Charge transport and glassy dynamics in ionic liquids

与经典分子溶剂中的反应相比，离子液体中的反应不仅对转化率有显著影响，而且对对映体选择性和区域选择性也有显著影响。这些影响涵盖了广泛的反应类型，主要包括酶催化反应、过渡金属催化反应，甚至非催化反应。目前，有近2600种关于离子液体的科学出版物，但大多数都是描述性的，这使得离子液体介导的反应的优化成为一项高度经验性的任务。相反，只有极少数的贡献在或多或少的分子水平上为观察到的宏观效应提供了合理的解释。在最初的研究中，我们评估了离子液体对多肽酰化反应的总体影响。由此得出结论，离子液体以高度不均匀的方式影响这些反应，不仅影响多肽的转化率，同时还影响反应的区域选择性。事实上，合适的离子液体提高了活性酯与含有两个亲核氨基的寡肽之间的非选择性生物素化反应的区域选择性，即N-端和赖氨酸-S N‘-氨基，分别高达75%(N-端)和82%的赖氨酸侧链。虽然这种意想不到的区域选择性效应可以使离子液体成为非常有用的“功能溶剂”，特别是在未来的肽甚至蛋白质合成中，但目前还没有在分子水平上对这种离子液体效应做出一致的解释。在所要求的项目中，应进行物理化学研究，最终目的是对观察到的离子液体效应作出合理解释，并了解这些特殊溶剂的区域选择性诱导行为的分子原因。自己的初步研究已经表明，离子液体会影响醋酸的pKa值。根据德拜-胡克尔理论，检测到的pKa漂移应与介电常数相对应。此外，我们还可以证明离子液体对该体系中氢键的强度有显着的调节作用。基于这些初步发现，S项目的主要任务是澄清已发现的离子液体的反应性和区域选择性诱导效应与离子液体引起的物理化学参数，特别是氢键性质、介电常数、pKa位移和热力学参数(RG、RH、RS)之间是否存在合理的相互作用。澄清这种因果关系对于合理使用离子液体进行合成将具有极其重要的意义。在实验上，介电常数应由相应的介电测量直接确定，而对AMID I键振动的FT-IR光谱监测则提供了相应离子液体和单个多肽氨基功能之间氢键强度的定量数据。根据Eyring方程，通过测定酰化反应速率常数k的温度依赖性，得到了离子液体对热力学活化参数RG、RH、RS的明显影响。最后，优化的FT-IR光谱技术将用于直接测定离子液体介导的不同氨基官能团的pKA-位移。

In comparison to reactions in classical molecular solvents, reactions in ionic liquids show pronounced effects not only on the conversion rates, but also on the enantio- and regioselectivities. Those effects cover a broad range of reaction types including mainly enzymatic, transition metal-catalyzed and even non-catalyzed reactions. Presently, there are nearly 2600 scientific publications about ionic liquids, but the majority of them are exclusively descriptive making the optimisation of ionic liquid-mediated reactions a highly empiric task. On the contrary, only a very few contributions provide rational explanations on a more or less molecular level for the macroscopic effects observed.In initial studies we evaluated the general influence of ionic liquids on peptide acylation reactions. Thereby it came out that ionic liquids surprisingly affect these reactions in a highly nonuniform manner influencing not only the conversion rate of the peptide, but simultaneously also the reactions¿ regioselectivity. In fact, proper ionic liquids increased the regioselectivity of an originally non-selective biotinylation reaction between an active ester and an oligopeptide bearing two nucleophilic amino functions, i.e. the N-terminal and lysine¿s N¿-amino group, up to 75% (N-terminus) and (82% lysine side-chain), respectively. Although this unexpected regioselectivity-effect could make ionic liquids highly useful as `functional solvents¿ in particular for peptide and probably even protein synthesis in the future, there is presently no coherent explanation for this ionic liquid-effect on a molecular level. In the project requested, physicochemical studies shall be conducted with the final aim to provide rational explanations of the ionic liquids-effects observed and to understand the molecular reasons of the regioselectivity-inducing behaviour of these special solvents. Own preliminary studies could already show that ionic liquids affect the pKa-value of acetic acid. According to the Debye-Hückel-theory, the pKa-shift detected should correspond to the dielectric permittivity ¿. Furthermore we could prove that ionic liquids modulate the strength of hydrogen bonds in this system significantly. Based on this preliminary findings, the project¿s major task is focused to clarify whether there is a rational interplay between the already found reactivity- and regioselectivity-inducing effect of ionic liquids on the one hand and physicochemical parameters, in particular hydrogen bond properties, dielectric permittivities ¿, pKa-shifts and thermodynamic parameters (¿RG¿, ¿RH¿, ¿RS¿) induced by ionic liquids, on the other hand. Clarification of such causal relationships would be of eminent importance to a rational use of ionic liquids for synthesis. Experimentally, dielectric permittivities shall be determined by respective dielectric measurements directly, while FT-IR spectroscopic monitoring of amid I bond vibrations provides quantitative data to the strength of hydrogen bonds between a respective ionic liquid and individual peptidic amino functions. Access to the distinct influence of ionic liquids on the thermodynamic activation parameters ¿RG¿, ¿RH¿, ¿RS¿ is given according to the Eyring-equation via measuring the temperature-dependency of the rate constant k of the acylation reaction. Finally, optimised FT-IR spectroscopic technologies shall be used to determine ionic liquids-mediated pKa-shifts of distinct amino functionalities directly.