Terahertz absorption of dilute aqueous solutions

Area of Science:

• Physical Chemistry
• Biophysics
• Spectroscopy

Background:

• Terahertz (THz) radiation absorption by aqueous solutions probes hydration shell polarization.
• At THz frequencies, solute dipolar relaxation is frozen, making interfacial water dominant in absorption changes.
• Existing dielectric theories do not fully explain observed solute-induced absorption variations.

Purpose of the Study:

• To propose and validate a model for solution dipolar response based on interfacial water's induced dipole moment.
• To investigate the THz absorption of hydrated sugars, amino acids, and proteins using this model.
• To compare experimental findings with theoretical predictions and molecular dynamics simulations.

Main Methods:

• Experimental measurement of THz absorption spectra for various solutes.
• Development of a theoretical model incorporating an interface dipole moment parameter.
• Molecular dynamics simulations of hydrated glucose and lambda repressor protein.

Main Results:

• Sugars exhibit near-zero interface dipoles; amino acids show diverse patterns (hydrophobic near-zero, hydrophilic positive).
• The model successfully describes experimental THz absorption data, contradicting standard dielectric theory predictions.
• Simulations confirm increased solution polarity with solute concentration and extended hydration layer correlations (4-5 nm).

Conclusions:

• The interface dipole moment is a key parameter for understanding THz absorption in solutions.
• Observed deviations from dielectric theory highlight the unique role of interfacial water.
• The study predicts a frequency-dependent inversion of absorption slope, offering a testable hypothesis for future research.