Unexpected structural preference with metallophilic Ag-Au contacts in silver(I)-N-heterocyclic carbene cluster; experimental and theoretical approach.

A novel synthetic donor-atom-selective approach has been adopted for the synthesis of a heterobimetallic cluster of a new NCN-pincer, 1,3-bis-(1-methyl-1H-benzo[d]imidazol-2-yl-methyl)-1H-imidazol-3-ium hexafluorophosphate (1·HPF6). The complex [Ag3(1)3][PF6]3 (2) has been prepared via the Ag2O route; which undergoes transmetallation to yield a cluster that seems to be the first example of the heterobimetallic trinuclear system [Au-Ag2(1)2Cl][PF6]2, 3. Finally, the trinuclear cluster geometries of 2 and 3 were confirmed via SCXRD studies. Interestingly, Au(I) binds preferentially with soft donor Ccarbene, which transmetallated from the cluster of 2. In both the cyclic trinuclear clusters, the M-M interactions were further inspected using gauge independent atomic orbital (GIAO) computations. Both 2 and 3 are luminescent and possess σ-aromaticity; the NICS values indicate that 3 is more aromatic than 2.

Deciphering the Role of Anions of Ionic Liquids in Modulating the Structure and Stability of ct-DNA in Aqueous Solutions.

Stabilizing biomolecules under ambient conditions can be extremely beneficial for various biological applications. In this context, the utilization of ionic liquids (ILs) in enhancing the stability and preservation of nucleic acids in aqueous solutions is found to be promising. While the role of the cationic moiety of ILs in the said event has been thoroughly explored, the importance of the anionic moiety in ILs, if any, is rather poorly understood. Herein, we examine the function of anions of ILs in nucleic acid stabilization by examining the stability and structure of calf thymus-DNA (ct-DNA) in the presence of various ILs composed of a common 1-ethyl-3-methylimidazolium cations (Emim+) and different anions, which includes Cl-, Br-, NO3-, Ac-,HSO4-and BF4- by employing various spectroscopic techniques as well as Molecular Dynamics (MD) simulation studies. Analysis of our data suggests that the chemical nature of anions including polarity, basicity, and hydrophilicity become an important factor in the overall DNA-IL interaction event. At lower concentrations, the interplay of intermolecular interaction between the IL anions with their respective cations and the solvent molecules becomes a very crucial factor in inducing their stabilizing effect on ct-DNA. However, at higher concentrations of ILs, the ct-DNA stabilization is additionally governed by specific-ion effect. MD simulation studies have also provided valuable insights into molecular-level understanding of the DNA-IL interaction event. Overall, the present study clearly demonstrated that along with the cationic moiety of ILs, the anions of ILs can play a significant role in deciding the stability of duplex DNA in aqueous solution. The findings of this study are expected to enhance our knowledge on understanding of IL-DNA interactions in a better manner and will be helpful in designing optimized IL systems for nucleic acid based applications.

Assessing the impact of choline chloride and benzyltrimethylammonium chloride-based deep eutectic solvents on the structure and conformational dynamics of bovine serum albumin: a combined steady-state, time-resolved fluorescence and fluorescence correlation spectroscopic study.

Although deep eutectic solvents (DESs) are regarded as useful substitutes for both ionic liquids and common organic solvents for storage and applications of biomolecules, it is still unclear whether all DESs or only specific types of DESs will be suitable for the said purpose. In view of this, the current study aims to report on the structure and conformational dynamics of BSA in the presence of two DESs, namely ethaline (choline chloride:ethylene glycol) and BMEG (benzyltrimethyl ammonium chloride:ethylene glycol), having the same hydrogen bond donor but with a distinct hydrogen bond acceptor, so that how small changes in one constituent of a DES alter the protein-DES interaction at the molecular level can be understood. The protein-DES interaction is investigated by exploiting both ensemble-averaged measurements like steady-state and time-resolved fluorescence spectroscopy, circular dichroism (CD) spectroscopy, and single-molecule sensitive techniques based on fluorescence correlation spectroscopy (FCS). Interestingly, the results obtained from these studies have demonstrated that while a very small quantity of BMEG completely unfolds the native structure of the protein, it remains in a partially unfolded state even at very high ethaline content. More interestingly, it has been found that at very high concentrations of BMEG, the unfolded protein undergoes enhanced protein-protein interaction resulting in the aggregation of BSA. All of the results obtained from these investigations have essentially suggested that both protein-DES interaction and interspecies interaction among the constituent of DESs play a crucial role in governing the overall stability and conformational dynamics of the protein in DESs.

