Effect Of Imidazolium Cation Research Articles

Experimental and DFT studies on micellization features of anionic surface active ionic liquid and nonionic surfactant mixtures: Effect of imidazolium cations

The micellization characteristics of nonionic surfactant (Triton TX-100, p-octyl polyethylene glycol phenyl ether) and two anionic surface active ionic liquids (SAILs) were investigated by surface tension measurements, steady state fluorescence tests, dynamic light scattering (DLS), HNuclear magnetic resonance, and density functional theory (DFT) calculations. The anionic SAILs were 1-ethoxycarbonyl-3-methylimidazolium dodecyl sulfate ([Etmim][C12H25SO4]) and 1-ethoxycarbonyl-3-butylimidazolium dodecyl sulfate ([Etbim][C12H25SO4]). CMCmix values of SAIL/TX-100 mixtures were between CMC values of individual surfactants, and decreased with the increase of the molar fraction of TX-100. The negative β values (interaction parameter) of SAIL/TX-100 mixtures indicated good synergistic effect amid SAIL/TX-100 mixtures. [Etbim][C12H25SO4] showed stronger attractive interactions than [Etmim][C12H25SO4] during SAIL/TX-100 micellization. The mixed micelles contained a larger molar fraction of TX-100 over the entire range of composition. At same molar fraction of TX-100, X1 values of [Etbim][C12H25SO4]/TX-100 mixtures were lower than those of [Etmim][C12H25SO4]/TX-100 mixtures. With addition of SAIL, the particle sizes of SAIL/TX-100 micelles decreased. With the increase of SAIL concentrations, the particle sizes of SAIL/TX-100 micelles decreased. In presence of SAILs ([Etmim][C12H25SO4] or [Etbim][C12H25SO4]), all protons of TX-100 moved downfield. The chemical shifts of protons in [Etmim][C12H25SO4] or [Etbim][C12H25SO4] slightly moved upfield. DFT simulations demonstrated the presence of hydrogen bonds between the imidazolium cation of SAILs and EO chain of TX-100, and between sulfate anion group(-SO4-) of SAILs and hydrogen atoms in TX-100. The information will shed light on the rapid screening of novel anionic SAIL/surfactant mixtures for practical applications.

The effect of imidazolium cations on the structure and activity of the Candida antarctica Lipase B enzyme in ionic liquids.

In order to understand how cations affect the structural changes and enzyme activity of Lipase B from Candida antarctica, we performed all-atom molecular dynamics simulations of CALB in four types of ionic liquids (ILs) with varying sizes of imidazolium cations and correlated these results with the experimentally determined CALB activity. The imidazolium cations under study differ in the alkyl tail length in the following order: [Emim](+) < [Bmim](+) < [Hmim](+) < [Omim](+). We observed that the best enzyme activity and structural stability of CALB are obtained in [Bmim][TfO] and [Hmim][TfO]. In contrast, in [Emim][TfO], bonding of [TfO](-) to LYS-290 disrupts the interactions between LYS-290 and ILE-285, which leads to a closed catalytic gate conformation with low accessibility of substrates to the catalytic triad. In [Omim][TfO], strong hydrophobic interactions between [Omim](+) and LEU-278 result in a significant loss of the secondary structure of the α-10 helix and cause the exposure of the catalytic triad to ILs, which affects the stability of the catalytic triad and consequently deteriorates the enzyme activity. Overall, our study indicates that a high ion coordination number ([Emim][TfO]) or the presence of a long hydrophobic tail ([Omim][TfO]) can facilitate ion-protein interactions that cause structural distortions and a decrease in CALB enzyme activity in ILs.