Computational Vibrational Spectroscopy of HDO in Osmolyte-Water Solutions.

The IR absorption and time-resolved IR spectroscopy of the OD stretch mode of HDO in water was successfully used to study osmolyte effects on water H-bonding network. Protecting osmolytes such as sorbitol and trimethylglycine (TMG) make the vibrational OD stretch band red-shifted, whereas urea affects the OD band marginally. Furthermore, we recently showed that, even though sorbitol and TMG cause a slow-down of HDO rotation in their aqueous solutions, urea does not induce any change in the rotational relaxation of HDO in aqueous urea solutions even at high concentrations. To clarify the underlying osmolyte effects on water H-bonding structure and dynamics, we performed molecular dynamics (MD) simulations of a variety of aqueous osmolyte solutions. Using the vibrational solvatochromism model for the OD stretch mode and taking into account the vibrational non-Condon and polarization effects on the OD transition dipole moment, we then calculated the IR absorption spectra and rotational anisotropy decay of the OD stretch mode of HDO for the sake of direct comparisons with our experimental results. The simulation results on the OD stretch IR absorption spectra and the rotational relaxation rate of HDO in osmolyte solutions are found to be in quantitative agreement with experimental data, which confirms the validity of the MD simulation and vibrational solvatochromism approaches. As a result, it becomes clear that the protecting osmolytes like sorbitol and TMG significantly modulate water H-bonding network structure, while urea perturbs water structure little. We anticipate that the computational approach discussed here will serve as an interpretive method with atomic-level chemical accuracy of current linear and nonlinear time-resolved IR spectroscopy of structure and dynamics of water near the surfaces of membranes and proteins under crowded environments.