Insight into the structure and transport properties of pyrrolidinium-based geminal dicationic-organic ionic crystals: unravelling the role of alkyl-chain length.

The present study has been undertaken with an aim to design and develop safer and more efficient all solid-state electrolytes, so that the issues associated with the use of conventional room temperature ionic liquid-based electrolytes can be tackled. To fulfil this objective, a series of geminal di-cationic Organic Ionic Crystals (OICs), based on C3-, C6-, C8- and C9-alkylbridged bis-(methylpyrrolidinium)bromide are synthesized, and the structural features, thermal properties and phase behaviours of these as synthesized OICs have been investigated. Additionally, a number of electro-analytical techniques have been employed to assess their suitability as an efficient electrolyte composite (OIC:I2:TBAI) for all solid-state dye sensitised solar cells (DSSCs). The structural analysis has revealed that along with excellent thermal stability and well-defined surface morphology, all thsese OICs exhibit a well-ordered three-dimensional network of cations and anions that can serve as a conducting channel for the diffusion of iodide ions. Electrochemical investigations have shown that OICs with an intermediate length of alkyl bridge (C6- and C8-alkyl bridged) show better electrolytic performance than those that are based on OICs with a relatively shorter (C3-) or longer (C9-) alkyl-bridge chain. A careful analysis of the above data has essentially demonstrated that the length of the alkyl bridge chain plays a significant role in determining the structural organisation, morphology and eventually the ionic conductivity of OICs. Overall, the comprehensive knowledge on OICs that has been extracted from the current study is expected to be helpful to explore further new types of OIC-based all solid-state electrolytes with improved electrolytic performance for targeted applications.

Assessing the Suitability of a Dicationic Ionic Liquid as a Stabilizing Material for the Storage of DNA in Aqueous Medium.

The present study has been undertaken with an objective to find out a suitable medium for the long-term stability and storage of the ct-DNA structure in aqueous solution. For this purpose, the potential of a pyrrolidinium-based dicationic ionic liquid (DIL) in stabilizing ct-DNA structure has been investigated by following the DNA-DIL interaction. Additionally, in order to understand the fundamental aspects regarding the DNA-DIL interaction in a comprehensive manner, studies are also done by employing structurally similar monocationic ionic liquids (MILs). The investigations have been carried out both at ensemble-average and single molecular level by using various spectroscopic techniques. The molecular docking study has also been performed to throw more light into the experimental observations. The combined steady-state and time-resolved fluorescence, fluorescence correlation spectroscopy, and circular dichroism measurements have demonstrated that DILs can effectively be used as better storage media for ct-DNA as compared to MILs. Investigations have also shown that the extra electrostatic interaction between the cationic head group of DIL and the phosphate backbone of DNA is primarily responsible for providing better stabilization to ct-DNA, retaining its native structure in aqueous medium. The outcomes of the present study are also expected to provide valuable insights in designing new polycationic IL systems that can be used in nucleic acid-based applications.

Choline chloride and ethylene glycol based deep eutectic solvent (DES) versus hydroxyl functionalized room temperature ionic liquids (RTILs): assessing the differences in microscopic behaviour between the DES and RTILs.

