Depicting a neoteric family of biocompatible ionic liquids: A look through a molecular dynamics prism

The organization of Ionic Liquids (ILs) at the nanoscale has been demonstrated to be invaluable knowledge to understand and even rationally develop novel applications. In this line, this work aims at the deeper structural comprehension of five model ionic solvents from the recently presented family of biocompatible ILs based on cholinium cation ([Ch]+) and peptide anions ([Pep]-). To do this, a molecular dynamics approach was employed. The use of different [Pep]- allowed us to evaluate the influence of (i) incrementing an oligopeptide chain through the glycyl (Gly) unit and (ii) employing a more complex functionalization based on the phenylalanyl (Phe) group. The simulations show that the additional peptide group from [Ch][Pep] adds a new anchoring point for inter-anionic association through H-bonding, and while the peptide hydrogen is interacting with other anions, the peptide oxygen seems to approach cation charged centers. This stronger H-bonding grid may explain the boost in viscosity observed when transitioning from [Ch][AA] to [Ch][Pep]. On the other hand, the nano-segregation of ILs built with Phe is distinguished by two continuous mesophases, polar and apolar. Both these networks have their impact on each other since, when the peptide chain is incremented from 1 to 2 Phe residues, (i) the polar domain is slightly less continuous and (ii) the apolar one presents different phenyl ring (Pher) solvation profiles depending on their position in the peptide chain. In addition, the edge-to-face configurations dominate the Pher-Pher aggregation, suggesting an electrostatic influence in the internal organization of this domain, and π-cation complexes appear to disturb apolar accumulation.