How peptides degrade: the routes that matter

The bonds holding the chain together can be cleaved by water. The result is shorter fragments, and it is the most direct form of degradation there is: the molecule is no longer the molecule.

It is strongly pH-dependent and accelerates at temperature. Certain sequences are far more susceptible than others — aspartic acid followed by proline is the classic weak point, cleaving under conditions the rest of the chain tolerates comfortably.

In analysis it shows as new earlier-eluting peaks and, by mass spectrometry, as fragments whose masses sum to the parent. That summing is the diagnostic: it tells you the chain came apart rather than something else having been added.

Asparagine and glutamine residues convert to aspartic and glutamic acid. The change is small — a gain of roughly one mass unit — and its consequences are out of proportion to its size, because it introduces a negative charge where none was.

Asparagine followed by glycine is the notorious pairing, deamidating quickly enough to be a design consideration in any sequence containing it. The reaction runs through a cyclic intermediate that can open two ways, so the products include an isomer with an altered backbone as well as the straightforward one.

It is the most common degradation route in peptides generally, and the easiest to miss: a one-unit mass shift is easily lost in a low-resolution measurement, while chromatography often separates the products cleanly because the charge change alters retention.

Methionine oxidizes readily, cysteine almost as readily, and tryptophan and histidine under harsher conditions. The mass increase is a clean sixteen units per oxygen added, which makes it one of the more recognizable signatures in mass spectrometry.

Trace metals catalyze it, dissolved oxygen supplies it, and light drives it. This is why containers are backfilled with inert gas, why amber glass exists, and why an elemental screen is more relevant to stability than it first appears — catalytic metal residue is a stability problem as much as a purity one.

Aggregation is physical rather than chemical: molecules associate with each other instead of remaining separate. No covalent bond changes, so a mass measurement of the material can look entirely normal.

It matters because aggregated material behaves differently — it may not fully dissolve, and the quantity actually available in a measurement is no longer what was weighed. It is also self-accelerating, since aggregates seed further aggregation.

Chromatography can miss it, because aggregates may not elute at all. Size-based methods are the ones that find it, which is why an unremarkable purity figure does not exclude it.

Peptides with more than one cysteine can form disulfide bonds in more than one arrangement. The correct arrangement is one of several possible, and under some conditions the bonds can rearrange into an incorrect pairing.

The scrambled product has exactly the same mass as the correct one, which makes mass spectrometry blind to it. Only methods sensitive to structure — chromatographic retention, or fragmentation of the intact molecule — distinguish them. It is the degradation route most likely to pass an ordinary certificate unnoticed.

Each route leaves a different mark, so degradation is legible rather than merely present. A mass sixteen units high suggests oxidation; one unit high suggests deamidation; fragments summing to the parent suggest hydrolysis; identical mass with shifted retention suggests scrambling.

It also explains why the storage conditions on a label are what they are. Cold slows every route listed here. Dry removes the reagent for two of them. Dark removes the driver for a third. Sealed under inert gas removes the oxygen. The instructions are not arbitrary caution — each one shuts down a specific reaction.