Protein Interactions Studied by SAXS:  Effect of Ionic Strength and Protein Concentration for BSA in Aqueous Solutions

• Published in: The journal of physical chemistry. B 2007-01, Vol.111 (1), p.251-259
• Main Authors: Zhang, Fajun, Skoda, Maximilian W. A, Jacobs, Robert M. J, Martin, Richard A, Martin, Christopher M, Schreiber, Frank
• Format: Article
• Language: English
• Subjects: 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
• ISSN: 1520-6106; 1520-5207
• 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 × 42 × 42 Å, combined with either a screened Coulomb, repulsive structure factor, S SC(q), or an attractive square-well structure factor, S SW(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 ≥ 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.