Peptide Sequencing by Mass Spectrometry: How MS/MS Reads Residues
An intact mass measurement confirms that a peptide weighs what its sequence predicts. It cannot confirm the order of the residues. Rearrange the same amino acids, or swap two neighbours, and the total mass does not change. Peptide sequencing by mass spectrometry closes that gap. In a tandem (MS/MS) experiment the instrument isolates one ion, breaks it into fragments and measures them, and the mass differences between related fragments reveal the sequence residue by residue. For QC teams receiving the same compound across many lots, it is the method that settles a question intact mass leaves open.
From intact mass to sequence: the two-stage idea
A tandem experiment has two mass analysis steps with a fragmentation step between them:
- Stage one surveys the ions entering the instrument and selects one of them, the precursor, based on its mass-to-charge ratio.
- Fragmentation adds energy to that selected ion until its backbone breaks.
- Stage two measures the masses of the resulting fragment ions.
Because only the chosen precursor is fragmented, the second spectrum belongs to a single species even if the sample contained several. That selectivity is useful when a sample is a mixture, for instance when a related impurity elutes close to the main peak.
Choosing and isolating the precursor ion
The precursor is isolated through a narrow mass-to-charge window. Window width is a compromise. A wide window risks letting a second, similar-mass species through, which then fragments alongside the target and produces a spectrum mixing two sequences. A very narrow window clips part of the isotope pattern and weakens the signal.
Charge state matters as well. Ions carrying two or more charges generally fragment into more informative ladders than singly charged ones. Electrospray ionisation tends to produce multiply charged peptide ions, which is one reason it is the usual source for sequencing work; the comparison with MALDI is covered in MALDI versus electrospray, and turning a charge series back into a neutral mass is explained in deconvoluting multiply charged spectra.
b ions, y ions and the fragment ladder
In collision-induced dissociation, the ion collides with an inert gas and the backbone tends to break at the amide (peptide) bond. Each break gives two pieces. If the charge stays with the N-terminal piece it is called a b ion; if it stays with the C-terminal piece it is a y ion.
When breaks occur at many positions along the chain, the result is a ladder of b ions growing from the N-terminus and a ladder of y ions growing from the C-terminus. The sequence is read from the spacing between neighbouring rungs of a ladder, not from any single peak. For singly charged ions from the same cleavage, a b ion and its matching y ion add up to the neutral peptide mass plus two protons, which helps confirm assignments.
The spacings correspond to residue masses (the amino acid mass minus one water, lost when the peptide bond formed). A few monoisotopic examples:
| Residue | Monoisotopic residue mass (Da) |
|---|---|
| Glycine | 57.021 |
| Alanine | 71.037 |
| Serine | 87.032 |
| Proline | 97.053 |
| Leucine / isoleucine | 113.084 |
| Glutamine | 128.059 |
| Lysine | 128.095 |
| Phenylalanine | 147.068 |
| Arginine | 156.101 |
Sequencing depends on monoisotopic values rather than average ones, because the differences between some residues are far smaller than the gap between the two conventions. See monoisotopic and average mass values.
Minor ion series and alternative fragmentation
Collisional spectra also contain minor series (a ions on the N-terminal side, x and z types on the C-terminal side), internal fragments from chains broken in two places, and immonium ions. Immonium ions are small, residue-specific signals that indicate which amino acids are present without placing them.
Electron-based methods such as electron transfer dissociation break a different backbone bond and produce c and z ions. Their practical advantage is that fragile modifications, such as phosphate groups, tend to stay attached while the backbone breaks, so the modification can be placed on a specific residue. They also behave differently towards disulfide bonds, which makes them useful for the connectivity questions discussed in disulfide bond formation and scrambling.
Peptide sequencing by mass spectrometry: ambiguities to expect
Some limitations come from chemistry rather than from the instrument:
- Leucine and isoleucine have identical elemental composition. Standard collisional spectra cannot tell them apart; a reported assignment usually comes from the expected sequence. Specialised fragmentation approaches can sometimes separate them through side-chain losses, but these are not routine.
- D- and L-amino acids have identical masses in every fragment. Chirality needs a separate chiral method; see racemisation and chiral purity.
- Near-isobaric pairs such as glutamine and lysine (about 0.036 Da apart) or oxidised methionine and phenylalanine (about 0.033 Da apart) can only be separated when mass accuracy is good enough, the subject of mass accuracy in ppm.
Incomplete ladders and honest reporting
Complete b and y ladders are the exception. Proline strongly favours cleavage on its N-terminal side and discourages it on the C-terminal side, creating gaps next to unusually intense peaks. Other bonds may resist breaking at the energy applied.
So a good report says which junctions were directly supported by fragment ions and which were inferred from the precursor mass. If a twelve-residue peptide yields evidence at eight junctions, eight are confirmed and four are assumed. This is the same coverage discipline used in peptide mapping with protease digestion, which is often paired with MS/MS for longer chains.
Where MS/MS fits in testing bulk material
MS/MS confirms identity. It does not measure purity: it tells you the selected ion has the expected residue order, not what share of the vial that ion represents. Purity remains the job of the chromatogram, with the caveats in what HPLC area percentage measures.
For a short peptide, an intact mass plus an HPLC purity trace is normally a proportionate level of evidence, which is why MS/MS rarely appears on routine certificates. Its value rises sharply in three situations: two plausible sequences with the same mass, a modification that has to be assigned to a single residue, or a chain long enough that intact mass no longer narrows things down. A lab that has queried an unexpected mass in one lot of a large order can also use it to find out exactly where that lot differs from the others.
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