Tandem Mass Spectrometry: Reading a Peptide Sequence From Fragment Ions

Nobody reads a sequence off an individual peak in a tandem spectrum. The sequence lives in the spaces between peaks. Break a peptide apart inside the instrument, weigh the fragments, and because those fragments all share one backbone, the arithmetic difference between neighbors names a residue. Weighing the intact molecule tells you only what it weighs; weighing the pieces tells you the order.

Gaps, not peaks

Take two consecutive members of a fragment series and subtract. A difference of 71.037 marks alanine. Land on 128.059 and the residue is glutamine, while 128.095 means lysine instead; 113.084 leaves you with either leucine or isoleucine. Walk the series and the chain spells itself out residue by residue.

These figures are residue masses, not the masses of free amino acids. Forming a peptide bond expels water, so a residue weighs the amino acid minus water. Monoisotopic and average values must also be kept strictly apart here. Telling glutamine from lysine demands the kind of resolution that only monoisotopic masses can supply, a point developed in monoisotopic versus average mass.

Why there is a ladder at all

Energy enters the ion through collision with an inert gas, and the backbone gives way preferentially at the amide bond. Two fragments result from each break, and whichever one holds onto the charge is the one the detector sees.

Break at every amide bond in turn and each type forms a complete series. Masses in the b series climb one residue at a time starting from the N-terminus; the y series climbs from the C-terminus. They complement one another, and the check is simple arithmetic: any b ion added to its partner y ion equals the precursor mass plus a proton and a molecule of water.

Isolating one molecule from a crowd

Why bother with a second stage? Because two peptides assembled from an identical set of residues weigh exactly the same, and no single-stage measurement can tell them apart. The second stage breaks that tie.

It works by pulling one ion out of the stream entering the instrument, fragmenting that ion alone, and recording what the pieces weigh. Since nothing else was fragmented, every signal in the resulting spectrum traces back to a single species, which is also the reason the technique copes with mixtures that a single-stage spectrum would render as unreadable overlap.

Selection is by mass, across a narrow window that is frequently one to three mass units wide, and the width is a genuine tradeoff. Widen it and a co-eluting species of comparable mass gets fragmented along with the target, blending two sequences into one spectrum. Narrow it and the isotope envelope is clipped, which costs signal. Charge state counts as well: ions carrying multiple charges fragment far more informatively than singly charged ones, which is part of why electrospray usually serves as the front end. The two ionization approaches are compared in MALDI versus electrospray for peptides, and the arithmetic that turns a multiply charged series back into a neutral mass is worked through in deconvoluting multiply charged spectra.

The rest of the alphabet

Collisional fragmentation is not tidy. Smaller quantities of a and c ions come off the N-terminal side, x and z ions off the C-terminal side. Chains that happen to break in two places give internal fragments. Immonium ions report which residues are present while saying nothing about where they sit.

Some methods change the pattern on purpose. Electron-based dissociation attacks a different backbone bond altogether, delivering c and z ions in place of b and y, and it manages this without dislodging labile modifications. A phosphate or a glycan that a collisional experiment would strip away before the backbone ever broke stays attached and can be localized. Disulfide bonds likewise survive intact, or are cleaved selectively, which explains the method turning up in connectivity work of the sort described in disulfide formation and scrambling.

Ambiguities no instrument setting resolves

Three limits are structural rather than a matter of equipment. Leucine and isoleucine are isomers, so ordinary fragmentation cannot separate them at all; when a reported sequence distinguishes the two, that distinction came from the expected sequence and not from the spectrum. D and L residues are likewise identical in mass at every stage of the experiment, which means chirality requires a wholly different method, described in racemization and chiral purity. Glutamine and lysine sit only 0.036 apart, and glutamate alongside an oxidized residue can raise comparable ambiguities; both are resolvable, but only where mass accuracy is good enough, which is the topic of mass accuracy in parts per million.

Missing rungs

Complete ion series are the exception. Proline suppresses cleavage on one side while enhancing it on the other, which produces both gaps and peaks of unusual intensity. Certain bonds just will not break at the energy applied. Recover eight of a possible twelve y ions and you have confirmed eight junctions, with the remaining four inferred through mass balance.

Report it the way a digest map should be reported, and for identical reasons, as covered in peptide mapping by protease digestion. Say which segments the fragment ions actually covered, and say which ones were assigned by difference rather than observed directly.

Identity is not purity

Sequencing by MS/MS settles what a molecule is. It does not measure how much of the vial that molecule accounts for. The statement it supports is that the selected species carries the expected residue order, and nothing beyond that; proportion remains the province of the chromatogram and the arithmetic explained in what area percentage measures.

Routine peptide certificates seldom include it, for the same proportionality reason digest mapping is usually absent. Intact mass plus a purity trace is the appropriate evidential level for a short chain. MS/MS becomes decisive in three situations: when two candidate sequences share a mass, when a modification has to be pinned to a particular residue, or when the chain is long enough that intact mass stops constraining the answer.