NMR studies on hydrophobic interactions in solution. Part 5. Effect of urea on the hydrophobic self-association of tert-butanol in water at different temperatures

Abstract

An NMR A₂₂ parameter study of the effect of urea on the hydrophobic self-association of tert-butanol in aqueous solutions is presented. One aim of the present paper is to contribute to the understanding of the urea effect in protein denaturation. The experimental data used for the computation of the A₂₂ parameters, namely the self-diffusion coefficients and the intermolecular ¹H relaxation rates of tert-butanol in the ternary system, together with the additionally measured self-diffusion coefficients of water, are also examined with respect to a possible structure-breaking effect of urea on the aqueous phase which has been discussed in the literature. We obtain the remarkable result that in a certain mixture range addition of urea can increase the translational mobility of the solute tert-butanol, whereas it always decreases the mobility of bulk water. Thus, we could not detect a structure-breaking effect of urea on the aqueous phase. The urea content was varied up to 12 mol% in the temperature range 10–40°C. It is found that low urea concentrations enhance the hydrophobic self-association in the water-rich region in the whole temperature range. With low tert-butanol concentrations this enhancement effect reaches a pronounced maximum. At higher urea concentrations a destabilisation of the hydrophobic interaction is observed. This latter result is in agreement with the need for relatively high urea concentrations to achieve protein denaturation. Obviously, the same basic mechanism controls the effect of urea not only on the stability of hydrophobic association of comparatively small molecules but also on the conformation of large biomolecules.