Analytical Techniques for Evaluating Protein Instability at Interfaces

Proteins at interfaces present a number of analytical challenges, particularly in the context of biotherapeutic formulations. Proteins typically adsorb to interfaces in thin films or monolayers while being present at high concentrations in the bulk liquid, making sensitivity to behavior directly at the interface an important feature. Additionally, there are multiple timescales of interest, from milliseconds to days, meaning the analytical techniques for probing interfacial behavior need to be adaptable with respect to time resolution. Also, biotherapeutic formulations are usually multicomponent systems, so the techniques need to distinguish between the behavior of the therapeutic protein and other surface-active excipients included to stabilize the protein (e.g., polysorbates and poloxamers). Finally, proteins form amorphous films at interfaces that make traditional interfacial techniques, usually applied to lipids and other well-defined surfactants, less sensitive to structural changes in protein films.