Mass spectrometry for proteins

Mass spectrometry answers the question no other routine method does: is this molecule what you think it is? For protein work there are three levels of analysis, and choosing the right one saves a great deal of time.

Intact mass

The whole protein is measured without digestion. Deconvoluting the charge envelope gives a mass for the intact molecule.

It is the fastest confirmation of identity, and it catches the errors that matter most: a construct that picked up a mutation, a signal peptide cleaved at an unexpected position, an unexpected modification, a truncation, and, for conjugates, the distribution of payload load.

Its limit is resolution of detail. A single deamidation on a large antibody is a small mass change against a broad glycan envelope.

Subunit analysis

The molecule is cut into a few large pieces, typically by reducing an antibody into heavy and light chains or by digesting with an enzyme that cuts in the hinge, and each piece is measured intact.

This localizes a difference to a chain or a domain and resolves modifications that are hidden in the whole molecule. For conjugates it shows where the payload sits at the subunit level.

Peptide mapping

The protein is digested into peptides, usually with trypsin, and the peptides are separated and measured. Coverage of the full sequence confirms identity residue by residue, and modifications are localized to individual sites.

This is the method for quantifying deamidation, oxidation and isomerization at specific positions, and it is what a forced degradation study needs. It is also the most work, and the sample preparation itself can introduce artifacts, notably deamidation and oxidation, so a matched control processed alongside is essential.

Practical notes

• Buffers matter. Non-volatile salts and detergents suppress signal. Plan a desalting step, or express and purify into a compatible buffer.
• Glycans broaden everything. For glycosylated proteins, an enzymatic deglycosylation step before intact analysis simplifies the spectrum when the glycan is not the question.
• Know your expected mass precisely, computed from the actual construct including tags, disulfides and any expected processing. Most "unexpected mass" results are an incorrect expectation.
• Disulfides change mass. Each one removes two hydrogens, which is enough to matter on a small domain.

Where it belongs in a campaign

Intact mass on every new construct, once, as part of build QC. Subunit or peptide mapping when a result demands it: an unexplained shift on a gel, a loss of potency on storage, a conjugate whose load has to be described, or a regulatory filing that will require full characterization.

Running intact mass early is cheap and prevents the worst outcome in a panel, which is a set of assay results collected on a molecule that was never the intended sequence.