With the aim of understanding the differences in the behavior of deep eutectic solvents (DESs) and room temperature ionic liquids (RTILs) in terms of their structure, dynamics, and intra- and intermolecular interactions, three different ILs and one DES having similar functionalities (hydroxyl) have been investigated by using both ensembled average and single-molecule spectroscopic techniques. Specifically, for this purpose, a choline chloride based DES (ethaline) and three hydroxyl functionalized ILs (1-(2-hydroxyethyl)-3-imidazolium bis(trifluoromethanesulfonyl)imide ([OHEMIM][NTF2]), N-(2-hydroxyl ethyl)-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide ([OHEMPy][NTf2]), and N-(2-hydroxyethyl)-N,N-dimethylpropan-1-aminium bis(trifluoromethanesulfonyl)imide ([OHC3CH][NTf2])) are employed and investigated by EPR, time-resolved fluorescence, NMR and FCS studies. Estimation of polarity through EPR spectroscopy has revealed that the hydroxyl ILs employed in these studies are hyper-polar (close to water) in nature, whereas the polarity of the DES is found to be close to those of aliphatic polyhydroxy-alcohols. Interestingly, both time-resolved fluorescence anisotropy and FCS studies on these systems have suggested that the hydroxyl ILs are more dynamically heterogeneous than the DES. More interestingly, PFG-NMR measurements have indicated that the fluid structure of ethaline is relatively more associated as compared to those of the ILs despite the fact that all the cations have the same hydroxyl functionalities. All these investigations have essentially demonstrated that, despite having similar functionalities, both the DES and hydroxyl ILs employed in the present study exhibit microscopic behaviours that are significantly different from each other, indicating the interplay of various intermolecular interactions within the constituent species in governing the behaviours of these solvent systems.

Effect of Lithium-Ion on the Structural Organization of Monocationic and Dicationic Ionic Liquids.

In recent times, ionic liquid-based (ILs) electrolytic system has emerged as suitable alternative to the conventional organic solvent-based electrolytic system. However, since, anion of ILs is known to form aggregates in the presence of lithium-ions (Li+), and this can influence the transport properties of Li+ ion in a significant manner, it is, therefore, important to understand how lithium-ions influence the structure and dynamics of ILs. With this objective, in the present study, intermolecular interaction, structural organization, and dynamics of monocationic ILs (MILs) and dicationic IL (DIL) have been studied in the absence and presence of lithium salt. Specifically, for this purpose, two MILs, 1-methyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C3C1im][NTf2]), 1-hexyl-3-propylimidazolium bis(trifluoromethylsulfonyl)imide ([C6C1im][NTf2]), and a DIL, 1,6-bis(3-methylimidazolium-1-yl)hexane bis(trifluoromethylsulfonyl)amide ([C6(mim)2][NTf2]2) have been chosen in such a way that either the alkyl chain of MILs becomes equal or half of the spacer chain length of DIL. To understand the effect of the addition of lithium-ion on the structural organization of MILs and DIL, steady-state absorption and fluorescence spectroscopies, time-resolved fluorescence anisotropy and nuclear magnetic resonance (NMR) techniques have been used. Structural organization in the apolar and polar domains of ILs has been probed by following the rotational diffusion of suitably chosen solute in the concerned media through time-resolved fluorescence anisotropy (TRFA) measurements. TRFA studies have revealed that with the addition of Li+ ion, coordination between the Li+ ions and anions of MILs and DILs takes place in the ionic region leading to a change in the structural organization of the apolar regions of the respective medium. In fact, upon adding lithium-ions, a reduction in the packing of alkyl chains has also been observed for the MILs. However, not much change in the structural organization of the apolar region of the DIL has been observed when Li+ ion is added to it. In the presence of Li+ ions, a similar trend in the change of structural organization of polar regions for both MILs and DIL has been observed. Further, measurements of the self-diffusion coefficient through NMR have also supported the observation that Li+ ion also perturbs the nanostructural organization of the MIL in a significant manner than that it does for the DIL. The behavior of DIL in the presence of Li+ ion, as revealed by the present study, has been rationalized by considering the folded arrangement of DIL in the fluid-structure. Essentially, all of these investigations have suggested that the addition of lithium-ion significantly alters the microscopic behavior of MILs in comparison to that of DIL. The outcome of this study is expected to be helpful in realizing the potentials of these media as electrolytes in battery applications.

