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The amphiphilic osmolyte trimethylamine-N-oxide (TMAO) is commonly found in natural organisms, where it counteracts biochemical stress associated with urea in aqueous environments. Despite the important role of TMAO as osmoprotectant, the mechanism behind TMAO's action has remained elusive. Here, we study the interaction between urea, TMAO, and water in solution using broadband (100 MHz-1.6 THz) dielectric spectroscopy. We find that the previously reported tight hydrogen bonds between 3 water molecules and the hydrophilic amine oxide group of TMAO, remain intact at all investigated concentrations of urea, showing that no significant hydrogen bonding occurs between the two co-solutes. Despite the absence of direct TMAO-urea interactions, the solute reorientation times of urea and TMAO show an anomalous nonlinear increase with concentration, for ternary mixtures containing equal amounts of TMAO and urea. The nonlinear increase of the reorientation correlates with changes in the viscosity, showing that the combination of TMAO and urea cooperatively enhances the hydrogen-bond structure of the ternary solutions. This nonlinear increase is indicative of water mediated interaction between the two solutes and is not observed if urea is combined with other amphiphilic solutes. [ABSTRACT FROM AUTHOR]

An understanding of the influence of hydrophilic and hydrophobic interactions on the dynamics of solvating water molecules is important in a diverse range of phenomena. The polarisability anisotropy relaxation dynamics of aqueous solutions of the amphiphiles TBA (t-butyl alcohol) and TMAO (trimethylamine N-oxide) have been measured as a function of concentration and temperature. TMAO is shown to have a greater effect on the picosecond relaxation dynamics of water than TBA. This result is consistent with hydrophilic interactions being mainly responsible for the slowing down the polarisability relaxation in aqueous solutions. The room temperature Raman spectral densities of the two solutions are remarkably similar to that of bulk water, an effect which is tentatively ascribed to the formation of nanoscale structure in the solutions, allowing the formation of bulk-like water pools. The temperature dependent spectral density of TMAO remains similar to that of bulk water at all temperatures, while that for TBA shows a marked decrease in the amplitude of the response usually ascribed to a water-water stretch with increasing temperature. This is discussed in terms of the temperature dependent structure of TBA aggregates in solution., (This journal is © the Owner Societies 2012)

Acetic acid is capable of forming strong multiple hydrogen bonds and therefore different dimeric H-bonded structures in neat liquid phase and in solutions. The low frequency Raman spectra of acetic acid (neat, in aqueous solution and as a function of temperature) were obtained by ultrafast time and polarization resolved optical Kerr effect (OKE) measurements. Isotropic OKE measurements clearly reveal a specific totally symmetric mode related to the dimeric structure H-bond stretching mode. The effects of isotope substitution, water dilution and temperature on this mode were investigated. These results together with anisotropic OKE measurements and density functional theory calculations for a number of possible dimers provide strong evidence for the cyclic dimer structure being the main structure in liquid phase persisting down to acetic acid concentrations of 10 M. Some information about the dimer structure and concentration dependence was inferred., (This journal is © the Owner Societies 2011)