Spectroscopy of solutions in the low frequency extended THz frequency range
EPJ Web of Conferences
One of the most important biological components of the living systems is water. It is the main informative feature and simultaneously the main obstacle for tissue and bio-solution spectroscopy in THz range. Its intensive absorption in this frequency range, on the one hand, limits its penetration into the samples, but, on the other hand, allows one to consider it as the subject of a separate fruitful study     . Water molecules form hydrogen bonds (HB) with their neighbors to
... eighbors to construct the water network. Currently, water can be divided on bulk water (it does not form bonds with biomolecules) and hydration or bounded water (it surrounds biomolecules and interacts with them). Thus, in the solution, a considerable part of water molecules are in the form of a hydration shell -Fig 1. Fig. 1. Components of the solution: free water (blue background), bounded water (gray circles), solute molecules (yellow circles). Dilute solution (a) and saturated solution (b). After  Bound or free water makes a valuable contribution to the THz response of biological objects. This allows us to use THz spectroscopy as a powerful tool to study different forms of water in a wide class of biological media, including the macromolecules in aqueous solutions. Actually, we do not measure the solute, its contribution is negligible. We measure how the solute modifies the solvent, the appearance and the structure of hydration shell around the solute molecules. To detect small changes in the solution smooth spectra, we should precisely describe solvent properties in a broad spectral range. We describe a number of models of a water solution dielectric permittivity, which are applicable in the THz frequency range. We give a detailed description of biological solutions (protein and sugar solutions, blood components), analyze modern measuring techniques. All known processes (relaxation and dumped oscillation) in polar solutions below the tens of THz have very broadband responses, so it is essential to combine several experimental techniques, each for its own spectral range, to obtain spectra over many octaves. From the low frequency side, it is well established dielectric spectroscopy; from the high frequen-cy side, it is FTIR and it is THz-time-domain spectroscopy (TDS) between them. The main relaxation process (at 0.02 THz) reflects the cooperative reorientational dynamics of the dipole moment. It is assigned to the cooperative reorientation time of hydrogen-bonded bulk water molecules involving HB switching events. The oscillation processes indicates the overdamped modes, which correspond to several known vibration modes in the THz region: the bending mode between two water molecules forming the hydrogen-bond (at 1.5-2 THz); the intermolecular stretching of water is assigned to a hindered O...O translation (at 5.4 THz) etc. Fig. 2. Slow relaxation shift of bounded water spectra. To fit dielectric function of water a well-known model with Debye-type relaxation and over-dumped oscillator components is usually applied     .