Broadband Dielectric Spectroscopy on Lysozyme in the Sub-Gigahertz to Terahertz Frequency Regions: Effects of Hydration and Thermal Excitation

Abstract

We have performed dielectric spectral measurements of lysozyme in a solid state to understand the effects of hydration and thermal excitation on the low-frequency dynamics of protein. Dielectric measurements were performed under changing hydration conditions at room temperature in the frequency region of 0.5 GHz to 1.8 THz. We also studied the temperature dependence (83 to 293 K) of the complex dielectric spectra in the THz frequency region (0.3 THz to 1.8 THz). Spectral analyses were performed using model functions for the complex dielectric constant. To reproduce the spectra, we found that two relaxational modes and two underdamped modes are necessary together with an ionic conductivity term in the model function. At room temperature, the two relaxational modes have relaxation times of ∼20 ps and ∼100 ps. The faster component has a major spectral intensity and is suggested to be due to coupled water-protein motion. The two underdamped modes are necessary to reproduce the temperature dependence of the spectra in the THz region satisfactorily. The protein dynamical transition is a well-known behavior in the neutron-scattering experiment for proteins, where the atomic mean-square displacement shows a sudden change in the temperature dependence at approximately 200 K, when the samples are hydrated. A similar behavior has also been observed in the temperature dependence of the absorption spectra of protein in the THz frequency region. From our broadband dielectric spectroscopic measurements, we conclude that the increase in the spectral intensities in the THz region at approximately 200 K is due to a spectral blue-shift of the fast relaxational mode.

Boson peak dynamics of natural polymer starch investigated by terahertz time-domain spectroscopy and low-frequency Raman scattering

“…On the other hand, the BP was not observed around room temperature due to the excess wing of the fast relaxation modes. The temperature dependence of ε'' during the heating process and cooling process showed a hysteresis below 230 K. A similar hysteresis has been observed in protein [62] and other glasses [63]. During the heating process, kinks were observed at 140 K and 230 K. These temperatures were in good agreement with the freezing temperatures [50] of the β-relaxation and the β wet -relaxation, respectively.…”

“…During the cooling process, the value of ε'' cool (1 THz, T) almost monotonically decreases. On the other hand, during the heating process, the bend points are observed at 140 K and 230 K, in the ε'' heat (1 THz, T); i.e., the complex dielectric constants of the starch shows a clear thermal hysteresis below 230 K. A similar behavior is observed in protein [62] and other glasses [63]. According to a previous study of natural polymer amber [63], this behavior is attributed to the existence of water in very small spaces of the starch.…”

Terahertz absorption of lysozyme in solution

“…[13][14][15][16][17][18][19][20][21][22] The absorption of light as the means of probing the solvation structure has a significant advantage compared to broad-band dielectric spectroscopy since the sensitivity of the response is improved by a factor roughly equal to the dielectric constant for the same magnitude of the liquid polarization or by a factor of dielectric constant squared for the same magnitude of the external electric field. 23 The difference is due to the transverse geometry of light absorption compared to the longitudinal geometry of the dielectric experiment.…”

Role of Disulfide Bonds and Topological Frustration in the Kinetic Partitioning of Lysozyme Folding Pathways

“…Lysozyme due to its antibacterial properties is one of the most widely studied proteins. [19][20][21][22] It is used as a model system to probe the effect of cosolvents [23][24][25][26][27][28][29][30] and salts 31 on protein folding and aggregation, 32-34 study the population of transient intermediates, 35 properties of water in the protein hydration shell, [36][37][38] and enzymatic activity. 39,40 Hen egg white lysozyme is a two domain protein with 129 residues (Figure 1).…”