Temperature and pH Effect on the Polymorphism of Aprotinin (BPTI) in Sodium Bromide Solutions

In this contribution, we present the phase transition of protein crystals in suspension as a function of temperature and pH. We measure the solubility of two polymorphs of aprotinin (BPTI) in 2 M NaBr solutions at different acidic pH values between 4.50 and 5.13 at 20 °C. We observe a major increase in solubility when the pH is increased from 4.5 to 5.13. The temperature effect between 5 and 35 °C on the solubility of the two BPTI polymorphs in 2 M NaBr solutions is investigated at pH 4.5 and 4.75. We observe two polymorphs in the same solution with direct and reverse solubility curves. We show that an adequate control of temperature around the transition temperature between the two polymorphs can be used to obtain the polymorph desired.