01A total can match while its arrangement differs

Consider the hypothetical unmodified linear sequences AGK and GAK with identical terminal chemistry. They contain the same residues, so rearranging their order does not change the intact molecular mass. A more precise measurement of that same total cannot recover the missing order.

This example concerns sequence arrangement, not the specialised question of distinguishing leucine from isoleucine. It shows why the ambiguity should be written down: different alternatives need different discriminating evidence.

A fragment that is identical for both candidates adds support for their shared region, but cannot choose between them. The most useful observation is one whose expected result differs between the alternatives.

02A complex-peptide example combines both levels

Gucinski and Boyne used high-resolution mass analysis with electron-transfer dissociation to characterise herring protamine sulfate. Their 2014 report combined accurate masses with fragment patterns to assign the peptide species.Gucinski and Boyne — Site-specific heterogeneity in peptide drugs by ETD-MS/MS (opens in a new tab)

The approach gave complete sequence coverage for three abundant peptides, identified a discrepancy with published sequences and characterised three lower-abundance related species. These findings concerned the tested protamine mixture, not a generic guarantee for all peptides.Gucinski and Boyne — Site-specific heterogeneity in peptide drugs by ETD-MS/MS (opens in a new tab)

The example illustrates why a cluster of intact masses and a set of sequence assignments are different deliverables. The second requires evidence explaining the composition of the first.

When a report says “confirmed by MS”, ask which level was actually performed. The phrase may refer to a mass match, a fragment comparison or a more extensive structural analysis.

03Local evidence can explain a global discrepancy

Wen and colleagues investigated unexpected serine incorporation at asparagine positions in recombinant proteins produced in CHO cells. Their 2009 study used intact mass measurements, peptide mapping and tandem-MS sequencing together to identify the substitutions.Wen and colleagues — Serine misincorporation at asparagine positions (opens in a new tab)

That case illustrates a useful sequence of questions: detect an overall discrepancy, locate candidate regions and examine evidence for a local assignment. The methods are complementary because they address different scales of the problem.

The interpretation still depends on which material was analysed. A digest fragment identifies a region within the mapped sample; it does not by itself resolve how every modification is combined on each intact molecule in a heterogeneous preparation.

04Specify what would distinguish the plausible alternatives

A strong evidence request names the competing identities and asks which observed fragments distinguish them. It also asks where the data are uninformative, rather than reporting only the percentage of a sequence coloured on a diagram.

If the alternatives remain indistinguishable by the chosen experiment, retain both possibilities or seek another discriminating method. Repeated confirmation of shared features does not resolve the feature on which they differ.

This gives an honest stopping rule: the evidence supports the identity at the level actually resolved. More detailed wording requires more detailed evidence, not simply a more confident interpretation of the same intact mass.