The nature of solvent-biomolecule interactions is generally weak and non-specific. The addition of ionic liquids (ILs), which have emerged as a new class of solvents, strengthen the stability of some proteins whereas the same ILs weaken the stability of some other proteins. Although ILs are commonly used for the stabilization of biomolecules, the bimolecular interactions of their stabilization-destabilization is still an active subject of considerable interest and studies on this topic have been limited. To reveal the impact of ILs on the stability of proteins, a series of protic ILs possessing a tetra-alkyl ammonium cation [R4N](+) with a hydroxide [OH](-) anion were synthesized. In this study, we report the structural stability of heme proteins such as myoglobin (Mb) and hemoglobin (Hb) in a series of ammonium-based ILs such as tetramethyl ammonium hydroxide [(CH3)4N](+)[OH](-) (TMAH), tetraethyl ammonium hydroxide [(C2H5)4N](+)[OH](-) (TEAH), tetrapropyl ammonium hydroxide [(C3H7)4N](+)[OH](-) (TPAH) and tetrabutyl ammonium hydroxide [(C4H9)4N](+)[OH](-) (TBAH) by fluorescence and circular dichroism (CD) spectroscopic studies. Our experimental results reveal that less viscous ILs carrying smaller alkyl chain such as TMAH are strong destabilizers of the heme proteins as compared to the ILs carrying bulkier alkyl chains which are more viscous ILs, such as TBAH. Therefore, our results demonstrate that the addition of these ILs to the heme proteins decreases their thermal stability allowing the protein to be in an unfolded state at lower temperatures. Further, we describe the molecular-structural interaction of the heme proteins with the ILs (molecule like a ligand) by the PatchDocking method.

No abstract

The large amount of attention earned by ionic liquids (ILs) in the various physical and chemical sciences has been attributed to their unique, designer nature. In the past few years, the role of ILs in protein folding/unfolding has been rapidly growing. In light of the increasing importance of ILs, it is desirable to systematize the ion effects on protein properties such as structure stability, activity and enantioselectivity. Various studies available in the literature show ILs as a potential solvent medium for many enzymatic reactions, as well as in various protein folding/unfolding studies. Various reviews by many researchers focus on the synthesis, application and general properties of the ILs, however a review focussing on the effect of various ILs on the activity, structure and stability of proteins is still missing. Also, according to the best of our knowledge there is no single review available throughout the literature that focuses on the effect of the same family of ILs on different proteins. Therefore, it is a priority to obtain complete knowledge of the biomolecules, particularly amino acids (AAs) and proteins in a particular IL family. The focus of the present perspective is to investigate the performance of a list of proteins and protein model compounds in the presence of ammonium-based ILs. This perspective presents a survey of all the key developments from the available reports and also our past and present experience related to proteins and ammonium-based ILs. Additionally, we have tried to put the available information in chronological order in most of the cases. The use of ammonium family ILs as a co-solvent for various proteins model compounds and proteins has been outlined. This perspective can act as a barometer for reckoning the various advancements made in this field and can also galvanize further investigation of various untouched aspects of this research area.

In this article, we have explored the influence of peculiar member of imidazolium-based ionic liquid (IL) 1-allyl-3-methylimidazolium chloride ([Amim][Cl]) on the stability of haemoglobin (Hb) by using fluorescence, thermal fluorescence, ANS fluorescence, circular dichroism (CD) spectroscopy and dynamic light scattering (DLS) measurements. In the attempt of searching of IL which can provide stability to Hb, for the first time, we have successfully shown the stability of Hb in [Amim][Cl]. Our results show that [Amim][Cl] stabilizes the Hb native structure in concentration dependent manner which indicates to fact that the concentration of IL is very crucial to determine the stabilization/destabilization of the protein in ILs. Here, we have observed that low concentration of IL provides stability to the protein while high concentration destabilizes the protein. The commendatory results obtained from the multi spectroscopic approaches provided some guidance regarding the mechanism of interaction between Hb and [Amim][Cl]. The predicted mechanism may be the accumulation of imidazolium cation on the protein surface and tendency of anion to remain in the bulk phase ultimately resulting in stabilization of the protein since; this can in direct/indirect way affect the hydrogen bonding of protein with the surrounding water. on the basis of our results [Amim][Cl] seems to be novel solvent for the stability of Hb, however, further studies with other proteins need to be carried out for further information. CONCLUSION Our results illustrate that interactions of ions of ILs with proteins are important for understanding the effects shown by them on proteins whether stabilization/destabilization. These interactions are governed by the ions present as well as the concentrations used. In this work [Amim][Cl] has been identified as a compatible solvent for Hb structure, nevertheless, the stabilization tendency being concentration dependent. Low concentration of [Amim][Cl] stabilized Hb while higher concentration destabilized Hb. Therefore, the results obtained here facilitate a much better understanding of protein folding/unfolding in IL. Hence, this IL was found to be stabilizing agent for Hb. Since, [Amim][Cl] is thermally stable as well as non-volatile, consequently it can be supposed to be promising green/biocompatible solvent for the proteins. However, further research with other proteins in the presence of this IL is required. Also, from our research this becomes completely clear that the role of alkyl group of imidazolium ILs is vital in determining the protein stability. Therefore, this work can encourage designing of novel ILs which can provide stability to many other proteins. ASSOCIATED CONTENT Supporting InformationThe supporting information is available free of charge on the ACS Publications website Thermal fluorescence spectra analysis of the Hb in buffer and in different concentrations of [Amim][Cl] (Figures S1-S7). Synopsis: The native structure of Hb concentration of [Amim][Cl] Graphical AbstractUnprecedented Improv...

No abstract