Water State in Hemoglobin Solutions: Microwave Dielectric Spectroscopy Study

In this article, we discuss the dielectric relaxation of water in the hemoglobin (Hb) aqueous solutions in different states [methemoglobin (MetHb), oxygenated hemoglobin (OxyHb), and deoxygenated hemoglobin (DeoxyHb)]. The interpretation of the results was performed by the 3-D trajectory approach, which considers dielectric parameters of water relaxation and protein concentration. It has been shown that the interaction of amino acid dipole residues, located on the surface of a protein macromolecule, with water dipoles determines the protein hydration. In addition, ions of the buffer, in which proteins are dissolved, are also hydrated. The transition from a dipole–ion interaction of water molecules to a dipole–dipole interaction of water with an increase in protein concentration is observed. We proposed a new approach to calculate Hb hydration shells. The theoretical model defines the change in the ratio between the content of free and bound water molecules (BWMs) as a function of MetHb concentration in ion-free and ion-containing aqueous solutions. It considers the number of positive and negative charges at the surface of the protein molecule, the number of BWMs in its hydration shells, and the partition of water molecules bound to MetHb and the inorganic ions. The theoretical evaluation of the ratio of free-to-bound water reveals that, in the absence of ions, MetHb binds about 1400 water molecules, which is in good agreement with experimental data. At MetHb concentrations close to physiological, Hb is dominating in binding water over the inorganic ions, and the amount of BWM remains at the level of ~20% of total cytosol water as the concentration of Hb increases above 15 g/dL. These results suggest that in the physiological concentration diapason (29–35 g/dL), molecules of MetHb are so close together that their hydration shells interact and shrink from four to one layer of water molecules. In contrast, Hb molecules aggregate to neutralize their surface charges mutually.