Optical Spectroscopic Studies on Structural Changes of Helical-rich Proteins in Aqueous Solutions of Ionic Liquids

We have investigated structural changes of myoglobin and cytochrome c, which are helical-rich proteins, in aqueous 1-butyl-3-methylimidazolium chloride (bmim]Cl]) solutions by Fourier transform infrared and circular dichroism spectroscopy. At low bmim]Cl] concentrations {X (mol% IL) < 10}, both proteins unfold. Remarkably, at high bmim]Cl] concentrations (X > 10), myoglobin aggregates whereas cytochrome c refolds its α-helical structure. The tertiary structures of both proteins are disrupted over the entire range of studied bmim]Cl] concentrations. Our results suggest that, in aqueous solutions at high bmim]Cl] concentrations, the differences in structural transitions between myoglobin and cytochrome c might be due to the difference in hydration between these proteins.