Protein interactions studied by SAXS: effect of ionic strength and protein concentration for BSA in aqueous solutions

J Phys Chem B. 2007 Jan 11;111(1):251-9. doi: 10.1021/jp0649955.


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.

Publication types

  • Research Support, Non-U.S. Gov't

MeSH terms

  • Animals
  • Biochemistry / methods*
  • Cattle
  • Chemistry, Physical / methods*
  • Hydrogen-Ion Concentration
  • Ions
  • Models, Chemical
  • Models, Statistical
  • Protein Binding
  • Protein Interaction Mapping*
  • Scattering, Radiation
  • Serum Albumin, Bovine / chemistry*
  • Spectrophotometry / methods*
  • Water
  • X-Rays


  • Ions
  • Water
  • Serum Albumin, Bovine