Pressure-Induced Unique Structural Changes in the Aqueous Solutions of Choline Dihydrogen Phosphate and Choline Iodide

Hydrated cholinium ([Chl]⁺)-based ionic liquids (ILs) are promising high-pressure (HP) preservation agents for biomolecules owing to their biocompatibility. However, their development for this application requires a fundamental understanding of the changes in their solution properties (crystallization and solution structure) under HP. Herein, we investigated the concentration- and pressure-induced structural changes of two [Chl]⁺-based ILs, namely, choline dihydrogen phosphate ([Chl][dhp]) and choline iodide ([Chl][I]), at mole fractions (<i>x</i>) of 0-0.20 in aqueous solution. Raman and ultraviolet-visible spectroscopic analyses and molecular dynamics simulations revealed that the solution structure of [Chl][dhp] at 0.1 MPa changed from a free-ion (FI) structure to an ion-pair structure ([Chl]⁺···associated [dhp]^-) with increasing <i>x</i>. In contrast, the solution structure of [Chl][I] changed from an FI structure to a structure in which I^- is located near [Chl]⁺. Based on these results, the effect of pressure on the solution structures of [Chl][dhp] and [Chl][I] under dilute (<i>x</i> = 0.02) and condensed (<i>x</i> = 0.15) conditions at approximately 1.0 GPa was investigated by Raman spectroscopy. Under both conditions, the aqueous [Chl][I] solution crystallized under HP, whereas the aqueous [Chl][dhp] solution remained in the liquid state up to 1.0 GPa. Notably, the [Chl]⁺ conformation of [Chl][dhp] under condensed conditions hardly changed at ∼1.0 GPa, and the associated structure of [dhp]^- at <i>x</i> = 0.15 did not change up to 300 MPa at least. Besides, 70% of the associated structure of [dhp]^- at 0.1 MPa was maintained at 1.0 GPa. This study indicates that a condensed [Chl][dhp] solution that can be maintained up to 1.0 GPa without crystallization has a potential application as an HP preservation agent for biomolecules.