Unveiling the three-step model for the interaction of imidazolium-based ionic liquids on albumin

The effect of the ionic liquids (ILs) 1-methyl-3-tetradecyl imidazolium chloride ([C14MIM][Cl]), 1-dodecyl-3-methylimidazolium chloride ([C12MIM][Cl]) and 1-decyl-methylimidazolium chloride ([C10MIM][Cl]) on the structure of bovine serum albumin (BSA) was investigated by fluorescence spectroscopic, UV-Vis spectroscopy, small an-gle X-ray scattering and molecular dynamics simulations. Concerning the fluorescence measurements, we observed a blue shift and a fluorescence quenching as IL concen-tration increased in the solution. Such behavior was observed for all three studied imidazolium-based IL, being larger as the number of methylene groups in the alkyl chain grew. UV-Vis absorbance measurements indicate that even at relatively small IL:protein ratios, like 1:1, or 1:2 ([C14MIM][Cl]) is able to change, at least partially, the sample turbidity. SAXS results agree with the spectroscopic techniques and sug-gest that the proteins underwent a partial unfolding, evidenced by an increase in the radius of gyration (Rg) of the scattering particle. In the absence and presence of ([C14MIM][Cl])=3mM BSA Rg, increases from 29.1 to 45.1 [A], respectively. Together, these results indicate that the interaction of BSA with IL is divided into three stages: the first stage is characterized by the protein in its native form. It takes place for IL:protein[≤] 1:2 and the interaction is predominantly due to the electrostatic forces, provided by the negative charges on the surface of the BSA and the cationic polar head of the ILs. In the second stage, higher IL concentrations induce the unfolding of the protein, most likely inducing the unfolding of domains I and III, in such a way that the proteins secondary structure is kept almost unaltered. In the last stage, IL micelles start to form and, therefore, interaction with protein reaches a saturation point and free micelles may be formed. We believe this work provides new information about the interaction of ILs with BSA. Graphical Abstract O_FIG O_LINKSMALLFIG WIDTH=200 HEIGHT=140 SRC="FIGDIR/small/542168v1_ufig1.gif" ALT="Figure 1"> View larger version (26K): org.highwire.dtl.DTLVardef@1e28360org.highwire.dtl.DTLVardef@1f1fe0org.highwire.dtl.DTLVardef@165b77forg.highwire.dtl.DTLVardef@f20500_HPS_FORMAT_FIGEXP M_FIG C_FIG