Hydrated proteins are intermediate in size between molecules of polar liquids and large dielectric bodies. They also usually carry significant dipole moments, 200-300 D. The combination of a large dipole moment with extended interface poses a theoretical challenge: standard theories of polar liquids do not recognize the existence of an extended polarized shell of the solvent, while dielectric models, operating with dielectric constants, do not correctly incorporate the microscopic polarization.  We propose a theoretical framework combining the molecular dipole moment of the protein with the polarization of a large sub-ensemble of solvent molecules at the solute-solvent interface. It represents the dielectric response of the solution as a mutually compensating combination of the response of the protein dipole and the response of its hydration layer. The formulation is applied to experimental frequency-dependent dielectric spectra of hydrated proteins and to terahertz (THz) absorption of protein solutions. The analysis of experimental results and Molecular Dynamics simulations shows that the polarization of hydration shells of proteins deviates in many significant ways from the polar response of small solutes in solution. It is recognized that the strong restructuring of water around polar/ionized residues results in a long-ranged perturbation of the hydration layer, with the polarization perturbation propagating 20-40 A into the bulk. A number of relaxation processes are also characterized, with a slow processes, from nanosecond to sub-microseconds, resulting from elastic deformations of the protein shape (on the faster scale) and motions of protein loops carrying ionized residues (on the slower scale).

Electrostatics of the protein-water interface

246th National Meeting (2013)

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