Proteins are complex macromolecules with dynamic conformations and spatially heterogeneous charge distributions but also bear striking similarity to colloids. We overturn pervasively established literature doctrine that uncritically treats proteins as colloidal hard-spheres by demonstrating the complex pH dependence of protein solution viscosity and by elucidating non-trivial molecular hydration effects on protein solution hydrodynamics. We measure the infinite shear viscosity of buffered Bovine Serum Albumin (BSA) solutions at protein concentrations between 2 mg/mL and 500 mg/mL, while adjusting pH between 3.0 and 7.4 to tune the competing long-range repulsions and short-range attractions. The pH-dependent BSA intrinsic viscosity, determined using the protein-specific definition of volume fraction that corrects for molecular hydration via the pH-dependent specific volume, never equals the classical hard-sphere result (2.5) here. We attempt to fit data to the colloidal rheology models of Russel, Saville, and Schowalter (RSS) and also Krieger and Dougherty (KD), which are typically, and successfully, applied to uniformly charged and hard-sphere suspensions, respectively. The RSS model captures the concentration dependence of viscosity at pH 3.0 and 5.0, but fails at pH 6.0 and 7.4, due to the sharp rise in viscosity with concentration. The KD model, when implemented with the intrinsic viscosity fixed from dilute solution data and maximum packing fraction as the single adjustable parameter, works only at pH 6 and 7.4. We propose the starting point of theoretical models for the composition-dependent viscosity of crowded protein solutions that account for critical protein-specific attributes like conformation, surface hydration, and surface charge distribution.
Citation: Biophysical Journal
Pub Type: Journals
protein rheology, colloid rheology, protein stability, biomanufacturing