Terahertz absorption of lysozyme in solution.

Area of Science:

• Physical Chemistry
• Dielectric Theory
• Biophysics

Background:

• Radiation absorption by solutions is governed by frequency-dependent dielectric functions, a key aspect of dielectric theory.
• For ideal solutions, dielectric theory separates polar response into void polarization and solute dipole response.
• In protein solutions, terahertz frequency dynamics show minimal protein dipole moment fluctuations, suggesting a 'dynamically frozen' state.

Purpose of the Study:

• To investigate the interfacial polarization contributing to radiation absorption in ideal protein solutions.
• To compare theoretical models with experimental data for lysozyme solutions in the terahertz frequency domain.
• To develop an analytical theory for solution absorption based on solute volume fraction.

Main Methods:

• Application of analytical theory to model radiation absorption in ideal lysozyme solutions.
• Utilizing computer simulations, including molecular dynamics, to calculate the cavity field response function.
• Comparison of theoretical predictions with experimental measurements of solution absorption.

Main Results:

• Maxwell electrostatics inaccurately describe protein-water interface polarization.
• The 'Lorentz void' model provides a better fit to experimental data by not assuming interface polarization by external fields.
• Analytical theory for absorption slope correlates well with molecular dynamics simulations and experimental results.

Conclusions:

• Interfacial polarization, not protein dipole dynamics, dominates terahertz radiation absorption in ideal protein solutions.
• The 'Lorentz void' model offers a more accurate framework than standard Maxwell electrostatics for this phenomenon.
• The protein hydration shell acts as a distinct sub-ensemble not captured by conventional dielectric electrostatics.