Water structure changes induced by hydrophobic and polar solutes revealed by simulations and infrared spectroscopy

A combination of simulations and Fourier transform infrared spectroscopy wa s used to examine the effect of three ionic solutes (KCI, NaCl, and KSCN), the polar solute urea, and the osmolyte trimethylamine-N-oxide (TMAO) on a water structure. The ionic solutes increase the mean water-water H-bond ang le in their first hydration shell concomitantly shifting the OH stretching mode to higher frequency, and shifting the HOH bending mode to lower freque ncy. TMAO decreases the mean water-water H-bond angle in its first hydratio n shell, shifts the OH stretching mode frequency down, and shifting the HOH bending mode frequency up. Urea has no effect on the mean H-bond angle, OH stretch, and HOH bend frequencies.' These results can be explained in term s of changes in the relative proportions of two H-bond angle populations: I onic solutes increase the population of more distorted (larger angle) H bon ds relative to the less distorted population, TMAO has the reverse effect, while urea does not affect the H-bond angle probability distribution. The n egligible effect of urea on water structure supports the direct binding mod el for urea-induced protein denaturation. (C) 2001 American Institute of Ph ysics.