Solubility and prenucleation of aprotinin (BPTI) molecules in sodium chloride solutions

In the present study we describe the first stage of the crystallization process of the bovine pancreatic trypsin inhibitor BPTI from sodium chloride solutions, with special attention to the polydispersity of the particle size distribution. First, we measured the solubility of BPTI at four NaCl concentrations (1.4 to 2.3 mol/l) and four temperatures (5 to 25°C). The solubility of the protein decreases with increasing temperature and increasing ionic strength, the effect of temperature being more pronounced at low ionic strength. Second, we investigated the behaviour of the BPTI molecules by quasi-elastic light scattering (QELS) measurements at different concentrations in protein (15 to 110 mg/ml), and ionic strengthes (0 to 2.3 mol/l NaCl). The solutions become monodisperse, but not monomeric, in the vicinity of the solubility curve. They are stable until nucleation takes place but it is impossible to decide whether these aggregates are the growth units of the crystals.