Pressure-Dependent Stability of Imidazolium-Based Ionic Liquid/DNA Materials Investigated by High-Pressure Infrared Spectroscopy

Abstract

1-Butyl-3-methylimidazolium hexafluorophosphate ([C4MIM][PF6])/DNA and 1-methyl-3-propylimidazolium hexafluorophosphate ([C3MIM][PF6])/DNA mixtures were prepared and characterized by high-pressure infrared spectroscopy. Under ambient pressure, the imidazolium C2–H and C4,5–H absorption bands of [C4MIM][PF6]/DNA mixture were red-shifted in comparison with those of pure [C4MIM][PF6]. This indicates that the C2–H and C4,5–H groups may have certain interactions with DNA that assist in the formation of the ionic liquid/DNA association. With the increase of pressure from ambient to 2.5 GPa, the C2–H and C4,5–H absorption bands of pure [C4MIM][PF6] displayed significant blue shifts. On the other hand, the imidazolium C–H absorption bands of [C4MIM][PF6]/DNA showed smaller frequency shift upon compression. This indicates that the associated [C4MIM][PF6]/DNA conformation may be stable under pressures up to 2.5 GPa. Under ambient pressure, the imidazolium C2–H and C4,5–H absorption bands of [C3MIM][PF6]/DNA mixture displayed negligible shifts in frequency compared with those of pure [C3MIM][PF6]. The pressure-dependent spectra of [C3MIM][PF6]/DNA mixture revealed spectral features similar to those of pure [C3MIM][PF6]. Our results indicate that the associated structures of [C4MIM][PF6]/DNA are more stable than those of [C3MIM][PF6]/DNA under high pressures.

Structural Reorganization of Imidazolium Ionic Liquids Induced by Pressure-Enhanced Ionic Liquid—Polyethylene Oxide Interactions

“…Figure 1 shows the IR spectra of (a) pure 1-butyl-3-methylimidazolium bromide ([BMIM]Br), (b) PEO (M.W.~900,000) with 75 wt% [BMIM]Br, and (c) pure PEO (M.W.~900,000) at ambient pressure. The absorption profile of pure [BMIM]Br ( Figure 1 a) exhibits five major bands at 3138, 3067, 2958, 2934, and 2869 cm −1 , corresponding to two imidazolium C–H bands (C 4,5 –H and C 2 –H) and three cation alkyl C–H peaks (methyl and butyl groups) [ 29 , 30 , 31 , 32 , 33 , 34 ]. Figure 1 c shows the IR spectrum of pure PEO exhibiting multiple C–H stretching absorptions in the 2950–2850 cm −1 region [ 7 , 35 ].…”

“…High-pressure IR spectroscopy is a direct method to investigate the pressure-induced changes in non-covalent interactions and intermolecular (or interionic) distances [ 29 , 30 , 31 , 32 , 33 , 34 ]. The local structures of the ILs can be disturbed by the polymers (PVdF-co-HFP [ 29 ], DNA [ 30 ]), β-cyclodextrin [ 31 ], porous silica [ 32 ], mica [ 33 ], and nano-alumina [ 33 , 34 ], as determined by the high-pressure method. For example, high-pressure studies indicate that the presence of PVdF-co-HFP mainly affects the local structures of the imidazolium C–H groups instead of the anionic groups [ 29 ].…”

“…For example, high-pressure studies indicate that the presence of PVdF-co-HFP mainly affects the local structures of the imidazolium C–H groups instead of the anionic groups [ 29 ]. Pressure-enhanced IL-polymer interactions are also observed [ 29 , 30 ]. In general, a variation in temperature may result in changes in both thermal energy and volume.…”

Characterization of Local Structures of Confined Imidazolium Ionic Liquids in PVdF-co-HFP Matrices by High Pressure Infrared Spectroscopy

“…High-pressure infrared spectroscopy is a straightforward method to study the adjusted vibrational signals by forcing molecules to come close to each other. Modulating the molecular environments and non-covalent interactions by high pressures may provide special insights into the thermodynamically stable conformations of chemical systems [ 27 , 28 , 29 , 30 , 31 ]. This connotes that, under high-pressure conditions, the IL ions and the PVdF-co-HFP may aggregate at hydrostatic pressures.…”

“…IR spectroscopy performed under high pressures may reveal whether it is the IL anions or the IL imidazolium cations that tend to associate with the polymers. In previous studies, IL cations have been known to form stable associations with deoxyribonucleic acid sodium salt (DNA) [ 27 ] under high pressures. Moreover, the results of previous studies imply that the IL hydrogen-bonding network or cluster structure may be disturbed on mixing additives such as porous silica [ 28 ], β-cyclodextrin [ 29 ], aluminum oxide [ 30 ], and mica [ 31 ] under high pressures.…”