title: Protein interactions studied by SAXS: effect of ionic strength and protein concentration for BSA in aqueous solutions.
abstract: We have studied a series of samples of bovine serum albumin (BSA) solutions with protein concentration, c, ranging from 2 to 500 mg/mL and ionic strength, I, from 0 to 2 M by small-angle X-ray scattering (SAXS). The scattering intensity distribution was compared to simulations using an oblate ellipsoid form factor with radii of 17 x 42 x 42 A, combined with either a screened Coulomb, repulsive structure factor, SSC(q), or an attractive square-well structure factor, SSW(q). At pH = 7, BSA is negatively charged. At low ionic strength, I < 0.3 M, the total interaction exhibits a decrease of the repulsive interaction when compared to the salt-free solution, as the net surface charge is screened, and the data can be fitted by assuming an ellipsoid form factor and screened Coulomb interaction. At moderate ionic strength (0.3-0.5 M), the interaction is rather weak, and a hard-sphere structure factor has been used to simulate the data with a higher volume fraction. Upon further increase of the ionic strength (I >or= 1.0 M), the overall interaction potential was dominated by an additional attractive potential, and the data could be successfully fitted by an ellipsoid form factor and a square-well potential model. The fit parameters, well depth and well width, indicate that the attractive potential caused by a high salt concentration is weak and long-ranged. Although the long-range, attractive potential dominated the protein interaction, no gelation or precipitation was observed in any of the samples. This is explained by the increase of a short-range, repulsive interaction between protein molecules by forming a hydration layer with increasing salt concentration. The competition between long-range, attractive and short-range, repulsive interactions accounted for the stability of concentrated BSA solution at high ionic strength.
authors.affiliation: Institut für Angewandte Physik, Universität Tübingen, Auf der Morgenstelle 10, 72076 Tübingen, Germany.
authors.forename: Fajun
authors.initials: F
authors.lastname: Zhang
authors.forename: Maximilian W A
authors.initials: MW
authors.lastname: Skoda
authors.forename: Robert M J
authors.initials: RM
authors.lastname: Jacobs
authors.forename: Richard A
authors.initials: RA
authors.lastname: Martin
authors.forename: Christopher M
authors.initials: CM
authors.lastname: Martin
authors.forename: Frank
authors.initials: F
authors.lastname: Schreiber
mesh_terms: D000818:Animals; D001671:Biochemistry / Q000379:methods*; D002417:Cattle; D002627:Chemistry, Physical / Q000379:methods*; D006863:Hydrogen-Ion Concentration; D007477:Ions; D008956:Models, Chemical; D015233:Models, Statistical; D011485:Protein Binding; D025941:Protein Interaction Mapping; D012542:Scattering, Radiation; D012710:Serum Albumin, Bovine / Q000737:chemistry*; D013053:Spectrophotometry / Q000379:methods*; D014867:Water; D014965:X-Rays
chemical_list: D007477:Ions; D014867:Water; D012710:Serum Albumin, Bovine