Petra Zalar, Matija Tomšič, Andrej Jamnik, Jurij Reščič *

University of Ljubljana, Faculty of Chemistry and Chemical Technology, Aškerčeva 5, SI-1000 Ljubljana, Slovenia.

Absrtract
Osmotic pressure measurements of human serum albumin (HSA) dissolved in 0.15M NaCl and in 0.1 M phosphate buffer at three values of pH are reported as a function of protein concentration. Experimentally obtained osmotic coefficients were well below one, indicating large deviations from ideality even for dilute protein solutions. This effect could be ascribed to formation of dimers or even higher aggregates in protein solution. The membrane equilibria measurements of osmotic pressures were therefore complemented by the small-angle X-ray scattering (SAXS) results that show no evidence of protein aggregates in solutions, leaving the question why is the osmotic coefficient so small unanswered.

Key words: human serum albumin, osmotic coefficient, small-angle X-ray scattering, indirect Fourier transformation