Osmotic pressure, small-angle X-ray, and dynamic light scattering studies of human serum albumin in aqueous solutions

Osmotic pressure measurements of human serum albumin (HSA) dissolved in wat er and in 0.01, 0.1, and 1.0 M phosphate buffer are reported as a function of the protein concentration, Two different forms of the protein were studi ed: defatted HSA (HSA1) and HSA with fatty acids (HSA2). The measured value s of the osmotic coefficient were well below 1, indicating large deviations from ideality even for dilute protein solutions. The measured values incre ased with increasing HSA concentration and the increase was a function of p H. For higher concentrations of added phosphate buffer, the pH of solution had less influence on the measured osmotic pressure. The osmotic pressure o f HSA1 in water was found to be considerably lower than that of the HSA2 mo dification. This effect was ascribed to formation of dimers in the HSA1 sol ution. The osmotic measurements were complemented by the small-angle X-ray scattering (SAXS) and dynamic light scattering (DLS) studies of dilute HSA solutions in water. The SAXS and DLS data confirmed the dimerization of HSA 1 molecules under these conditions. Detailed analysis of the SAXS data sugg ested a parallel orientation of two protein molecules in a dimer, (C) 2001 Academic Press.