How peptides degrade, and what slows itWhat the studies show

Asparagine and, more slowly, glutamine carry an amide on their side chain that can attack the backbone next to it, forming a five-membered ring that then opens into either aspartate or its isomer, isoaspartate. The peptide gains a negative charge and often a kink, and on a chromatogram it appears as a new peak just ahead of the parent. The speed depends on the next residue: with glycine following asparagine, the half-life at 37 °C and neutral pH can be a day or two; with a bulky residue following, it can be over a year ². Robinson and Robinson measured these rates across a large set of model peptides and proposed that deamidation acts as a molecular clock, which is a way of saying that it runs whether anyone wants it to or not ². It is fastest at neutral to alkaline pH, slower in mildly acidic solution, and it needs water: in the dry solid it all but stops.

Three side chains are vulnerable to oxygen and its reactive forms: methionine, which becomes methionine sulfoxide; cysteine, which pairs into disulfides or oxidises further; and tryptophan, with histidine and tyrosine behind it ³. The oxidants are ordinary — dissolved oxygen, traces of peroxide in excipients, metal ions that catalyse radical formation — and light drives a separate route, photo-oxidation, in which tryptophan absorbs ultraviolet and passes the energy to oxygen ³. A 1995 review from Borchardt's group catalogues these mechanisms and the standard defences used in formulation: keeping oxygen and metals out, keeping light off, and keeping the peptide out of solution ³.

The amide bonds of the backbone themselves hydrolyse, slowly, and some much faster than others: the bond between aspartate and proline is the weak point, breaking in acid at rates that leave other bonds untouched, and the bonds on either side of aspartate come next ¹. Hydrolysis is driven by acid and heat and, like deamidation, cannot happen without water.

Aggregation is physical rather than chemical. Chains that have partly unfolded expose hydrophobic patches and stick to one another, forming dimers, then soluble oligomers, then visible particles. A 2003 review from Carpenter and Randolph's group describes the process as a competition between the native fold and partly unfolded states, pushed toward the latter by heat, by surfaces such as the air–water interface, by agitation and by freeze–thaw cycles ⁴. Aggregates can be reversible or not, and they are the form of damage most likely to change what a peptide does biologically rather than just how much of it is left.

Every reaction above runs faster as temperature rises, roughly doubling or tripling for each 10 °C, which is ordinary chemistry ¹. But the single variable that touches all of them is water. In a lyophilised solid the molecules are locked in a glass, cannot diffuse to find each other or an oxidant, and have no solvent in which to hydrolyse or deamidate; Lai and Topp's 1999 review of solid-state stability shows degradation rates falling by orders of magnitude from solution to dry powder, and rising again as residual moisture creeps up and the glass softens ⁵. That is why a peptide that would last days in solution at room temperature lasts months or years as a cake, why residual moisture is a release specification, and why the words cold, dark and dry on a shipping label are three different chemical statements rather than one piece of caution.