  A theory of radiation absorption by dielectric mixtures is presented. The
coarse-grained formulation is based on the wavevector-dependent correlation
functions of molecular dipoles of the host polar liquid and a density-density
structure factor of the positions of the solutes. A nonlinear dependence of the
absorption coefficient on the solute concentration is predicted and originates
from the mutual polarization of the liquid surrounding the solutes by the
collective field of the solute dipoles aligned along the radiation field. The
theory is applied to terahertz absorption of hydrated saccharides and proteins.
While the theory gives an excellent account of the observations for saccharides
without additional assumptions and fitting parameters, experimental absorption
coefficient of protein solutions significantly exceeds theoretical calculations
within standard dielectric models and shows a peak against the protein
concentration. A substantial polarization of protein's hydration shell is
required to explain the differences between standard theories and experiment.
When the correlation function of the total dipole moment of the protein with
its hydration shell from numerical simulations is used in the present
analytical model an absorption peak similar to that seen is experiment is
obtained. The result is sensitive to the specifics of protein-protein
interactions in solution. Numerical testing of the theory requires the
combination of terahertz dielectric and small-angle scattering measurements.

Biological Physics;Chemical Physics