Influence of polydispersity on protein crystallization: a quasi-elastic light-scattering study applied to α-amylase

The early stages of the crystallization process of porcine pancreatic α-amylase were investigated by quasi-elastic light scattering. It is shown that at 288 and 293 K the diffusion coefficient does not monotonically change with increasing protein concentration but passes through a maximum at 10 mg ml⁻¹. In supersaturated solutions, prior to nucleation, the protein is strictly monodisperse. Nucleation induces the formation of aggregates and a polydispersity of, for example, 18% for an initial supersaturation C/C_e = 5.8. Monodispersity is restored after the nuclei have grown and partially consumed the solute. On the other hand, polydispersity increases up to 20% at 298 K if the protein concentration decreases to 3–4 mg ml⁻¹, values at which the solutions are under-saturated. When the protein concentration exceeds 5–6 mg ml⁻¹ the protein becomes monodisperse again. These results, confirmed by those of another system we are studying (bovine pancreatic trypsin inhibitor), are at variance with the statements that supersaturation is always at the origin of aggregation and polydispersity, and that in undersaturated solutions the diffusion coefficient should remain constant for obtaining crystals once the solutions are supersaturated.