Charge transport and glassy dynamics in ionic liquids

• Applicant: Professor Dr. Friedrich Kremer
• Subject Area: Physical Chemistry of Molecules, Liquids and Interfaces, Biophysical Chemistry
• Term: from 2006 to 2013
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 29405857

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.

• DFG Programme: Priority Programmes
• Subproject of: SPP 1191: Ionic Liquids