Shaking and Interfacial Denaturation

Peptides and proteins are surface-active. At an air-liquid or liquid-solid boundary they adsorb, then partially unfold so hydrophobic residues face the non-aqueous side, forming a film of misfolded molecules. On its own that film is limited by the area available. Shaking is damaging because it constantly renews the interface: every bubble created and collapsed strips the film back into the bulk as aggregation-prone material and offers a fresh surface. Damage therefore scales with interfacial turnover, not with mechanical shear.

This explains a near-universal formulation choice. Almost every biologic solution contains a small amount of a non-ionic surfactant, usually a polysorbate, whose job is to occupy the interface faster and more tenaciously than the peptide can. Stability programmes include agitation stress because it is the quickest way to expose this weakness, and the readouts are subvisible particle counts and turbidity rather than the main-peak purity a chromatogram reports. Transport and pneumatic-tube studies show the same particle rise.

Interfacial stability governs how a product can be handled from filling line to bedside, and it is why handling instructions read as they do. It also limits how much appearance conveys, because the aggregates formed this way begin far below the visible range and a solution can be substantially altered while looking untouched.

The concrete misjudgement is treating a clear vial as evidence of an intact product after rough handling; the sub-micron and low-micron species are invisible and are the ones most associated with immunogenicity risk. The second is treating research vials as robust because no handling instructions came with them, then vortexing them to speed dissolution, which is close to the worst achievable agitation.