Department of Physics and INFM CRS-SOFT, La Sapienza University of Rome, Rome, Italy.
J Phys Chem B. 2011 Jun 2;115(21):7144-53. doi: 10.1021/jp2019389. Epub 2011 May 10.
The dielectric properties of lysozyme aqueous solutions have been investigated over a wide frequency range, from 1 MHz to 50 GHz, where different polarization mechanisms, at a molecular level, manifest. The dielectric relaxation spectra show a multimodal structure, reflecting the complexity of the protein-water interactions, made even more intricate with the increase of the protein concentration. The deconvolution of the spectra into their different components is not unambiguous and is generally a delicate process which requires caution. We have analyzed the whole relaxation region, on the basis of the sum of simple Debye-type relaxation functions, considering three main contributions. Particular attention has been payed to the δ-dispersion, intermediate between the β-dispersion (rotational dynamics of the protein) and the γ-dispersion (orientational polarization of the water molecules). This intermediate contribution to the dielectric spectrum is attributed to the orientational polarization of water molecules in the immediate vicinity of the protein surface (hydration water). Our measurements clearly demonstrate that, at least at high protein concentrations, the δ-dispersion has a bimodal structure associated with two kinds of hydration water, i.e., tightly bound and loosely bound hydration water. In the concentration range investigated, the existence of a three-modal δ-dispersion, as recently suggested, is not supported, on the basis of statistical tests, by the analysis of the dielectric relaxations we have performed and a bimodal dispersion is accurate enough to describe the experimental data. The amount of the hydration water has been evaluated both from the dielectric parameters associated with the δ-dispersion and from the decrement of the loss peak of the γ-dispersion. The relative weight of tightly bound and loosely bound hydration water is briefly discussed.
溶菌酶水溶液的介电性质在很宽的频率范围内（从 1MHz 到 50GHz）进行了研究，在分子水平上出现了不同的极化机制。介电弛豫谱呈现出多模态结构，反映了蛋白质-水相互作用的复杂性，随着蛋白质浓度的增加变得更加复杂。对谱图进行分解为不同的成分并不明确，通常是一个需要谨慎处理的微妙过程。我们根据简单的德拜型弛豫函数的和，分析了整个弛豫区域，考虑了三个主要贡献。特别关注了δ-弥散，它处于β-弥散（蛋白质的旋转动力学）和γ-弥散（水分子的取向极化）之间。这种介电谱的中间贡献归因于蛋白质表面附近水分子的取向极化（水合水）。我们的测量结果清楚地表明，至少在高蛋白质浓度下，δ-弥散具有与两种水合水相关的双峰结构，即紧密结合和松散结合的水合水。在研究的浓度范围内，基于统计测试，我们进行的介电弛豫分析不支持最近提出的三模态δ-弥散的存在，双峰弥散足以准确描述实验数据。通过与δ-弥散相关的介电参数和γ-弥散损耗峰的衰减，评估了水合水量。简要讨论了紧密结合和松散结合水合水的相对重量。
