Room temperature molecular dynamics (MD) simulations
of the globular
protein α-lactalbumin in aqueous solutions containing BMIM (1-butyl-3-methylimidazolium)
based ionic liquids (ILs) with a series of Hofmeister anions have
been carried out. In particular, effects of anions of different shapes/sizes
and hydrophobic/hydrophilic characters, namely, thiocyanate (SCN–), dicyanamide (DCA–), methyl sulfate
(MS–), triflate (TFO–), and bis(trifluoromethane)
sulfonimide (TF2N–) on the heterogeneous
dynamic environment at the interface around the protein have been
explored. The calculations revealed exchange of population between
water and IL cation–anion components beyond the first layer
of bound water molecules at the protein surface. Further, increasingly
restricted diffusivity of the IL components and water around the protein
has been found to be associated with a longer time scale for the onset
of dynamic heterogeneity at the interface. Restricted diffusivity
of water molecules at the interface in the presence of the ILs has
been found to be correlated with the longer time scale of structural
relaxations of protein–water hydrogen bonds at the interface.
More importantly, the time scale associated with the reorientations
of the anions has been found to be anticorrelated with their translational
diffusivity, with the effect being more at the interface as compared
to the bulk IL solutions. It is demonstrated that the nonuniform ability
of the anions to form hydrogen bonds with water due to their differential
shapes and hydrophilic characters is the origin of such anticorrelation