Does Urea Alter the Collective Hydrogen-Bond Dynamics in Water? A Dielectric Relaxation Study in the Terahertz-Frequency Region

We report the ultrafast collective hydrogen-bond dynamics of water in the extended hydration layer of urea by using terahertz time-domain spectroscopy in the frequency region of 0.3-2.0THz. The complex dielectric function has been fitted using a Debye relaxation model, and the timescales obtained are in the order of approximately 9ps and 200fs for bulk water; this exhibits a considerable acceleration beyond the 4M urea concentration and indicates a possible disruption in the collective hydrogen-bonded water-network structure, which, in turn, provides an indirect support for the water structure-breaking ability of urea. With 5M urea in the presence of different concentrations of trimethylamine-N-oxide (TMAO), it was found that these parameters essentially follow the trend observed for TMAO itself, which signifies that any possible disruption of the water structure by urea is outdone by the strong hydrogen-bonding ability of TMAO, which explains its ability to revive urea-denatured proteins to their respective native states.