How water manifests the structural regimes in ionic liquids

Toward Greater DNA Stability by Leveraging the Proton-Donating Ability of Protic Ionic Liquids

Deoxyribonucleic acid (DNA) stability is a prerequisite in many applications, ranging from DNA-based vaccines and data storage to gene therapy. However, the strategies to enhance DNA stability are limited, and the underlying mechanisms are poorly understood. Ionic liquids (ILs), molten salts of organic cations and organic/inorganic anions, are showing tremendous prospects in myriads of applications. With a judicious choice of constituent ions, the protic nature of ILs can be tuned. In this work, we investigate the relative stability of full-length genomic DNA in aqueous IL solutions of increasing protic nature. Our experimental measurements show that the protic ionic liquids (PILs) enhance the DNA melting temperature significantly while unaltering its native B-conformation. Molecular dynamics simulations and quantum mechanical calculation results suggest that the intramolecular Watson-Crick H-bonding in DNA remains unaffected and, in addition, the PILs induce stronger H-bonding networks in solution through their ability to make multiple intermolecular H-bonds with the nucleobases and among its constituent ions, thus aiding greater DNA stability. The detailed understanding obtained from this study could bring about the much-awaited breakthrough in improved DNA stability for its sustained use in the aforesaid applications!

Ionic Liquids: New Forms of Active Pharmaceutical Ingredients with Unique, Tunable Properties

This Review aims to summarize advances over the last 15 years in the development of active pharmaceutical ingredient ionic liquids (API-ILs), which make up a prospective game-changing strategy to overcome multiple problems with conventional solid-state drugs, for example, polymorphism. A critical part of the present Review is the collection of API-ILs and deep eutectic solvents (DESs) prepared to date. The Review covers rules for rational design of API-ILs and tools for API-IL formation, syntheses, and characterization. Nomenclature and ionic speciation, and the confusion that these may cause, are highlighted, particularly for speciation in both ILs and DESs of intermediate ionicity. We also highlight in vivo and in vitro pharmaceutical activity studies, with differences in pharmacokinetic/pharmacodynamic depending on ionicity of API-ILs. A brief overview is provided for the ILs used to deliver drugs, and the Review concludes with key prospects and roadblocks in translating API-ILs into pharmaceutical manufacturing.

Effects of Ionic Liquids on Aqueous Urea Solutions: Insights into the Ionic Liquid-Assisted Protein Renaturation

Ionic liquids (ILs) are designer solvents that find wide applications in various areas. Recently, ILs have been shown to induce the refolding of certain proteins that were previously denatured under the treatment of urea. A molecular-level understanding of the counteracting mechanism of ILs on urea-induced protein denaturation remains elusive. In this study, we employ atomistic molecular dynamics simulations to investigate the ternary urea-water-IL solution in comparison to the aqueous urea solution to understand how the presence of ILs can modulate the structure, energetics, and dynamics of urea-water solutions. Our results show that the ions of the IL used, ethylammonium nitrate (EAN), interact strongly with urea and disrupt the urea aggregates that were known to stabilize the unfolded state of the proteins. Results also suggest a disruption in urea-water interaction that releases more free water molecules in solution. We subsequently strengthened these findings by simulating a model peptide in the absence and presence of EAN, which showed broken versus intact secondary structure in urea solution. Analyses show that these changes were accomplished by the added IL, which enforced a gradual displacement of urea from the peptide surface by water. We propose that the ILs facilitate protein renaturation by breaking down the urea aggregates and increasing the amount of free water molecules around the protein.

Extended H-Bonding through Protic Ionic Liquids Facilitates the Growth and Stability of Water Domains in Hydrophobic Environment

Discrete water domains in hydrophobic environment find relevance in aerosols, oil refinery, the human body, etc. The interfacial microstructure plays a crucial role in the stability of such water domains. Over the decades, the amphiphile-induced electrostatic interaction is considered to be the major stabilizing factor operating at these interfaces. Here we take the representative water/AOT/oil microemulsion to show that creating a strong H-bonding network through suitable additive, such as protic ionic liquid (IL) at the interface, helps both the growth and stability of water domains in the hydrophobic phase. On the other hand, common electrolytes and aprotic ILs fail to replicate such behavior as seen by Raman, Fourier transform infrared spectroscopy, dynamic light scattering (DLS), and electron microscopy measurements. Experimental results are further supported by the all-atomic molecular dynamics (MD) simulations that showed extended H-bonding mediated by the protic IL cations that were localized at the interface. High temperature DLS and rheology studies have shown greater thermal stability and mechanical strengths of our biocompatible microemulsions, which have potential to become suitable templates for in situ synthesis of nanoparticle and various organic compounds.

