Institut für Festkörperphysik, Technische Universität Darmstadt, Hochschulstr. 6, 64289 Darmstadt, Germany.
Centro de Fisica Materiales (CSIC-UPV/EHU) - Material Physics Centre (MPC), Paseo Manuel de Lardizabal 5, 20018 San Sebastian, Spain.
J Chem Phys. 2020 Jun 21;152(23):234503. doi: 10.1063/5.0010312.
We combine broadband dielectric spectroscopy (BDS) with H and H nuclear magnetic resonance (NMR) to study molecular dynamics in mixtures of ε-polylysine with HO or DO. In BDS, four relaxation processes can be attributed to molecular dynamics. While the fastest process P1 obeys the Arrhenius law, the slowest process P4 shows prominent non-Arrhenius behavior typical of structural α relaxation. For the intermediate processes P2 and P3, the temperature dependence changes at the glass transition temperature T. The H and H NMR results yield insights into the molecular origins of these relaxation phenomena. In these NMR analyses, we exploit, in addition to the isotope selectivity of the method, the possibility to distinguish between various types of motion based on their respective line-shape effects and the capability to single out specific molecular moieties based on different spin-lattice relaxation behaviors. In this way, we reveal that process P1 results from the rotation of side and end groups of the peptide, while process P2 is caused by a reorientation of essentially all water molecules, which are quasi-isotropic and survive well below T. As for the peptide backbone dynamics, we find evidence that rotational motion of polar groups is involved in process P3 and that nonpolar regions show a dynamical process, which is located between P3 and P4. Thus, the NMR analyses do not yield evidence for coexisting fast peptide-decoupled and slow peptide-coupled water species, which contribute to BDS processes P2 and P3, respectively, but minor bimodality of water motion may remain undetected. Finally, it is demonstrated that the proton/deuteron exchange needs to be considered when interpreting experimental results for molecular dynamics in aqueous peptide solutions.
我们将宽带介电谱（BDS）与 H 和 H 核磁共振（NMR）相结合，研究 ε-聚赖氨酸与 HO 或 DO 混合物中的分子动力学。在 BDS 中，可以将四个弛豫过程归因于分子动力学。虽然最快的过程 P1 遵循阿仑尼乌斯定律，但最慢的过程 P4 表现出显著的非阿仑尼乌斯行为，这是结构 α弛豫的典型特征。对于中间过程 P2 和 P3，温度依赖性在玻璃化转变温度 T 处发生变化。H 和 H NMR 的结果提供了对这些弛豫现象分子起源的深入了解。在这些 NMR 分析中，除了该方法的同位素选择性之外，我们还利用了基于各自的线形状效应区分各种类型运动的可能性，以及根据不同的自旋晶格弛豫行为分离特定分子部分的能力。通过这种方式，我们揭示了过程 P1 是由肽的侧基和末端基团的旋转引起的，而过程 P2 是由基本上所有水分子的重新取向引起的，这些水分子是准各向同性的，并且在 T 以下很好地存活。至于肽骨架的动力学，我们发现证据表明极性基团的旋转运动参与了过程 P3，并且非极性区域显示出一种动力学过程，该过程位于 P3 和 P4 之间。因此，NMR 分析没有证据表明存在共存的快速肽去耦和缓慢肽耦联的水物种，它们分别导致 BDS 过程 P2 和 P3，但水运动的次要双峰性可能未被检测到。最后，证明在解释水溶液中肽溶液的分子动力学实验结果时需要考虑质子/氘核交换。
