An X-ray and neutron scattering study of the equilibrium between trimethylamine N-oxide and urea in aqueous solution

Abstract

The interaction of the osmolytes trimethylamine N-oxide (TMAO) and urea in aqueous solutions at 40 °C was investigated by isotopic substitution neutron scattering at a TMAO mole fraction of 0.05 and TMAO/urea concentration ratios of 1 : 2 and 1 : 4. The partial pair distribution functions obtained by the empirical potential structure refinement method are consistent with those obtained previously for similar pure TMAO and 1 : 1 TMAO–urea solutions and indicate that urea progressively replaces the water molecules in the first coordination shell of the TMAO oxygen atom. The apparent association constant for the TMAO : urea complex (K₁) was calculated to be 0.14 M⁻¹, which is of the same order as the experimental urea–protein binding constants per site reported in the literature. This confirms that the two osmolytes act independently at least in the physiological range.