Molecular Dynamics Insights into the Nanoscale Structural Organization and Local Interaction of Aqueous Solutions of Ionic Liquid 1-Butyl-3-methylimidazolium Nitrate

Considering the growing number of applications of the aqueous ionic liquids (ILs), atomistic molecular dynamics (MD) simulations were used to probe the effect of water molar fraction, x_w, ranging from 0.00 to 0.90, on the nanoscale local structure of 1-butyl-3-methylimidazolium nitrate, [bmim][NO₃], IL. The results prove that, with water addition, the cation-anion, cation-cation, and anion-anion structural correlations are weakened, while strong anion-water and unconventional cation-water hydrogen bonds are formed in the solutions. Water molecules were detected as bridges between nitrate anions, and the water cluster size distribution at different x_w's was investigated. Simulation shows a similar pattern of probability densities for water and anion around the acidic hydrogen atoms of the reference cation ring, while both species move away from the cation butyl chain. Increasing the water concentration to x_w = 0.90 causes decreasing of the local arrangement of the nearest-neighboring cations, because of the weakening of cation-cation π-π stacking. In addition, this dilution reduces the probability of the in-plane cation-anion conformation, disrupts both the polar ionic network and nonpolar domains, and diminishes the nanoaggregation of the cation butyl chains compared to those of the neat IL. These results can rationalize the origins of the fluidity enhancements and transport property trends upon adding water to the imidazolium-based ILs. The current study proposes a deep atomistic-level insight into the complex coupling between water concentration, microscopic structure, and local interactions of aqueous imidazolium-based ILs with hydrophilic anions.

Spectroscopic and computational insights into the ion-solvent interactions in hydrated aprotic and protic ionic liquids

Ionic liquids (ILs) and their aqueous solutions are emerging media for solving and manipulating biochemical molecules such as proteins. Unleashing the full potential however requires a detailed mechanistic understanding of how suitable protic and aprotic ILs behave in the presence of water in the first place. The present work aims at making an important step by performing a combined experimental and computational study of two selected ILs and their mixtures with water: the aprotic cholinium propionate ([Chl][Pro]) and the protic N-methyl-2-pyrrolidonium propionate ([NMP][Pro]). IR and Raman spectroscopy reveal stronger ion-solvent interactions in [Chl][Pro]-H₂O systems compared to [NMP][Pro]-H₂O mixtures. This can be explained by the tightly packed ion-pair associations in [NMP][Pro] comprising the protic -N⁺-H counterpart, which allows the establishment of highly directional and strong interionic hydrogen bonds. The spectral decomposition of the O-D stretching band into three sub-peaks showed that the protic [NMP][Pro] favors the self-association of water molecules. On the other hand, the predominant fraction of water-anion/cation aggregates exists in aprotic [Chl][Pro]. These hydrated systems can be envisaged using quantum-chemical calculations in the following way: H₂O[Chl]⁺H₂O[Pro]^-H₂O and H₂O[NMP]⁺[Pro]^-H₂O, which implied preferable solvent-shared ion-pair (SIP) configurations for [Chl][Pro]-H₂O systems, whereas the contact ion-pair (CIP) state prevails for the [NMP][Pro]-H₂O systems. The latter holds even in the water-rich regime. In future work, these findings will be the basis for an understanding of the underlying principles that govern the interactions of ions with bio-molecules in aqueous solutions.

Anomalous and Not-So-Common Behavior in Common Ionic Liquids and Ionic Liquid-Containing Systems

This work highlights unexpected, not so well known responses of ionic liquids and ionic liquid-containing systems, which are reported in a collective manner, as a short review. Examples include: (i) Minima in the temperature dependence of the isobaric thermal expansion coefficient of some ILs; (ii) Viscosity Minima in binary mixtures of IL + Molecular solvents; (iii) Anomalies in the surface tension within a family of ILs; (iv) The constancy among IL substitution of C_p/V_m at and around room temperature; (v) ILs as glass forming liquids; (vi) Alternate odd-even side alkyl chain length effects; (vii) Absolute negative pressures in ILs and IL-containing systems; (viii) Reversed-charged ionic liquid pairs; (ix) LCST immiscibility behavior in IL + solvent systems.