Comparison between pyrrolidinium-based and imidazolium-based dicationic ionic liquids: intermolecular interaction, structural organization, and solute dynamics.

With an aim to understand the difference in the behaviour of imidazolium and pyrrolidinium-based dicationic ionic liquids (DILs) in terms of the intermolecular interactions, microscopic-structure and dynamics, two DILs, the imidazolium-based 1,9-bis(3-methylimidazolium-1-yl)nonane bis(trifluoromethanesulfonyl)imide and the pyrrolidinium-based 1,9-bis(1-methylpyrrolidinium-1-yl)nonane bis(trifluoromethanesulfonyl)imide, have been synthesized and subsequently investigated by exploiting combined steady sate and time resolved fluorescence, electron paramagnetic resonance (EPR) and nuclear magnetic resonance (NMR) spectroscopic techniques. Data obtained for DILs have also been compared with their corresponding mono-cationic counterpart (MILs) to evaluate and understand the distinctive characteristics of the DILs in contrast with the corresponding MILs. Steady state emission and EPR data have revealed that the pyrrolidinium-based DIL is slightly less polar than the imidazolium-based DIL. Temperature-dependent fluorescence anisotropy decay of two probes, perylene and MPTS (8-methoxypyrene-1,3,6-trisulfonate), has been measured in DILs as well as in MILs. Solute-solvent coupling constants obtained from the experimentally measured rotational correlation times with the aid of Stokes-Einstein-Debye hydrodynamic theory have indicated appreciable differences in the dynamics of both the solutes on going from MILs to DILs. More interestingly, the outcome of the NMR study has suggested that the alkyl spacer chain in the imidazolium-based DIL exists in the folded form, but the pyrrolidinium-based DIL remains in the straight chain conformation. Inherently, the outcomes of all of these studies have depicted that the microscopic structural organisations in imidazolium and pyrrolidinium-based DILs are different from each other as well as from their respective mono-cationic counterparts.

Binary mixtures of ionic liquids: Ideal, non-ideal, or quasi-ideal?

The mixing of ILs provides an opportunity for fine tuning the physiochemical properties of ILs for various applications. However, a suitable mixture having desired properties can only be designed when the physiochemical properties of the mixtures of ILs along with their spectroscopic properties are well understood. With an aim to achieve this objective, three different mixtures with a common anion, namely, [C2C1im][C4C1im][NTf2], [C3C1pyr][C4C1pyr][NTf2], and [C3C1im][C3C1pyr][NTf2], have been investigated in the current study. Investigations have been carried out at the macroscopic level by observing the thermophysical properties, such as molar volume and thermal expansion coefficient, and at the microscopic level with time-resolved fluorescence measurements and the pulse field gradient nuclear magnetic resonance (NMR) technique. The results obtained from the thermophysical study have indicated that excess molar volume for imidazolium-based IL-IL mixtures may be linked to the free volume created by the alkyl chain of the imidazolium cation whereas for the mixture of pyrrolidinium ILs, lowering of density can give rise to free volume. Analysis of time-resolved fluorescence anisotropy data has provided clear evidence in favor of the presence of free volume in the binary mixture of ILs. NMR studies have also supported the fluorescence anisotropy data. The outcome of the present investigation reveals that the mixtures show appreciable deviation from ideal behavior and the deviation from the ideal behavior is caused due to the generation of free volume in the resultant mixture, describing these IL mixtures as quasi-ideal rather than ideal or non-ideal.

How Does Addition of Lithium Salt Influence the Structure and Dynamics of Choline Chloride-Based Deep Eutectic Solvents?

