SOLUBILITY AND PRENUCLEATION OF APROTININ (BPTI) MOLECULES IN SODIUM-CHLORIDE SOLUTIONS

In the present study we describe the first stage of the crystallizatio n process of the bovine pancreatic trypsin inhibitor BPTI from sodium chloride solutions, with special attention to the polydispersity of th e particle size distribution. First, we measured the solubility of BPT I at four NaCl concentrations (1.4 to 2.3 mol/l) and four temperatures (5 to 25 degrees C). The solubility of the protein decreases with inc reasing temperature and increasing ionic strength, the effect of tempe rature being more pronounced at low ionic strength. Second, we investi gated the behaviour of the BPTI molecules by quasi-elastic light scatt ering (QELS) measurements at different concentrations in protein (15 t o 110 mg/ml), and ionic strengthes (0 to 2.3 mol/l NaCl). The solution s become monodisperse, but not monomeric, in the vicinity of the solub ility curve. They are stable until nucleation takes place but it is im possible to decide whether these aggregates are the growth units of th e crystals.