The control of protein crystallisation

The objective of the studies was to learn how to control protein crystallisation to ease the separation of crystals from the mother liquor. The significance of the work is that bulk protein crystallisation could become a cheap alternative to chromatography in obtaining high purity proteins while not compromising the product yield. Simultaneous product capture and purification are achieved using protein crystallisation and that often with yields as high as 80-90%. The process is cheap both in operational and fixed cost and is reported to be easily scaleable. Typically, the crystals produced are small (max. 100μm) and some crystal habit are fragile. Recovering the crystals can hence be difficult. Compared to for example protein precipitation, crystallisation is a slow process allowing time for manipulation during the process and can potentially be based on on-line measurements. The first part of the study focuses on the characterisation of the crystallisation process in terms of nucleation and growth rates, crystal size distribution and filterability of the final crystal suspension. The aim was to produce large crystals of a narrow size distribution with a view of improving their filterability. Different approaches to achieve this are described in the second part of the thesis. These include addition of surfactant, coating of the impeller, and control of the rate of supersaturation generation combined with seeding. All the approaches proved to have good potentials in the control of the crystal size distribution. Bulk protein crystallisation for product capture and purification has proven successful. The present method of bulk crystallisation of proteins can however be improved. For example, the seeding technique should be further developed. An understanding of the reason behind the observed high dependency of the growth rate on the degree of supersaturation could also guide the design of a better process. Finally, a better grasp of the interplay between the two observed crystals habits could lead to large reduction in the filtration time combined with a high yield.