What LC-MS can establish about peptide identity

Leucine and isoleucine provide a concrete example: they are isomeric residues whose distinction requires information beyond their mass. An intact-mass match therefore does not, by itself, establish which occupies a position in the chain.

A 2022 study compared charge-transfer fragmentation with collision-induced dissociation in model peptides. The ability to distinguish the residues depended on informative fragments, not simply the name of the spectrometer or precursor mass. Study.

Work on bradykinin variants and peptides from a mussel adhesion protein studied hydroxyproline isomers and isobars. The authors sought fragmentation pathways that would separate explanations left open by residue mass.

The contribution was a specific experimental discrimination. It does not mean that every MS/MS record automatically contains the same diagnostic fragments. The question is whether the spectrum obtained provides the signals needed to distinguish the candidate structures in the case examined. Study.

A 2017 investigation examined ambiguous cases of leucine–isoleucine discrimination. Alongside comparison of useful signals, it explained one signal through an alternative fragmentation pathway. This matters because assigning a fragment solely by its position can lead to a premature interpretation.

Identity evidence must consider how the signal could have been generated, not only whether it matches an expected mass difference. The study used specific dissociation modes; its conclusions are not a guarantee for every acquisition configuration. Study.

A useful conclusion distinguishes compatible mass, sequence coverage and discrimination of a specific variant. If the question was to distinguish two candidates sharing a mass, the report should identify the observation that ruled one out.

For example, the wording ‘mass compatible with the candidate’ preserves an uncertainty that ‘complete identity confirmed’ conceals in words. Changing the phrase does not change the experiment. The broader conclusion requires additional evidence, particularly when the relevant alternative is an isomer.

The three studies do not propose one evidence level for every sample. They present particular structural problems and ways to investigate them. A method may be suitable for recognizing a material among very different candidates yet insufficient for closely related variants.

Scope is therefore better expressed as a relationship between question and evidence: which structures were considered, which signals were observed and which remained compatible. This approach makes use of LC-MS without attributing unlimited identification capability to it.