How peptides degrade: oxidation, deamidation, hydrolysis and aggregation

Methionine and cysteine are the primary targets, with tryptophan, tyrosine and histidine susceptible under harsher conditions. Methionine oxidises to the sulfoxide, adding about 16 daltons; a second oxidation to the sulfone adds another 16.

Cysteine has a richer set of outcomes. Free thiols oxidise to sulfenic and higher acids, and two cysteines can form a disulfide — intramolecular if the geometry allows, intermolecular if it does not, the latter producing covalent dimers that appear as a species at roughly twice the expected mass.

Oxidation is driven by dissolved and headspace oxygen, accelerated by trace transition metals and by light. Inert headspace, intact seals, chelators where appropriate, and minimal solution-state time are the practical controls. On an assay, look for the +16 pattern and, for cysteine-containing sequences, for dimer species.

Asparagine and, more slowly, glutamine convert their side-chain amide to a carboxylic acid, adding roughly 1 dalton and introducing a negative charge. The reaction proceeds through a cyclic succinimide intermediate, which is why the sequence context matters so much.

Asparagine followed by glycine is the classic labile motif: the small, flexible glycine allows the backbone to reach the conformation the intermediate requires. Asn-Ser and Asn-His are also elevated. The same asparagine in a sterically crowded context can be orders of magnitude more stable.

The succinimide intermediate opens to a mixture of the normal aspartate linkage and an isoaspartate linkage in which the backbone runs through the side chain. Isoaspartate has the same mass as aspartate but different chromatographic behaviour, so it shows up as peak splitting rather than a mass shift. Deamidation is strongly pH-dependent, accelerating markedly above neutral, and is essentially arrested in the dry state.

Backbone amide bonds hydrolyse under acidic or basic conditions, cleaving the peptide into fragments and producing masses corresponding to sub-sequences. The aspartate-proline bond is notably labile under acidic conditions, and Asp-Gly is also elevated.

This is the pathway most directly suppressed by lyophilisation: no water, no hydrolysis. It is also the reason strongly acidic or basic reconstitution solvents should be used at the minimum effective strength and not left standing longer than necessary.

On an assay, hydrolysis presents as multiple lower-mass species whose masses sum, with corresponding early-eluting peaks in the chromatogram.

Aggregation is physical rather than covalent, and it is the pathway most likely to be missed because the molecules are individually intact. The peptide leaves solution as ordered assemblies, typically beta-sheet, and once formed these are often practically irreversible.

Drivers are concentration, ionic strength, temperature, agitation and air-water interface exposure. This is the mechanism behind the advice not to vortex: foam is a large interfacial area, and interfaces nucleate aggregation efficiently.

Because aggregated material is frequently insoluble, it does not appear in an HPLC injection at all — it is removed during sample preparation or trapped on the column frit. The result is a chromatogram that looks clean while the solution concentration has quietly dropped. Visible cloudiness, unexpected loss of activity at unchanged nominal concentration, and irreproducibility between aliquots all point at aggregation rather than chemistry.

All four pathways slow in the dry state and all four accelerate in solution, which is the entire justification for shipping and storing lyophilised material and reconstituting immediately before use.

Where a solution must be kept, the controls follow from the mechanisms: aliquot to avoid repeated interface and freeze-thaw stress, keep pH near neutral to limit deamidation and hydrolysis, exclude oxygen and light for sequences carrying methionine, cysteine or tryptophan, and avoid concentrations near the solubility limit where aggregation nucleates.