Degradation, oxidation and aggregation: how they show up in the data

When a peptide changes, it rarely disappears. It turns into something closely related: a slightly heavier version, a slightly different charge, a fragment, or a clump. Each of those leaves a fingerprint in the analytical data, and knowing the patterns makes a chromatogram much easier to read.

The main pathways

Pathway	What happens	Residues involved	Mass change
Oxidation	Oxygen adds to a side chain	Methionine (to sulfoxide), tryptophan, cysteine	+16 Da per oxygen
Deamidation	An amide side chain becomes an acid	Asparagine, less often glutamine	about +1 Da
Pyroglutamate formation	N-terminal glutamine cyclizes	N-terminal Gln	about -17 Da
Hydrolysis	A peptide bond breaks	Often next to aspartic acid	Two smaller fragments
Disulfide changes	Bonds form, break or swap partners	Cysteine	Varies; dimers double the mass
Aggregation	Molecules stick together	Hydrophobic stretches, many sequences	Multiples of the mass, or non-covalent clumps

Where each one shows up on reverse-phase HPLC

Oxidation. A methionine sulfoxide is more polar than methionine, so the oxidized form usually elutes a little earlier than the main peak. It often appears as a small peak just ahead of it, growing over time if the material is degrading.

Deamidation. Adds a negative charge and can produce two products (aspartate and isoaspartate). These often show up as small peaks very close to the main peak, sometimes as shoulders on it.

Hydrolysis. Fragments are smaller and usually elute at noticeably different times from the main peak, often earlier.

Disulfide changes and dimers. Usually shift retention time; dimers are often more hydrophobic and come off later.

Aggregation. The awkward one. Large aggregates may not elute normally from a reverse-phase column, may stick to it, or may come apart in the acidic, organic mobile phase. So reverse-phase HPLC can under-report aggregation. Size-exclusion chromatography (SEC), which separates by size, is the usual tool for it.

Confirming with mass spectrometry

A new peak on HPLC says something changed. LC-MS says what. A peak 16 Da heavier than the main species points to oxidation. A peak about 1 Da heavier, close to the main peak, points to deamidation. A peak at twice the mass points to a dimer.

Comparing a lot over time

The most useful comparison is the same lot, same method, at two points in time:

1. Main-peak area percent: has it dropped?
2. Known impurity peaks: have they grown?
3. New peaks: where are they, and do they match a known pathway by retention time and mass?

A single chromatogram can show impurities. Only a comparison shows degradation.

What drives these reactions in dry material

Moisture, heat, light and oxygen, roughly in that order of importance for lyophilized material. That is the practical reason behind dry, cold, dark and sealed storage.