Ionic liquid induced dehydration and domain closure in lysozyme: FCS and MD simulation

摘要

Effect of a room temperature ionic liquid (RTIL, [pmim][Br]) on the structure and dynamics of the protein, lysozyme, is investigated by fluorescence correlation spectroscopy (FCS) and molecular dynamic (MD) simulation. The FCS data indicate that addition of the RTIL ([pmim][Br]) leads to reduction in size and faster conformational dynamics of the protein. The hydrodynamic radius (r(H)) of lysozyme decreases from 18 angstrom in 0 M [pmim][Br] to 11 angstrom in 1.5 M [pmim][Br] while the conformational relaxation time decreases from 65 mu s to 5 mu s. Molecular origin of the collapse (size reduction) of lysozyme in aqueous RTIL is analyzed by MD simulation. The radial distribution function of water, RTIL cation, and RTIL anion from protein clearly indicates that addition of RTIL causes replacement of interfacial water by RTIL cation ([pmim](+)) from the first solvation layer of the protein providing a comparatively dehydrated environment. This preferential solvation of the protein by the RTIL cation extends up to 30 angstrom from the protein surface giving rise to a nanoscopic cage of overall radius 42 angstrom. In the nanoscopic cage of the RTIL (42 angstrom), volume fraction of the protein (radius 12 angstrom) is only about 2%. RTIL anion does not show any preferential solvation near protein surface. Comparison of effective radius obtained from simulation and from FCS data suggests that the dry protein (radius 12 angstrom) alone diffuses in a nanoscopic cage of RTIL (radius 42 angstrom). MD simulation further reveals a decrease in distance (domain closure) between the two domains (alpha and beta) of the protein leading to a more compact structure compared to that in the native state. (C) 2015 AIP Publishing LLC.