(265g) Viscosity and Short Time Dynamics of Concentrated Solutions of Proteins Interacting with a Short Range Attractive and Long Range Repulsive Interaction

Concentrated colloidal dispersions interacting with a short range attractive and long range repulsive interaction are currently of significant scientific and technological interest as they can exhibit intermediate range order as well as significant viscosities.  Such systems with competing interactions are capable of producing, among a wide array of microstructures, clustered fluid phases in which reversible, thermodynamically stable aggregates of finite size exist in equilibrium with monomers. Clustered fluids are formed by the balance of these competing potential features and are thought to be especially prevalent in concentrated protein solutions. Of particular importance in biotechnology is the effect of this fluid structure on the viscosity and diffusivity of proteins in solution. Recent studies have linked the formation of clusters in solutions of lysozyme protein and monoclonal antibodies (mAbs) to a substantial increase in solution viscosity (Biophys J. 105:720, 2013). The goal of our work is to quantify the effect of intermediate range order and cluster formation on the viscosity of model protein solutions. Here we present experimental results for lysozyme, which is selected because of its availability, stability, and globular structure, the latter enabling quantitative comparison to models. Zero shear viscosity is obtained by microrheological measurements to avoid artifacts of interfacial rheological effects. A strong divergence of zero shear viscosity is observed for volume fractions significantly below that typical for hard sphere dispersions. The fluid microstructure and protein short time-self diffusion are measured across a broad range of conditions  by small angle neutron scattering (SANS) and neutron spin echo (NSE), respectively. Previously validated models that include explicit hydrodynamic, Brownian, and interaction contributions to the viscosity fail to account for the large viscosity rise with concentration.  However, this excessive viscosity rise can be semi-quantitatively predicted when protein clustering is properly accounted for and effective cluster-cluster interactions properly included.  The implications for understanding anomalous viscosities observed in some concentrated monoclonal antibody solutions are discussed.