Hydrophobic solvation: a 2D IR spectroscopic inquest

Area of Science:

• Physical Chemistry
• Chemical Physics
• Biophysical Chemistry

Background:

• The hydrophobic effect is crucial for protein structure and function.
• Previous theories, like Frank and Evans'
• iceberg
• model, proposed specific water structuring around hydrophobic solutes.
• Understanding hydrophobic interactions requires detailed insights into water dynamics.

Purpose of the Study:

• To investigate the dynamics of water molecules near hydrophobic groups in concentrated solutions.
• To elucidate the role of hydrophobic solutes on hydrogen-bond network dynamics.
• To compare experimental spectroscopic data with molecular dynamics simulations for a microscopic understanding.

Main Methods:

• Utilized two-dimensional infrared (2D IR) spectroscopy to probe the OH-stretch vibrational mode of water.
• Employed molecular dynamics (MD) simulations to model water/tetramethylurea (TMU) mixtures.
• Analyzed time correlations of vibrational frequencies on femtosecond timescales.

Main Results:

• Observed two distinct hydrogen-bond dynamics: fast (approx. 100 fs) water reorientations and slow (>1 ps) translational motions.
• Found that hydrophobic solutes (TMU) significantly suppress water translational mobility.
• Demonstrated that molecular-jump reorientations are inhibited near hydrophobic groups due to restricted access of a fifth water molecule.

Conclusions:

• Water molecules in the hydrophobic solvation shell exhibit altered hydrogen-bond dynamics compared to bulk water.
• The suppressed dynamics suggest that water reorientation is hindered near hydrophobic groups.
• The study highlights the importance of considering hydrogen-bond dynamics for defining hydrophobic interactions.