Solid-liquid equilibrium for proteins in solutions with an unconventional salt (ammonium carbamate): Phase behavior analysis

In this manuscript, we report the precipitation and crystallization of three proteins (chicken egg-white lysozyme, porcine insulin and bovine insulin) using ammonium carbamate as salting-out agent. For these proteins, data on solubility, metastability limits, and osmotic second virial coefficient (B22) as a function of salt concentration at constant temperature (25.0 °C) were obtained. The value of osmotic second virial coefficient (B22) can be regarded as a selection criterion for protein crystallization, as it is related to the interaction potential between protein molecules. Negative values of B22 and large values of the salting-out constant showed that ammonium carbamate is a good precipitating agent for these proteins. Crystallization trials conducted under specific conditions showed that both insulins form amorphous precipitates in ammonium carbamate solutions, which is compatible with the large negative values of B22 measured. Conversely, lysozyme crystals were obtained under all conditions studied, and B22 values for this enzyme were within or close to the crystallization slot.