Protein–solvent interaction in urea–water systems studied by dielectric spectroscopy

Dielectric spectroscopy measurements were performed at 298 K for bovine serum albumin (BSA) dissolved in aqueous solutions of urea (0–9  M with steps of 1 M). It was shown that unfolding of BSA molecules by urea influenced the dynamic structures of water and urea molecules, and consequently changed their dielectric responses. At low urea concentrations (0–5 M), unfolding of BSA molecules is not very significant and mainly the globular surface of the BSA interacts with the solvent molecules. In comparison with urea solutions without BSA, the high frequency dielectric spectrum (γ-dispersion) was significantly broadened with addition of the BSA, although the characteristic frequency was not affected much. Such behavior is similar to that reported for aqueous solutions of synthetic polyelectrolytes. At high urea concentrations (6–9 M), significant unfolding of the BSA molecules occurred, exposing the hydrophobic side chains to the solvent and likely increasing the effects of hydrophobic hydration. As a result, the dielectric response of the solvent molecules was found to be more complicated.