On the Interaction of Alkyl-Functionalized Ionic Liquids with Model Proteins: A Spectroscopic and Structural Study

Abstract

Ionic liquids (ILs) are salts that are liquid at temperatures smaller than 100°C and are gaining prominence in the so-called green chemistry. In order to understand its interaction with biologically relevant systems, we conducted a systematic study of the interaction of three different ILs ([C10mim][Cl], [C12mim][Cl] and [C14mim][Cl]) with three proteins (BSA, HSA and Lysozyme), by means of UV-vis absorption, fluorescence, circular dicrhoism (CD) and small angle X-ray scattering (SAXS). We observed fluorescence quenching of all studied proteins, the decrease were (for BSA, HSA and lysozyme, respectively): (55±3)%, (16.1±0.8)% to (4.1±0.2)% in the presence of 0.6mm [C14mim][Cl], similar trend were obtained for [C12mim][Cl] and [C10mim][Cl]. We also note the shift of the fluorescent peak of BSA and HSA for shorter wavelengths (blue-shift), as the IL content was increased. The maximum shift (Δλ) achieved corresponded to (21±1) nm for both albumins, whereas no significant displacement was observed for lysozyme. SAXS data indicate an increasing in the proteins radius of gyration (Rg) as ILs was added in the solution. Rg of BSA, HSA and lysozyme in the absence of IL are (29±1) A, (30±1) A and (15±1) A, respectively, and go to (46±1) A, (44±1) A and (20±1) A, respectively, in the presence of 0.6mm [C14mim][Cl]. CD data suggest a slight loss of secondary structure of both albumins (BSA and HSA), from 80 to 65% of α-helix in the absence and presence of 0.6mm [C14mim][ Cl], respectively. Taking together, our data suggest that the interaction between IL and the proteins is initially driven by electrostatic forces, having also a major hydrophobic contribution. We believe this work provides new information about the interaction of ILs with model proteins, indicating its ability to alter the conformation of the same.