Optimization of protein solution by a novel experimental design method using thermodynamic properties

In this study, the structural stability of hen egg white lysozyme in solution at various pH levels and in different types of buffers, including acetate, phosphate, histidine, and Tris, was investigated by means of differential scanning calorimetry (DSC). Reasonable pH values were selected from the buffer ranges and were analyzed statistically through design of experiment (DoE). Four factors were used to characterize the thermograms: calorimetric enthalpy (ΔH), temperature at maximum heat flux (T m ), van’t Hoff enthalpy (ΔH V ), and apparent activation energy of protein solution (Eapp). It was possible to calculate Eapp through mathematical elaboration from the Lumry-Eyring model by changing the scan rate. The transition temperature of protein solution, T m , increased when the scan rate was faster. When comparing the T m , ΔH V , ΔH, and Eapp of lysozyme in various pH ranges and buffers with different priorities, lysozyme in acetate buffer at pH 4.767 (scenario 9) to pH 4.969 (scenario 11) exhibited the highest thermodynamic stability. Through this experiment, we found a significant difference in the thermal stability of lysozyme in various pH ranges and buffers and also a new approach to investigate the physical stability of protein by DoE.