  Absorption of radiation by solution is described by the solution dielectric
constant and can be viewed as a specific application of the dielectric theory
of solutions. For ideal solutions, the dielectric boundary value problem
separates the polar response into the polarization of the void in the liquid
created by the solute and the response of the solute dipole. In the case of a
protein as a solute, its nuclear dynamics do not project on significant
fluctuations of the dipole moment in the terahertz domain of frequencies and
the protein dipole can be viewed as dynamically frozen. Absorption of radiation
then reflects the interfacial polarization. Here we apply an analytical theory
and computer simulation to absorption of radiation by ideal solutions of
lysozyme. Comparison with experiment shows that Maxwell electrostatics fails to
describe the polarization of the protein-water interface and the "Lorentz
void", which does not anticipate polarization of the solute void by the
external field (no surface charges), better represents the data. An analytical
theory for the slope of the solution absorption against the volume fraction of
the solute is formulated in terms of the cavity field response function. It is
calculated from molecular dynamics simulations in good agreement with
experiment. The protein hydration shell emerges as a separate sub-ensemble,
which collectively is not described by the standard electrostatics of
dielectrics.
