Hydrogen bonding of water in aqueous solutions of trimethylamine-N-oxide and tert-butyl alcohol:: A near-infrared spectroscopy study

The hydration properties of two biologically relevant molecules, trimethylamine-N-oxide (TMAO) and tertbutyl alcohol (TBA), were investigated by monitoring the effects of these two solutes on the near-infrared (NIR) spectra of water. In particular, the 1450-nm nu(1) + nu(3) water combination band (nu(1) is the symmetric stretching and nu(3) is the asymmetric stretching) and the 1928-nm nu(2) + nu(3) band (nu(2) is the bending) were recorded at 25degreesC in aqueous solutions of TBA and TMAO over the 0-0.1 and 0-0.05 solute mole fraction intervals for TBA and TMAO, respectively. NIR data show, in agreement with molecular dynamics simulations and other suggestions found in the literature, that on the whole water molecules are more tightly coordinated by TMAO than by TBA. Furthermore, nonadditive perturbations of the water's H-bond network are observed for TBA and are absent in the TMAO case. These results are discussed in connection to the significantly different action exerted by these two solutes on typical processes governed by hydrophobic interactions, such as protein folding and micellization of a surfactant. In these processes, the data support the assumption that the presence of TMAO or TBA modifies the extent of the free-energy contribution associated with structural reorganization of water.