Protein crystallization: Contribution of small angle X-ray scattering (SAXS)

Abstract
Small Angle X-ray Scattering (SAXS) experiments together with crystallization experiments have shown the importance of studying macromolecular associations and interactions involved in nucleation and crystal growth mechanisms. In the study by SAXS of Bovine pancreatic trypsin inhibitor (BPTI) and Aspergillus flavus Urate oxidase (UOX) in crystallization conditions, we characterize the crystal growth unit, which is a decamer in the case of BPTI and a tetramer in the case of UOX. We study interactions in solution, showing that salt, added alone to small protein solutions, is efficient to induce attractive interactions leading to crystallization, whereas for large proteins, the addition of non absorbing polymers is necessary to induce attractive interactions, which are characterized by negative values of second virial coefficient (A₂).