Nonlinear Dielectric Response of Dilute Protein Solutions

A theory for the nonlinear dielectric response of dilute protein solutions is presented. The field-dependent dielectric function of the protein solution changes linearly with the electric field squared in the lowest order. The slope of this dependence is expressed in terms of the protein dipole moment $M_0$, its volume fraction in solution $\eta_0$, and the second osmotic virial coefficient. For practical conditions, the nonlinear dielectric response scales as $\eta_0^3 M_0^8$. This strong dependence on the protein dipole moment and concentration establishes a sharp contrast between the nonlinear response of solvated proteins relative to the surrounding polar solvent. Nonlinear dielectric response can serve as a sensitive tool for monitoring protein conformations and physiological activity.