Reentrant Condensation and Counterion Distriubtion around Charged Proteins in Solution Studied by ASAXS

A detailed understanding of the ion and counterion distribution and its effect on protein-protein interactions are essential for many biological systems. We have performed normal SAXS studies on a model globular protein, bovine serum albumin (BSA) in solution as a function of protein concentration, ionic strength and also the nature of the added salt [1,2]. We recently observed a reentrant condensation phase behavior for globular proteins in the presence of multivalent cations [2], as shown in Figure 1. Negatively charged globular protein in solution undergoes a phase-separation upon adding trivalent counterions up to a critical concentration C*. Further increasing the salt concentration above a second critical value, C**, causes the precipitate to dissolve and the system turns back to a homogeneous solution [2]. Using normal/anomalous small-angle X-ray scattering (ASAXS), we have also studied the ion distribution around a globular model protein in water [3]. In particular, differences in the ion distribution for diand tri-valent ions have been studied. In this beamtime (14 May – 22 May 2009), we study the protein-protein interactions in the re-entrant regime as well as the counterion distribution around charged proteins in solution by ASAXS. In order to improve the sensitivity and resolution for ASAXS measurements, a Pilatus 300k detector was borrowed from S.V. Roth's MINAXS beamline.