Effect of Temperature and Hydration Level on Purple Membrane Dynamics Studied Using Broadband Dielectric Spectroscopy from Sub-GHz to THz Regions.
Effect of Temperature and Hydration Level on Purple Membrane Dynamics Studied Using Broadband Dielectric Spectroscopy from Sub-GHz to THz Regions.
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DOI：
10.1021/acs.jpcb.7b10077
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发表时间：
2018-01
期刊：
影响因子：
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通讯作者：
Naoki Yamamoto;S. Ito;M. Nakanishi;E. Chatani;Keiichi Inoue;H. Kandori;K. Tominaga
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文献类型：
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作者：
Naoki Yamamoto;S. Ito;M. Nakanishi;E. Chatani;Keiichi Inoue;H. Kandori;K. Tominaga
为了研究温度和水化对紫色膜动力学的影响，我们使用矢量网络分析仪测量了0.5 GHz至2.3太赫兹的宽带复介电谱和233至293 K的太赫兹时域谱。在低至83 K的低温区，也得到了太赫兹区的复介电谱。采用多种模型函数对复介电谱进行曲线拟合分析。我们发现一个弛豫模式的水合态，被指定为水分子与PM表面的耦合运动，在大约230k时开始与太赫兹区重叠。另一方面，在脱水状态下没有观察到弛豫模态。基于这一结果，我们得出结论，蛋白质在太赫兹区域的动态跃迁行为是由于弛豫模式与太赫兹区域重叠的开始。在263 K时，当水化水平较高时，介电谱出现了温度滞后现象。在该温度下，水化水的行为与过冷液体相似。第三水化层可以部分形成以观察这种现象。我们还发现，PM的弛豫时间比球状蛋白、溶菌酶的弛豫时间要慢，并且PM表面附近的微观环境比溶菌酶更不均匀。提出了弛豫模式和低频振动模式的频谱重叠是蛋白质大构象变化的必要条件。
To investigate the effects of temperature and hydration on the dynamics of purple membrane (PM), we measured the broadband complex dielectric spectra from 0.5 GHz to 2.3 THz using a vector network analyzer and terahertz time-domain spectroscopy from 233 to 293 K. In the lower temperature region down to 83 K, the complex dielectric spectra in the THz region were also obtained. The complex dielectric spectra were analyzed through curve fitting using several model functions. We found that the hydrated states of one relaxational mode, which was assigned as the coupled motion of water molecules with the PM surface, began to overlap with the THz region at approximately 230 K. On the other hand, the relaxational mode was not observed for the dehydrated state. On the basis of this result, we conclude that the protein-dynamical-transition-like behavior in the THz region is due to the onset of the overlap of the relaxational mode with the THz region. Temperature hysteresis was observed in the dielectric spectrum at 263 K when the hydration level was high. It is suggested that the hydration water behaves similarly to supercooled liquid at that temperature. The third hydration layer may be partly formed to observe such a phenomenon. We also found that the relaxation time is slower than that of a globular protein, lysozyme, and the microscopic environment in the vicinity of the PM surface is suggested to be more heterogeneous than lysozyme. It is proposed that the spectral overlap of the relaxational mode and the low-frequency vibrational mode is necessary for the large conformational change of protein.