Insulin nucleation kinetics in an oscillatory flow-based platform: Protein crystallization as a highly reproducible separation process

Abstract

Insulin is a human therapeutic protein that has been used to treat diabetes for almost a century. Its crystallization has been of significant interest since the protein is commonly administered by subcutaneous injections of crystalline formulations, where crystal size distribution (CSD) has a critical role in product release and injectability. Herein, insulin crystallization was investigated in a unique platform based on oscillatory flow mixing technology. Assays were carried out at different supersaturation ratios (i.e., insulin concentrations) in the presence and absence of acetone, and turbidity of the crystallization solution was monitored over time by UV-Vis spectrophotometry. The results show the impact of both supersaturation ratio and acetone on nucleation kinetics, as well as on CSD and growth rate. As the initial supersaturation increases, the nucleation rate increases, and the growth rate and mean crystal size decrease. The presence of acetone allows a faster nucleation event, a narrower CSD and a larger mean crystal size. The kinetic parameter A derived from the Classical Nucleation Theory (CNT) also indicates the kinetics of molecular attachment acceleration in the presence of acetone. These findings contribute to a better understanding of insulin crystallization processes under oscillatory flow. Thus, the described strategy and oscillatory flow-based systems are very promising for optimizing protein crystallization processes to be used during the downstream separation of bioproducts.