Effect of urea on the structural dynamics of water.

We use polarization-resolved mid-infrared pump-probe spectroscopy to study the effect of urea on the structure and dynamics of water. Surprisingly, we find that, even at high concentrations of urea (8 M), the orientational dynamics of most water molecules are the same as in pure liquid water, showing that urea has a negligible effect on the hydrogen-bond dynamics of these molecules. However, a small fraction of the water molecules (approximately one water molecule per urea molecule) turns out to be strongly immobilized by urea, displaying orientational dynamics that are more than six times slower than in bulk water. A likely explanation is that these water molecules are tightly associated with urea, forming specific urea-water complexes. We discuss these results in light of the protein denaturing ability of aqueous urea.