample it represents.
Key takeaways
- A peptide's mass can be calculated from its sequence and compared with measurement
- Monoisotopic and average mass are different conventions and must be compared consistently
- Electrospray ionisation produces multiply charged ions that require charge-state interpretation
- Recurring mass differences such as +16 or −2 point to specific chemical changes
- Intact mass cannot resolve sequence order; fragmentation methods are needed for that
