Impact of ionic liquids on the structure of peptides proved by HR-MAS NMR spectroscopy

Abstract

The present work reports on the first systematic study on the impact of ionic liquids on the structure of peptides via high-resolution magic angle spinning (HR-MAS) NMR spectroscopy. The investigation is based on the use of six tetrapeptides of the general structure Ala-Xaa-Pro-Phe (Xaa: Ala, Asp, Glu, Gly, Lys and Phe) dissolved in three aqueous N,N′-dialkylimidazolium-derived ionic liquids. Analyses of individual chemical shift differences based on highly resolved one- and two-dimensional NMR spectra revealed a broad spectrum of strong peptide/ionic liquid interactions depending on the nature of the peptide and, most importantly, the composition of the molten salt. Alignments of ionic liquid specific interaction pattern with the effect of the respective ionic liquid on the cis/trans equilibrium state of the peptides' Xaa-Pro bond allowed for initial conclusions on the importance of individual modes of peptide/ionic liquid interactions on the structure of peptides at a molecular level. The reliability of the results obtained could be proven by a clear concentration dependency of individual peptide/ionic liquid interactions and cis/trans ratios of Xaa-Pro bonds as well as initial 2D ROESY NMR experiments.