Department of Physics, Arizona State University, P.O. Box 871504, Tempe, Arizona 85287, USA.
Department of Physics and School of Molecular Sciences, Arizona State University, P.O. Box 871504, Tempe, Arizona 85287, USA.
J Chem Phys. 2017 Aug 28;147(8):084502. doi: 10.1063/1.4989641.
Absorption of radiation by solution is described by its frequency-dependent dielectric function and can be viewed as a specific application of the dielectric theory of solutions. For ideal solutions, the dielectric boundary-value problem separates the polar response into the polarization of the void in the liquid, created by the solute, and the response of the solute dipole. In the case of a protein as a solute, protein nuclear dynamics do not project on significant fluctuations of the dipole moment in the terahertz domain of frequencies and the protein dipole can be viewed as dynamically frozen. Absorption of radiation then reflects the interfacial polarization. Here we apply an analytical theory and computer simulations to absorption of radiation by an ideal solution of lysozyme. Comparison with the experiment shows that Maxwell electrostatics fails to describe the polarization of the protein-water interface and the "Lorentz void," which does not anticipate polarization of the interface by the external field (no surface charges), better represents the data. An analytical theory for the slope of the solution absorption against the volume fraction of the solute is formulated in terms of the cavity field response function. It is calculated from molecular dynamics simulations in good agreement with the experiment. The protein hydration shell emerges as a separate sub-ensemble, which, collectively, is not described by the standard electrostatics of dielectrics.
溶液中辐射的吸收由其频率相关介电函数描述，可视为溶液介电理论的具体应用。对于理想溶液，介电边值问题将极性响应分为由溶质产生的液体中空腔的极化和溶质偶极子的响应。在蛋白质作为溶质的情况下，蛋白质核动力学不会在太赫兹频率范围内的偶极矩的显著涨落上投影，并且可以认为蛋白质偶极子是动态冻结的。因此，辐射的吸收反映了界面极化。在这里，我们应用分析理论和计算机模拟来研究溶菌酶理想溶液对辐射的吸收。与实验的比较表明，麦克斯韦静电学无法描述蛋白质-水界面和“洛伦兹空穴”的极化，后者不预期外场（无表面电荷）对界面的极化，更好地代表了数据。根据腔场响应函数，提出了一种用于描述溶液吸收随溶质体积分数变化的斜率的分析理论。它是从分子动力学模拟中计算得出的，与实验结果吻合良好。蛋白质水合壳作为一个单独的子集合出现，而标准的介电静电学无法描述该集合。
