A comparative study of the effects of the Hofmeister series anions of the ionic salts and ionic liquids on the stability of α-chymotrypsin

Abstract

In this article, we have compared the anions of sodium salts (Is) and ionic liquids (ILs) with the stability and structure of αchymotrypsin (CT), through fluorescence, thermal fluorescence 10 analysis and circular dichroism (CD) spectroscopy. The experimental results revealed that the Hofmeister series of anions such as SCN‾, SO 4 2¯, Cl‾, Br‾, CH 3 COO‾ and I‾ of Is destabilized the native structure of the CT. On the contrary, the anions such as CH 3 COO‾, Cl‾ and Br‾ of imidazolium-based IL with fixed cation 15 such as 1-butyl-3-methylimidazolium, [Bmim] + stabilized the native structure of the CT. The remaining anions of ILs such as SCN‾, HSO 4 ‾, and I‾ acted as denaturing agents for the native structure of CT. Furthermore, molecular docking results show that the imidazolium-cation of the IL enters the sub-domains of 20 the CT and interacts with the ionic residues of the CT, that is Ser217 close to Trp215. This interaction is in well agreement with the fluorescence quenching observed for CT in the presence of [Bmim] + . On the other hand, the destabilizing anion such as SO 4 2¯w as observed to be directly interacting with Ser195 in the 25 active site of the CT. We have observed that the Hofmeister series effects of anions of either Is or ILs are entirely based on the interaction of the anions with its counterion, that is the cation, with solvent molecules, as well as with the protein surface. Evidently, these interactions vary with co-solvent system and the 30 type of the protein. Hence, the stability of a biomolecule in the presence of the anions may or may not obey Hofmeister series. 65 80

Endeavour to simplify the frustrated concept of protein-ammonium family ionic liquid interactions

“…Complex formation between heme coordination site and imidazolium cation was supposed to occur on dissolution. 138 As already explained in the preceding section, in 2011, Wei and Danielson 84 showed MAF and EAF to improve the structural stability of cyt c. As per our experience, the protein CT was stabilized in ammonium family ILs such as DEAA, DEAS, DEAP, TMAA, TMAS and TMAP 83,91 however, CT was destabilized in the presence of Hofmeister series of anions such as SCN -, HSO 4 -, I -with [Bmim] + cation 139. Mb was also stabilized in ammonium-based ILs88 although it was destabilized in the presence of SCN -, I -, Br -, Cl -, COO -with [Bmim] + cation 140.…”

On the Use of a Protic Ionic Liquid with a Novel Cation To Study Anion Basicity

“…[7] One of the most important factorsd etermining the properties of an ionicl iquid (IL) is the basicity of its anion. [8][9][10][11][12][13] Their basicities can to some extent be inferred from the equilibrium constant of their corresponding acids' dissociation at low concentrations in water [2,14] (pK a values) but the accuracyo fs uch deduction is limited by solvent (medium)a nd concentratione ffects. [8][9][10][11][12][13] Their basicities can to some extent be inferred from the equilibrium constant of their corresponding acids' dissociation at low concentrations in water [2,14] (pK a values) but the accuracyo fs uch deduction is limited by solvent (medium)a nd concentratione ffects.…”

“…DMI, which to the best of our knowledge has not been used to make PILs before,s hows larger changes in the 15 Na nd 13 Cc hemical shifts upon interaction with acids, compared to aliphatic amines for example, and the presenceo ft he amide bond also makes it suitable and conveniently responsive for vibrational spectroscopicc haracterization. DMI, which to the best of our knowledge has not been used to make PILs before,s hows larger changes in the 15 Na nd 13 Cc hemical shifts upon interaction with acids, compared to aliphatic amines for example, and the presenceo ft he amide bond also makes it suitable and conveniently responsive for vibrational spectroscopicc haracterization.…”

“…DMI, which to the best of our knowledge has not been used to make PILs before,s hows larger changes in the 15 Na nd 13 Cc hemical shifts upon interaction with acids, compared to aliphatic amines for example, and the presenceo ft he amide bond also makes it suitable and conveniently responsive for vibrational spectroscopicc haracterization. This change in the electronic structure of the base is what causes the change in the directly involved 15 Na nd 13 Cn uclei chemicals hifts,a nd also the disappearance of the C=Os tretching band upon protonation (explained further below). This change in the electronic structure of the base is what causes the change in the directly involved 15 Na nd 13 Cn uclei chemicals hifts,a nd also the disappearance of the C=Os tretching band upon protonation (explained further below).…”

“…It is known that amides interactw ith electrophiles, [57] specifically proton, [58][59][60] through their oxygen atom (Figure 1), putting ap ositive chargeo nt he nitrogen atom andg iving CÀN bond more, and CÀOb ond less, of ad ouble-bond character. The change in the 15 Na nd 13 Cc hemical shifts of DMI is carefully monitored in its mixtures with two acids, sulfuric acid (H 2 SO 4 ) and methanesulfonic acid (HOMs), over the full range of compositions and then measured at composition1 :1 (i.e.,f or DMIbased PILs) using different acids of different character and strength.I nt he Results section it will be shown how this systematicc hange can give aq ualitative order of the basicity of IL anions. The change in the 15 Na nd 13 Cc hemical shifts of DMI is carefully monitored in its mixtures with two acids, sulfuric acid (H 2 SO 4 ) and methanesulfonic acid (HOMs), over the full range of compositions and then measured at composition1 :1 (i.e.,f or DMIbased PILs) using different acids of different character and strength.I nt he Results section it will be shown how this systematicc hange can give aq ualitative order of the basicity of IL anions.…”