Relaxation dynamics of a protein solution investigated by dielectric spectroscopy

TL;DR

This study investigates the dielectric relaxation processes in lysozyme protein solutions across various frequencies and temperatures, revealing insights into molecular motions, hydration effects, and deviations from classical diffusion models.

Contribution

It provides detailed temperature-dependent dielectric data on three relaxation processes in protein solutions, highlighting the connection between hydration shell dynamics and ionic conductivity.

Findings

Beta- and gamma-relaxations are temperature-dependent and correlated.

Significant changes in protein dipole moments indicate conformational shifts.

Breakdown of the Debye-Stokes-Einstein relation suggests hydration shell influence on conductivity.

Abstract

In the present work, we provide a dielectric study on two differently concentrated aqueous lysozyme solutions in the frequency range from 1 MHz to 40 GHz and for temperatures from 275 to 330 K. We analyze the three dispersion regions, commonly found in protein solutions, usually termed beta-, gamma-, and delta-relaxation. The beta-relaxation, occurring in the frequency range around 10 MHz and the gamma-relaxation around 20 GHz (at room temperature) can be attributed to the rotation of the polar protein molecules in their aqueous medium and the reorientational motion of the free water molecules, respectively. The nature of the delta-relaxation, which often is ascribed to the motion of bound water molecules, is not yet fully understood. Here we provide data on the temperature dependence of the relaxation times and relaxation strengths of all three detected processes and on the dc conductivity arising from ionic charge transport. The temperature dependences of the beta- and gamma-relaxations are closely correlated. We found a significant temperature dependence of the dipole moment of the protein, indicating conformational changes. Moreover we find a breakdown of the Debye-Stokes-Einstein relation in this protein solution, i.e., the dc conductivity is not completely governed by the mobility of the solvent molecules. Instead it seems that the dc conductivity is closely connected to the hydration shell dynamics.