Multi-component protein precipitation modelling based on the phase equilibrium evaluated by statistical tests

The mechanism of multi-component protein precipitation is very complicated and not well understood. In this paper, a new model describing how the protein solubility changes in the protein precipitation and based on the phase equilibrium of the light liquid phase and dense solid phase is proposed to aid the preparation of modern therapeutic compounds. This non-linear model is derived from the thermodynamic phase equilibrium and involves eight parameters to account for the impact of salt concentration and pH on precipitation. Two feedstocks of a pure fragment antibody (Fab’) solution obtained by chromatographic purification and a clarified Fab’ homogenate solution from E. coli were used to examine the effect of ammonium sulphate concentrations and pH conditions on precipitation. It was found that the model can describe pure Fab’ precipitation well, and identify the non-ideal behaviour of Fab’ precipitation in multi-component homogenate. The structure of the model was proven to be robust, and the quality of the model is such that errors were within the acceptable statistical confidence limits, even when applied to multi-component impurity precipitation. The new model structure is generic and reflects the non-linearity of the protein precipitation better than existing empirical models. The model can be applied directly to industrial processes for protein precipitation process design, model-based optimisation and control after appropriate calibration with the operating conditions of pH and salt concentration