Dielectrophoresis of Proteins in Solution.

A nonionic particle placed in the gradient of an electric field experiences the dielectrophoretic force which scales linearly with the gradient of the electric field squared. The proportionality constant is the dielectrophoretic susceptibility, that is, a linear transport coefficient. For proteins in solution, it is mostly affected by the following two parameters: the squared dipole moment and the cavity susceptibility accounting for cross-correlations of the protein dipole with the hydration shell (protein-water Kirkwood factor). Both of these parameters enter the dielectric increment of the solution which fully specifies the dielectrophoretic susceptibility. The link between these two measurable properties is proven here to hold using molecular dynamics simulations of solvated proteins. The dielectrophoretic susceptibility for proteins is in the range of 10, significantly exceeding traditional estimates limiting it to values below unity. Part of this large magnitude of the dielectrophoretic response is the cavity susceptibility of the protein-water interface, which significantly exceeds dielectric estimates. The study analyzes local fields inside the protein in terms of the reaction-field and directing-field components. We find that the local field exceeds the external field by a substantial factor described by the local field susceptibility. The electric field produced by water inside the protein is retarded by 3-4 orders of magnitude compared to the bulk.