To someone who works on measurements of both polymers and proteins with the Viscotek and Zetasizer instruments, the similarities between the two samples are very clear. Both are made from repeating monomeric units and the function of both is highly dependent on their molecular weight, size and structure. In polymers, the repeating unit is typically a single monomer or co-monomer whereas in proteins the repeating units are amino acids, of which there are about 20 that can be joined in any sequence and this, of course, is one of the primary determinants of a protein’s structure and function.
Research into proteins is continually growing with the rapidly expanding number of protein drugs, or biopharmaceuticals on the market. With the development of biopharmaceuticals, the behaviour of proteins in formulation, and in particular their stability and viscosity, is being ever more widely studied. An ideal biopharmaceutical product should have high stability giving it a long shelf life and low viscosity to make formulating higher concentrations easier and subsequent delivery less problematic.
Dynamic light scattering and size exclusion chromatography (SEC) allow us to study the hydrodynamic size and the molecular weight of particles and molecules, respectively. SEC also allows us to measure intrinsic viscosity, which is a measure of changing structure. This is a property that has been used by polymer chemists for many years but by biochemists only more recently and only in a limited number of studies. On top of these, polymer chemists also use rheology to measure the shear viscosity, and even the viscoelastic properties, of polymer solutions in different concentration regimes. Intermolecular interactions have a huge effect on solution viscosity and this is something that has been clear to polymer chemists for a long time but biochemists are only recently starting to use rheology to study the viscosity of high concentration protein formulations.
The boundary between techniques that are used to characterise polymers and proteins is becoming more and more blurred. So next time you are thinking about how to characterise and understand protein formulation behaviour, ask yourself…..what would the polymer chemist do?