Water Clathrates in Nanostructural Organization of Hydrated Ionic Liquids Manifest a Peculiar Density Trend

Ionic liquid-water binary solutions have significantly expanded the applications of ionic liquids (ILs) in chemical and biological research. Therefore, considerable research has focused on measuring the thermophysical properties of these binary mixtures. From low-to-moderate concentrations of water, several IL/water mixtures exhibit deviations from expected trends in thermophysical behavior. One such example is a unique density trend observed for certain IL classes, which exhibit a characteristic increase in density with the addition of small amounts of water. Since water primarily interacts with the IL anion, such deviations have always been explained in the context of anion-water associations. Surprisingly, however, IL/water mixtures containing different cations but a common lactate anion exhibit similar peculiarities in density trends. Using atomistic level molecular dynamics simulations, we show that diverse density trends are caused by cation-mediated modulations in the IL nanostructure. Depending on its nature, the IL cation can play a dual role in modulating the IL nanostructure: (i) resist water-mediated breakdown of the nanostructure by interacting with the anion very strongly, (ii) further strengthen the nanostructure by incorporating water in the IL framework. The [emim] cation fails to play both roles resulting in the density decrease, while the [tmg] cation fulfills both roles leading to a density rise. The choline cation resists the density fall by inducing the formation of "water-clathrates" in the solution. Such occurrence of clathrates in IL/water binary mixtures, reported for the first time in this study, further emphasizes that the properties of ILs and its mixtures are not merely determined by the chemical nature of the component ions, but also by their unique nanostructural organizations. These unique nanostructural organizations also manifest in their unusual dynamics.

Ionic liquids make DNA rigid

Persistence length of double-stranded DNA (dsDNA) is known to decrease with an increase in ionic concentration of the solution. In contrast to this, here we show that the persistence length of dsDNA increases dramatically as a function of ionic liquid (IL) concentration. Using all atom explicit solvent molecular dynamics simulations and theoretical models, we present, for the first time, a systematic study to determine the mechanical properties of dsDNA in various hydrated ILs at different concentrations. We find that dsDNA in 50 wt % ILs have lower persistence length and stretch modulus in comparison to 80 wt % ILs. We further observe that both the persistence length and stretch modulus of dsDNA increase as we increase the concentration of ILs. The present trend of the stretch modulus and persistence length of dsDNA with IL concentration supports the predictions of the macroscopic elastic theory, in contrast to the behavior exhibited by dsDNA in monovalent salt. Our study further suggests the preferable ILs that can be used for maintaining DNA stability during long-term storage.

Effect of molecular solvents of varying polarity on the self-assembly of 1-n-dodecyl-3-methylimidazolium octylsulfate ionic liquid

Large-scale molecular dynamics simulations consisting of more than 88,000–106,000 atoms for approximately 250 ns (including equilibration and production) were conducted to assess the effect of polar, nonpolar and amphiphilic molecular solvents on the nanoscale structuring of 1-[Formula: see text]-dodecyl-3-methylimidazolium [C[Formula: see text]mim] octylsulfate [C8SO4] ionic liquid (IL). Water [H2O], [Formula: see text]-octane [C8H[Formula: see text]] and 1-octanol [C8H[Formula: see text]OH] are employed as examples of polar, nonpolar, and amphiphilic molecules, respectively. The results indicate that each of these molecular solvents modify the nanosegregation behavior of the ionic liquid in a unique way. Water induces a high order of structuring of the ionic liquid as indicated by extremely high nematic order parameter for the system. In addition, the morphology of the neat ionic liquid is transformed from layer-like to that of bilayer-like in which the polar and nonpolar domains alternate. The presence of water also causes the stretching of the nonpolar domain, thus, increasing its size. At the concentration examined in this work, [Formula: see text]-octane is found to be only partially miscible with the ionic liquid. The polar network is maintained; however, the continuous cationic nonpolar domain is split into multiple domains. [Formula: see text]-octane is accommodated in the ionic liquid nonpolar domain. Similarly, the amphiphilicity of 1-octanol leads to an increase in the number of cationic as well as anionic domains. The overall nonpolar domain length, however, remains nearly identical to that found for the pure ionic liquid. Additional characterization of structural features of the three systems is discussed in terms of one-dimensional number densities, nematic order parameters for the overall systems and their components and structure factors.

Environmentally benign tetramethylguanidinium cation based ionic liquids

Ionic liquids (ILs) are being considered as greener alternatives to conventional organic solvents.