Salt-Specific Effects in Solutions of Human Serum Albumin: Small-Angle X-Ray Scattering and Osmometry

Small-angle X-ray scattering (SAXS) method and membrane osmometry were used to study salt-specific effects of various 1:1 salts at pH 4.0 and 8.0 on interparticle interactions and thermodynamic quantities in aqueous solutions of human serum albumin (HSA) at 25 °C. Our results show that Donnan pressures of HSA solutions exhibit stronger dependence on the type of anion than on the type of cation in the solution. However, the presence of different salts in the HAS solutions only weakly affects the SAXS curves in the accessible q regime (0.1 nm–1 < q < 7 nm–1). SAXS data were analyzed utilizing generalized indirect Fourier transformation, which indicated weak repulsive interactions between HSA molecules in all cases. Further, this analysis of the experimental SAXS curves strongly suggests that HSA molecules in the studied solutions have the shape of oblate ellipsoid with dimensions of approximately 8.5 × 8.5 × 4 nm. Both SAXS and membrane osmometry results can be satisfactorily explained in terms of the strength of the inter-particle interactions and conform to the trends of the Hofmeister series.