Intermolecular organization in aqueous mixtures of choline lysinate studied via NMR and molecular dynamics/quantum mechanics

Abstract

Aiming to scrutinize intermolecular organization in aqueous mixtures of the choline lysinate, [Cho][Lys], ionic liquid (IL), the dependence of the ¹H NMR chemical shifts and diffusion coefficients on their composition was measured. To rationalize experimental findings, extensive molecular dynamics (MD) simulations and linear response quantum mechanics/molecular mechanics (QM/MM) computations of NMR shielding constants were performed. Analysis of MD trajectories reveals that the extent of intermolecular contacts between cations and anions intensifies with the increasing content of the IL in the mixture. Moreover, the tendency of choline cations and the side chains of lysinate anions to self-aggregate was observed as well, leading to the formation of a continuous, highly polar domain composed of choline cations and the carboxylate groups of lysinate anions, as well as a less polar domain formed by the side chains of the anions in IL-rich mixtures. Under these circumstances, isolated water pockets are found to be situated at the interface of the polar and nonpolar ionic domains. The dependence of the measured diffusion coefficients on the composition of the mixture reveals the existence of two dynamical regimes – fast and slow regimes below and above molar fraction of the IL of 11%, respectively. Results of MD simulations suggest that – at this specific molar composition of aqueous [Cho][Lys] mixture – continuous water network ceases giving way to the continuous structure of ionic domains being formed. The QM/MM results for the ¹H NMR chemical shifts of aqueous IL mixtures generally agree well with experimental findings and corroborate structural results. The prominent upfield shift of the NMR signal of protons in fast exchange with the rising content of the IL was successfully rationalized.