Counteracting Effects of Trimethylamine N -Oxide against Urea in Aqueous Solutions: Insights from Theoretical Two-Dimensional Infrared Spectroscopy.

The study of small osmolytes in their aqueous solutions has gained significant attention because of their relevance to structure and thermodynamics of proteins in aqueous media. Special attention has been given to the binary and ternary aqueous solutions of urea and trimethylamine N -oxide (TMAO). Urea is a well-known protein denaturant, while TMAO protects proteins in their native states. Interestingly, TMAO counteracts urea's ability to denature proteins when present in solutions with approximately half of the concentration of urea. Vibrational spectroscopy can improve our understanding of the molecular origin of this counteracting effect because of its sensitivity to local structure and dynamics. We present results of theoretical linear vibrational and two-dimensional infrared (2DIR) spectroscopy of water in the binary and ternary aqueous solutions of TMAO and urea. The 2DIR spectra are calculated using the electronic structure/molecular dynamics approach. The non-Condon effects in spectral transitions are incorporated in the theoretical calculations of 2DIR spectra. It is found that TMAO disrupts the local structure of water, while urea leaves it essentially unaffected. The 2DIR results show that both TMAO and urea slow down the dynamics of spectral diffusion of water. The extent of slowing down is found to be particularly significant for both hydration and bulk water in the presence of TMAO which can be attributed to strong hydrogen bonds between the water and TMAO molecules. The water molecules present in the hydration layer of the solutes in the ternary solutions are found to relax at even slower rates compared to that in their binary solutions in water. The hydrogen bonds between TMAO and urea are found to be not stable. Thus, the counteracting effect of TMAO against urea is seen to take place mainly through water-mediated interactions. Such TMAO-induced effects giving rise to more structured and slower hydrogen-bonded network are successfully captured through 2DIR spectroscopic calculations.