Lois MORANDEIRA1,2, Ángeles SANROMÁN 1, Adilson A. FREITAS 2, José N. CANONGIA LOPES 2, Karina SHIMIZU2
1Department of Chemical Engineering, University of Vigo, Vigo, Spain
2Centro de Química Estrutural, Institute of Molecular Sciences, Instituto Superior Técnico, Universidade de Lisboa, Lisbon, Portugal
Ionic Liquids (ILs) derived from natural renewable resources are calling great attention because of their potential biocompatibility and low toxicity in comparison to conventional ILs. In this context, our research group has recently proposed the use of ILs based on cholinium cation, [Ch]+, and amino acid anions, which expands exponentially this family of green solvents.
The results for pure ILs suggest that peptides with chains up to 3 residues of glycine (Gly) exhibit a continuous polar domain characterized by a complex hydrogen bonding network. For phenylalanine (Phe) up to 2 residues, the ionic liquids [Ch][Phe] and [Ch][PhePhe] present two continuous phases, one polar and other apolar. In such cases, each phenyl ring is surrounded by one cholinium cation, suggesting the presence of π-cation complexes. The conformational preference between the phenyl rings is the T-shaped geometry, with some presence of unfavourable eclipsed-cofacial and offset-stacked geometries
In aqueous mixtures, the water molecules are dispersed into the polar network, acting as a hydrogen bonding intermediator and that ends up separating the ion pair. For the 5 studied ILs, the aggregate size distribution of the charged centres suggests that the ion pair starts being isolated about an 80%n/n, and the ion pair is fully dissociated beyond 95%n/n. Moreover, water facilitates the organization of the phenyl rings in [Ch][Phe] and [Ch][PhePhe] since the relative conformation evolves towards the more favourable T-shaped geometry.