In recent times, deep eutectic solvents (DESs) have emerged as an environment-friendly alternative to both common organic solvents and ionic liquids (ILs). The present study has been undertaken with an objective to understand the intermolecular interaction, structural organization, and dynamics of two DES systems in the absence and presence of lithium salt so that the potential of these mixtures in electrochemical application is realized. For this purpose, the steady-state, time-resolved fluorescence, electron paramagnetic resonance (EPR), and nuclear magnetic resonance (NMR) behavior of two DESs (ethaline and glyceline) and their mixture with lithium bis(trifluoromethylsulfonyl) imide (LiNTf2) has been investigated. Measurements of polarity through EPR technique have revealed that the polarities of DESs are close to aliphatic polyhydroxy alcohol and the polarities of the medium increase with the increase in lithium salt concentration. Studies on solvation dynamics have indicated that there is an increase in average solvation time with the increase in lithium salt concentration. Investigation of rotational dynamics of some selected fluorophore in these media has shown that addition of lithium salt significantly alters the nano/microstructural organization of both DESs. Further, measurements of the self-diffusion coefficient through NMR have also supported the perturbation of the nanostructural organization of the solvent systems by addition of lithium salts. Essentially, all of these investigations have suggested that addition of lithium salt significantly alters the microscopic behavior of DESs. The outcome of this study is expected to be helpful in realizing the potential of these media for various electrochemical applications including application in lithium-ion battery.

Interaction of Lysozyme with Monocationic and Dicationic Ionic Liquids: Toward Finding a Suitable Medium for Biomacromolecules.

With an objective to understand the differences in the behavior of monocationic and dicationic ionic liquids (ILs) in their interaction with protein, we have investigated the binding interaction of lysozyme enzyme with two monocation ionic liquids (MILs), [C3MIm][Br], [C6MIm][Br], and one dicationic ionic liquid (DIL), [C6(MIm)2][Br]2, by exploiting various experimental methods. These ILs are purposefully chosen so that the effect of both hydrophobicity and structural arrangements of the cationic moiety of ionic liquids (ILs), if any, on the interaction event is understood. Both average ensemble and single molecule pathways have been adopted to obtain a comprehensive picture. For ensemble averaged measurements, the interaction events have been investigated by steady-state and time-resolved fluorescence spectroscopy, whereas for single molecule measurements, fluorescence correlation spectroscopy (FCS) has been utilized. Additionally, the behavior of protein in the absence and presence of ILs has also been investigated through circular dichroism (CD) measurements. The investigations have revealed that MILs and DIL interact differently with the protein. In particular, as compared to MILs, the influence of DIL toward protein is observed to be significantly less in terms of change in the structure and dynamics of protein. The outcome of the present work has demonstrated that imidazolium-based DIL can be a better choice over MILs for retaining native structure of protein in aqueous medium.

Probing the Interactions of 1-Alkyl-3-methylimidazolium Tetrafluoroborate (Alkyl = Octyl, Hexyl, Butyl, and Ethyl) Ionic Liquids with Bovine Serum Albumin: An Alkyl Chain Length-Dependent Study.

Herein, we have investigated the binding interaction of bovine serum albumin (BSA) with a series of 1-alkyl-3-methylimidazolium tetrafluoroborate (alkyl = ethyl, butyl, hexyl, and octyl) ionic liquids (ILs) in physiological buffer medium. The ILs are chosen basically to understand the effect of alkyl chain length on IL-protein interaction. Experiments have shown that the quenching of fluorescence of BSA is induced by relatively longer alkyl chain-containing ILs, [OMIM][BF4] and [HMIM][BF4]. The enthalpy-driven spontaneous binding (-ve Δ G) of hexyl and octyl chain-containing ILs with the protein is mediated by both hydrogen-bonding and van der Waals interactions. The experimental data have categorically explained the denaturation of protein conformation upon interaction with both [OMIM][BF4] and [HMIM][BF4]. The molecular docking calculation nicely corroborates the experimentally obtained results. The present study reveals that neither a smaller alkyl group-containing IL nor a very large alkyl group-containing IL is necessary to have effective protein-IL interactions. The study also reveals the influence of hydrophobic interaction over and above the hydrogen-bonding interaction on protein-IL binding events and essentially gives an idea about the optimum hydrophobic character of the ILs that is necessary to induce protein-IL interaction and consequently the denaturation of the protein structure.