Comparison of solubilities and molecular interactions of BPTI molecules giving different polymorphs

Abstract

The solubility of the monoclinic modification of BPTI was determined at different temperatures in solutions at three KSCN concentrations (250, 300 and 350 mM) and five (NH4)2SO4 concentrations (1.4, 1.5, 1.6, 1.75 and 1.85 M). In both cases, the solubility decreases with increasing ionic strength. Moreover, in KSCN solutions it increases with increasing temperature whereas it decreases in (NH4)2SO4 solutions. Quasi-elastic light scattering and small-angle X-ray scattering experiments were carried out at constant protein concentration (15 mg ml−1) for KSCN and (NH4)2SO4 concentrations ranging from 0 to 350 mM and from 0 to 1.85 M, respectively. They showed that the solutions are monodispersed, but not monomeric, in the vicinity of the solubility curve, whereas they are polydispersed at lower concentration. Under crystallization conditions, the molecular interactions change from repulsive to attractive once the KSCN concentration was higher or equal to 250 mM. In KSCN, (NH4)2SO4 and also in NaCl solutions the scatterers have the same size (Rh ≈ 23 Å) suggesting that they are BPTI tetramers. As a consequence, it was not possible to predict which polymorph was going to nucleate and grow on the basis of the scatterers size since in NaCl solutions a hexagonal modification occurs instead of the monoclinic modification which occurs in the two other solutions.