Temperature dependence of the reorientational dynamics and low-frequency response of aqueous urea solutions investigated by femtosecond optical Kerr-effect spectroscopy and molecular-dynamics simulation

The time resolved optical Kerr effect (OKE) has been applied to aqueous urea (chi(urea) = 0.18) at different temperatures ranging from 283 to 333 K in order to investigate the behaviour of the different reorientational relaxation times and the low frequency OKE response of the system. The relaxation times follow the Debye-Stokes-Einstein (DSE) theory. These results reveal that at a microscopic level the reorientation of a molecule of urea is anisotropic. This anisotropy is expressed as an orientational correlation between urea molecules, as revealed by both the asymptotic decay of the N-urea-N-urea radial distribution function and by the angular distribution probability at different distances between urea molecules. It is speculated that this anisotropy plays an important role in the denaturation of proteins in aqueous urea solutions. As the molecular dynamics simulations suggest, the low-frequency OKE response is analysed in terms of the translational dynamics of the molecules in the solution. The slight shift of the position of the peak associated with the hydrogen bond interactions between water molecules suggests that urea acts as a slight structure breaker.